Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Power Amplifier

IntroductionIt is rare that an electronic system will be able to operate with a single amplifier. But in

general more than amplifier is used. This amplifier are so connected that output of one stage is connected as input to next stage. In such system small signal is applied to first of amplifier , this amplified output is applied as input to next stage and so on. This process is continued until the output signal from the final amplifier is of sufficient amplitude as per the requirement is available.

We generally require that a weak signal (i.e. voice) should be heard loudly, it may be signal applied to picture tube , or a signal which may be applied antenna for transmission. This becomes possible only if the voltage and power level of the weak signal is raised and supplied to the load. Hence practical amplifier always consists of a number of stages that amplify a weak signal until sufficient power is available to operate a loudspeaker or other output device or circuit. The first few stages in this multi stage amplifier have the function of only voltage amplifier. However, the last stage is designed to provide maximum power. This final stage is known as power stage or driver stage.Fig.shows block diagram of such power amplifier.

Practically before final stage, their is driver stage, which is also one type of power amplifier. Driver stage supplies the necessary power to the output stage.

A transistor amplifier which raises the power level of the signals that have audio frequency range is known as transistor audio power amplifier. Block Diagram of Practical Audio Amplifier

Fig Block Diagram of Audio AmplifierAmplifier

An Amplifier is the electronic device which is used for raising the strength of a weak signal is called an amplifier.

When only one transistor with associated circuitry is used for amplifying a weak signal that circuit is known as single stage amplifier.Classification of amplifier on the basis of different factors.

Linear amplifiers may be classified according to their mode of operations i.e. the way they operate or the predetermined set of values. Their descriptions are based on the following factors

1. According to input signal(a) Small signal amplifier,(b) Large signal amplifier.

2. According to output:(a) Voltage amplifier,(b) Power amplifier.

3. According to transistor configuration(a) Common-emitter (CE) amplifier, (b) Common-base (CB) amplifier,(c) Common-collector (CC) amplifier.

4. According to biasing conditions(a) Class-A – amplify complete Cycle ,(b) Class - B, - amplify Half cycle(c) Class – AB – amplify More than half but less than Full cycle(d) Class – C – amplify less than half cycle

5 According to frequency response :(a)direct current (DC) amplifier,(b) Audio frequency (AF) amplifier,(c) Radio frequency (RF) amplifier,(d) Ultra-high frequency (UHF) and micro-wave frequency amplifier.

6.According to number of stages:(a) Single-stage amplifier(b) Multi-stage amplifier,

7.According to coupling methods:(a) Direct coupled (DC) amplifier,(b) Resistance-capacitance (AC) coupled amplifier,(c) Transformer coupled (TC) amplifier.

Classification Of Power Amplifier

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VoltageAmplifier

VoltageAmplifier

DriverStage

PowerStage

Microphone

Speaker

RC Coupling TransformerCoupling

Weak Signal

Amplified Signal

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Power Amplifiers are classified as per their position of d.c operating point. Each class

has its own merits and demerits. As per our requirement, we can select any one of them.They are classified as:

(i) Class A(ii) Class B(iii) Class AB(iv) Class C

(i) Class A Power AmplifierFig.. Class A power Amplifier.If the collector current flows at all times during the entire cycle of input signal, such amplifier is known as class A power amplifier. DC load lines and operating point of class A is shown in Fig.7.2 Transistor is not allowed to enter saturation and cutoff region. Variation of collector is for completer3600 of input. Hence output will contain the entire input signal.

In this amplifier operating point is located at the centre of the load line and in this distortion is minimum and collector efficiency is approximately equal to 35%.Class A amps are often used for "signal" level circuits (where power requirements are small)

(ii) Class B Power AmplifierFig.Class B Power AmplifierIf the collector current flows only during positive half

cycle of input signal, it is called class B power amplifier. In this operating point is located on the X-axis of DC load line as shown in Fig. ie in cutoff region.

In this amplifier distortion is maximum but collector efficiency is approximately equal to 60%.Transistor is biased at cutoff so that when there is no input there is not conduction. Hence the conduction during exactly 50% of the time that the AC input is applied.

class B amplifiers were common in clock radio circuits, pocket transistor radios, or other applications where quality of sound is not that critical

(iii) Class AB power AmplifierFig.Class AB Power AmplifierIn this amplifier collector current flows for more than half

cycle of input signal. In this amplifier operating point lies between class A and class B as shown in Fig.

For such amplifier the Operating(Q) point is selected near the cut off region of the load line , such that collector current flow for more half cycle but less than full of the applied signal.

An amplifier biased in AB mode it will conduct more than 50% and less than 100% of the time that an input signal is present. In this case distortion is more but less than class B and collector efficiency is approximately equal to 55%

Class AB is probably the most common amplifier class currently used in home stereo and similar amplifiers.

(iv) Class C power AmplifierFig.Class C Power Amplifier

In this amplifier collector current flows for half of the half cycle of the input signal. In this amplifier operating point lies below the X-axis of DC-load line.

For such amplifier the Operating(Q) point is selected below cut off region of the load line , such that collector current flow for less than half cycle of the applied signal. An amplifier is biased below cutoff and that conducts less than 50% of the time during which an AC signal is applied.

In this case distortion is maximum but collector efficiency is approximately equal to 75%.

Class C amps are never used for audio circuits. They are commonly used in RF circuits

Single Stage Class A Power Amplifier – Resistive Load

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)

Fig.Single stage Class A Power amplifier

Fig.Input and Output Waveform with resistive load

Above it is already explained what is meant by Class A amplifier.The output current flows during the entire cycle of the ac. input signal. The operation of the amplifier is limited to smaller central region of the load line, so that it can operate in the linear region of the load line. The large-signals may shift the Q-point into non-linear regions near saturation or cut-off and hence produce amplitude distortion.Fig. shows circuit diagram and respective waveform.

Since the transistor operates over the linear region of the load line, therefore the output waveform is almost similar to the input waveform. The ac. power output per active device (or transistor) is smaller than that of class-B or class-C amplifier.

The overall efficiency or circuit efficiency of the amplifier circuit is an important parameter. It is defined as the ratio of ac. power delivered to the load to the total power supplied by the d.c. source. Mathematically, the overall efficiency,

The maximum possible overall efficiency of a class-A amplifier with series fed resistive load is 25%.

The collector efficiency of the amplifier circuit is another important parameter. It is de fined as the ratio of a.c. power delivered to the load, to the power supplied by the d.c. source to the transistor. Mathematically, collector circuit efficiency,

The maximum possible value of collector efficiency for a series fed resistive load is 50%. If a transformer coupled load instead of a direct coupled resistive load, the maximum possible overall efficiency increases to 50%.

Single Stage Class A Power Amplifier – Transformer CoupledFig. shows the Circuit of single stage class A power amplifier. It is also called as driver stage or driver amplifier.

The output from the last voltage amplification stage is fed to the driver stage. It supplies the necessary power to the final stage. The driver stage generally employs class A transformer coupled power amplifier. Here, concentrated effort is made to obtain maximum power gain.

Fig. shows the transformer coupled Class A power amplifier. Designing is done in such a way that its operating point lie exactly at the centre of DC load line. In this amplifier output of driver stage is given to the input of it, for further power amplification and output of it is given to the speaker. Since it is Class A power amplifier, distortion will be minimum, but collector efficiency is less.

Function of Components

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ηo = A.C. power delivered to the load . Total power supplied by the d.c. source = Average ac. output power Average d.c. input power

ηC = AC. power delivered the load . Power supplied by the d.c. source to the transistor

= Average a.c. output power . Average d.c. input power to the transistor

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)1. R1, R2 resistors: They are used for proper biasing. The biasing circuit must establish a

proper operating point.2. RE, resistor: It establishes the operating point from temperature. 3. Input Capacitor (Cin): It is used for coupling the signal to the base emitter junction if this

capacitor is not used R2 will come in parallel with the internal resistance of source and hence change the biasing, also the operating point.

4. By pass capacitor (CE): It is used to provide a low resistance path to amplified a.c signal, if it is not used, there will be more voltage drop across RE because a.c and d.c both will pass through RE and hence change the biasing and reduces the gain.

5. Centre tapped Step down transformer (T1): It is used for impedance matching so that maximum power should be transferred from driver stage to the final power stage.

6. Transistor (Q1): Its basic function is to amplify the weak input signal.Working

Driver stage is a class A power Amplifier. The output of final voltage amplifier is the input of driver stage through Cin. The driver stage provides the drive for push pull stage. (i.e. Power stage)

During the positive half cycle of input, the base emitter junction becomes more forward biased this will increase collector current. Hence for positive cycle of input, we are getting amplified +ve half cycle at the primary. Due to the transformer action negative (-ve) cycle is induced across secondary.

During negative half cycle of input forward bias between base and emitter junction decreases, this will decrease collector current, hence we get amplified -ve half cycle across primary, but at secondary, we get amplified +ve half cycle in the output. Thus their is phase reversal but input and output.

Transformer Impedance MatchingThe Function of a transformer is to match the low impedance load (Such as speaker) to

that of the output impedance of the amplifier. The impedance matching property follows from the relation

Let V1 , I1 and N1 be primary Voltage , current and number of turns on Primary Winding Let V2 , I2 and N2 be secondary Voltage , current and number of turns on secondary

Winding V1 = N1 * V2 and I1 = N2 * I1 N2 N1

The ratio of two equation V1 = 1 * V2I1 n2

I2It may be noted that if the value of secondary turns (N2) is less than primary turns (N1)

then the above equation shows that the transformer reduces the voltage in proportion to turns ratio (N2/N1) equal to n and step up the current in the same ratio.

Here V1/I1 is called as effective load resistance or the resistance from the primary of the transformer and is designated as R’L. N1/N2 is called as step down ratio.

V2/I2 is the resistance of the load connected across the transformer secondary. Hence this transformation of load resistance offer an opportunity to match the actual load resistance RL to the desired value R’L by proper choice of n.

Frequency response power

Amplifier gain is more as compared to voltage amplifier but frequency response is poor i.e. gain falls for both low and high frequency as explained.

We know that output voltage is equal to collector current multiplied by reactance of primary. At low frequencies, the reactance is very less hence output voltage is also very less. But for higher frequencies the capacitance between turns of winding act as by pass capacitor, due to this output voltage decreases, for higher- frequencies also. The above curve shows that gain changes for almost all frequencies.

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R’L = 1 * RL

n2 R’L = V1 and RL = V2

I1 I2

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Fig. shows frequency response of single ended class A power Amplifier.It is clear from

frequency response that is poor i.e. gain falls for both low and high frequency as explained.We know that output voltage is equal to collector current multiplied by reactance of

primaryAt low frequencies the reactance is very less (XL = 2FL) hence output voltage is also very

less. But for higher frequencies the capacitance between turns of winding act as by pass capacitor, due to this output voltage decreases, for higher frequencies also. The above curve shows that gain changes for almost all frequencies.

AdvantagesThe advantages of Class-A amplifier are:

1. It has least amplitude distortion.2. It has no hum.

DisadvantagesThe disadvantages of Class A amplifier are:

1. It has poor efficiency2. It provides low a.c. power output.

ApplicationsThe important applications of Class A amplifier are:

1. It is used where distortion less output is required e.g. audio power amplifier.2. It is used as voltage amplifier for audio, radio and video frequencies.

Class B AmplifierAlready discussed above what is class B amplifier. The output current flows only for one-

half of the cycle (i.e180°) of the input signal. Shown in fig.The transistor dissipates no power with zero input signal. However, it increases with the

increase in the amplitude of input signal. It is contrary to class-A amplifier operation, where the transistor dissipation is maximum with no input signal and minimum with the largest input signal.Fig.Q-point Waveform for Class B Power Amplifier

The average current drawn by the circuit in class-B operation is smaller than that in class A. Fig.shows the load line and q point look of the characteristic. As a result of this, the amount of power dissipated by the transistor is less in class-B. Thus the overall efficiency of the circuit is higher than that of class-A. Its maxi mum value has been found to be equal to 78.5%.

Class B Push Pull Power Amplifier Fig.shows the circuit of Class B push

pull Amplifier. It is used when high power at high collector efficiency is required but in this some distortion is present in the final output which is known as cross over distortion. This distortion can be easily eliminated by class AB or class A push pull amplifier.

In this circuit two transistors Q1 and Q2 are connected back to back. Both transistors are operated in class B operation i.e. collector current is nearly Zero in the absence of the signal. The centre-tapped secondary of driver transformer T1 supplies equal and opposite voltage to the base circuits of two transistors.

The output transformer T2 has the centre-tapped primary winding. The supply voltage VCC is connected between the bases and its centre-tap. Emitters of both the transistors are shorted and connected to the negative end of VCC. The speaker is connected across the Secondary of transformer T2.

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)VS1 and VS2 are the source input voltage from driver stage of transformer T1 but both these voltages are out of phase due to centre-tap property of transformer. When +ve half cycle of VS1

arrives at the base of the transistor Q1, at that instant -ve half cycle arrives at the base of Q2

transistor. Due to this Q1 is forward-biased and Q2 is reverse biased, hence iC1 flows as shown in the

circuit diagram. When -ve half-cycle of VS1 arrives at the base of Q1 transistor, at that moment +ve half cycle arrives at the base of Q2 transistor. So now Q2 transistor is forward biased and Q1

transistor is reverse biased, now iC2 flows through the circuit but in opposite direction. But the actual current through the transformer T2 is (iC1 - iC2) because iC1 is in anti-clock

wise direction and iC2 is in the clockwise direction. Since it is class B amplifier. Some distortion is present in the final output. This distortion is known as crossover distortions as shown in the output wave forms of Fig. This distortion can be overcome in class AB push pull by providing little forward biasing to the base emitter junction of transistors which are used in them.

Input and output wave forms of Class B push amplifier is shown in Fig.

aAdvantagesA push pull amplifier possesses many advantages over a single ended power amplifier. For

this reason, to get a given output power, we prefer using two transistors in push pull connection rather than using a single large power transistor in a single ended circuit. These advantages are given below:

1. The circuit efficiency of a class-B push-pull amplifier is 78.5%, which is much higher than that of class-A whose value is 25%. It is mainly due to the reason that no power is drawn from the d.c. supply under no signal condition.

2. The use of push-pull system in the class-B amplifier eliminates even order harmonics in the a.c. output signal.

3. Because of the absence of even harmonics, the circuit gives more output, per device, for a given amount of distortion.

4. There is no d.c. component in the output signal. It is because of the fact, that d.c. components of two collector currents, through the two-halves of the primary of the Transformer flow in opposite directions. As a result of this, there is no possibility of the core saturation of the T even at the peak value of the signals. Thus we can use smaller sized cores in the transformers, without affecting the circuit performance.

5. It has comparatively larger a.c. power output than Class A.6. For the same collector dissipation , an output that is larger than a single-ended amplifier

can be obtained per transistor.7. It give more output per transistor for a given amount of distortion.8. As each transistor amplifies only half of cycle, the system can be operated in Class B.

This is not possible in single-ended amplifier.9. We can use Class-AB/Class B operation to get high efficiency without producing much

distortion. However, if a single-ended amplifier used in Class AB/Class B, a larger distortion results.

10.An output transformer and even the driver transformer can be eliminated. This brings a considerable saving in the cost.

DisadvantagesThe main disadvantages of Class B amplifier

1. It produces more amplitude distortion. It gives more distortion.2. In class B push pull amplifier operation here is Harmonic Distortion. 3. Two transistors have to be used.4. It requires the use of driver stage to furnish two equal and opposite voltages at the input.5. The parameters of the two transistor should be equal, otherwise it causes unequal

amplification of two halves of the input signal.6. It requires a bulky and expensive transformer.

Applications1. It is used as tuned power amplifiers for RF signals.2. It is mostly used for power amplification in push pull arrangement.3. It is used as a power output stage of audio amplifier system in radio receivers, public

address system etc.Cross Over Distortion

In Class B push pull Amplifiers, some distortion is there because the transistors do not get ON at 0V of Input but at 0.3V for Ge and 0.7V for Si transistors, hence there is distortion at corners of output as shown in Fig.

This distortion can be overcome by forward biasing the base emitter junctions of transistors by cut in voltages of them.

The transistors in a class-B push-pull amplifier are biased at cut-off. It means that when

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)the d.c. bias voltage is zero, the input signal voltage must exceed the barrier voltage before a transistor conducts.

The transistor does not conduct, until the input signal voltage exceeds 0.7 V for silicon and 0.3 V for germanium transistors. Because of this, there is a time interval between the positive and negative alternations of the input signal when neither transistor is conducting as shown in Figure The resulting distortion in the output signal is quite common and is called crossover distortion.

The crossover distortion may be avoided by applying a slight- forward bias (equal to 0.7 V for silicon and 0.3 V for germanium transistors) to the base-emitter junction of both the transistors of the amplifier circuit. It causes the transistor to conduct immediately, when the a.c. input signal is applied. The application of slight forward bias shifts the Q-point slightly above the cut-off. In that case, each transistor operates for more than one half cycle. The resulting operation of the transistor is called class-AB operation.

Complementary Symmetry Class B Amplifier.Fig.shows the circuit of

complementary symmetry class B Amplifier. The name is complementary because one transistor is NPN and other is PNP. Fig. shows the waveform.

OperationIn this type of amplifier NPN

and PNP transistors are used, hence the name is complementary. In this same signal voltage but without phase reversal is applied to the bases of two transistors. On the positive half cycle the NPN transistor is forward biased but PNP is reverse biased.

This shows that Q1 transistor amplify half positive cycle. But when -ve half cycle arrives Q1 is reversed biased while Q2 is forward biased, so Q2 transistor amplifies negative half cycle. In this circuit there is complete balance so no D.C current flows through the load i.e. speaker. This shows that across load we get amplified a.c. signal. Since it is a class B symmetry amplifier cross over distortion will be present. To avoid this distortion each transistor should be slightly forward biased i.e. it should be operated in class AB mode.AdvantagesThe advantages of complementary symmetry Class B push pull amplifier are:

1. It has a unity gain because of the emitter follower configuration .

2. There is no phase inversion of the input signal.3. It eliminates the bulky and expensive transformers.4. It has less distortion due to the absence of transformers.5. Due to elimination of transformer both the high and low frequency responses of the

circuit are extended.6. It requires less space.7. It is light in weight.8. It is comparatively cheap.

DisadvantagesThe main disadvantage of the circuit is that it requires twp power supplies. This increases

the cost. If cells are used, thus this also increases the weight.

Class C amplifierThe class-C amplifier is the most efficient power amplifier, which can produce more load

power than that of either class-A or class-B amplifier. To amplify a sinusoidal signal, a class-C amplifier has to be tuned to the sinusoidal frequency. Because of this fact, the class-C amplifier is called a tuned amplifier or a narrow band circuit.

It means that it can amplifier only the resonant frequency and those frequencies which are closer to it. In order to avoid the need for large. inductors and capacitors, in the resonant circuit, the class-C amplifiers are used to amplify the signals at radio frequencies (i.e., frequencies above 20 kHz). Thus class-C amplifier is basically a radio-frequency (RF) power amplifier and not an audio power amplifier like class-A and class-B amplifier.Following are some of the important characteristics of class-C amplifier

1. The output current flows only during a part of the positive (or negative) half cycle of the input signal. This condition is achieved by biasing the transistor below cut-off.

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)2. The output signal does not resemble the input signal because it consists of narrow

pulses.3. The class-C amplifier is the most efficient power amplifier and ts overall efficiency, under

certain conditions, may approach even 100%.AdvantagesThe advantages of Class-C amplifier are

1. The overall efficiency, under certain conditions, may approach even 100%.2. The collector efficiency ranges between 60 to 80%.3. The ac power output is very large.

DisadvantagesThe disadvantages of Class C amplifier is that it produces maximum amplitude distortion.

ApplicationsThe important applications of Class A amplifier are:

1. It is used as tuned amplifier to amplify narrow band frequencies near the resonant frequency

2. It is used as RF power amplifier is transmitter or some industrial circuits.Heat Sink

If the temperature of the collector and base junction increases , the collector leakage current ICO increases. Due to this collector current increases. The increase in collector current produce an increase in power dissipated at the collector junction. This in turn further increases the temperature and process is cumulative. Hence it may lead to self destruction called as thermal runaway.

For the transistors handling small signals, the power dissipated at the collector is small. Such transistors have little chances of thermal runaway. How ever in power transistors, the power dissipated at the collector junction is larger. This may cause the junction temperature to rise to a dangerous level. We can increase the power handling capacity of a transistor if we make a suitable provision for rapid conduction of heat away from the junction. This is achieved by using a sheet of metal called heat As the power dissipation a sheet within a transistor is predominantly the power dissipated at its collector metallic junction, sometimes the collector of the power transistor is connected Is. its metallic case. The case of the transistor is then bolted on to a sheet

A perfect black body radiates maximum amount of heat. Therefore, a dull black heat sink is nearest approximation to a perfect black body. Hence, a heat sink is painted with a black color to radiate maximum amount of heat from power transistor.

A heat sink is made of aluminum with beryllium oxide washer beryllium, aluminum and copper are used for a heat sink.

The power that can be safely handled by a power transistor can be increased by decreasing the difference in temperature between the case and the air. By increasing the effective area of the case, the thermal resistance (θJA) can be decreased. For this reason, a heat sink with large finned black metallic cover is used.

Heat sink - constructionThe metal sheet that serves to dissipate the additional heat from the power transistor is

called heat sink.Fig.shows same type of heat sink.As power transistors handle large currents, they always get heated during operation.

Since transistor is a temperature dependent device, the heat produced at the collector junction must be dissipated to the surroundings in order to keep the temperature within the permissible units. It becomes necessary to cool them.

Generally, the case of a power transistor is electrically and thermally connected to the collector internally and emitter and base terminals come out through separate leads. This case is fixed on a metal sheet so that the heat produced at the collector is transferred to the metal sheet. This metal sheet is usually of aluminum and is called heat sink.

The heat sink increases the surface area and allows heat to escape from the collector junction easily by conduction, convention and radiation. The result is that the temperature of a transistor is sufficiently lowered. As a matter of fact, modern power transistors are generally mounted in thermal contact with the chassis. Now the entire chassis becomes the heat sink.

The metal sheet that serves to dissipate the additional heat from power transistor is known as heat sink.

PD = TJ - TA/θJA

WherePD - Power dissipation capability of power transistorTJ - Temperature of junction (i.e. collector)TA - Temperature of atmosphereθJA - Thermal resistance from .junction to atmosphereTo stabilize the operating point in power transistor from thermal runaway, we have to use

heat sink. Heat sink conducts heat away from collector junction. Mostly dull black body is used as heat sink because perfect black body radiates maximum heat which it has absorbed. Beryllium, aluminum and copper are used for heat sink.

PD = TJ-TA/ (θJH || θHA)

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)After using heat sink, the total thermal resistance is decreasing because θJH|θHA hence

power dissipation of transistor is increasing.

Definition of Thermal resistance: The resistance offered by heat sink in dissipating power from collector junction to atmosphere is known as thermal resistance. (θJA). Fig. shows one type of heat sink which is commonly used.

Fig. Types of Heat SinkComparison Between Voltage And Power Amplifier

Particular Voltage Amplifier Power AmplifierVoltage Gain High ( > 100) Low ( 20 to 50)RC High (2-10Kohm) Low (5 to 20ohm)Coupling RC-Coupling Transformer couplingPower Output Low High Output Impedance High (≅12Kohm) Low (≅200ohm)Impedance Matching Poor ExcellentInput Voltage Low (few mv) High (2 to 4V)Collector Current Low (≅1 mA) High (> 100mA)

Specifications Of Power AmplifiersThe main purpose for a power amplifier is to obtain maximum output ac power. Since a

transistor like any other electronic device namely has voltage current and power dissipation limit using which performance of power amplifier can be defined. There are three characteristic those are

1. Collector Efficiency2. Distortion3. Power Dissipation Capability

(i) Collector Efficiency(η) Collector efficiency describes, the ability of a power amplifier to convert d.c power

from supply into a.c output power , It is a measure of its effectiveness. It is also called as overall efficiency or circuit efficiency.

Collector efficiency defined as the ratio of a.c output power to the Zero signal power (i.e. d.c power) supplied by the battery of a power amplifier.

The maximum theoretical values for circuit efficiency depend upon the way in which the load is coupled to the transistor and the class of operation of the amplifier. Its value may lie, anywhere, from 25% to 90%.

(ii) Distortion A good amplifier should produce an output, which does not differ from the input in any

respect, except amplitude. In other words, the amplifier output is expected to be enlarged but faithful reproduction of the input. However, in actual practice, it is not possible to construct such an ideal amplifier, whose output is exact reproduction of the input. The output is always found to be different from the input either in its waveform or frequency content. This difference between the output and input of an amplifier is called distortion.

Two types of distortions namely 1. Amplitude (or harmonic) distortion and 2. Crossover distortion

are considered to be important in power amplifiers. The amplitude distortion results from the non-linearity of the transistor. It occurs

because of the fact that transistor output may not increase, equally, for all portions of the input signal during positive and negative half cycles.

The crossover distortion (which usually exists in class B amplifiers) occurs when transistors do not operate in the correct phase with each other.

(iii) Power Dissipation Capability Power transistor are used in power amplifiers and they handles large currents and heats

up during operation, this heat should be totally dissipated in the atmosphere otherwise it will burn the transistor. So while selecting the power transistor we should consider its power dissipation specification.

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Collector Efficiency η = Maximum AC output Power DC input Power

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Power dissipation is defined as the ability of a power transistor to dissipate heat. It is

also called as power ratingFor complete dissipation of heat, generally a heat sink (a metal case) is attached to a

power transistor case. The increased surface areas allows heat to escape easily and keeps the case temperature of the transistor within limits.

Comparison Between Different Power AmplifiersSr. Particular Class A Class B Class AB Class Ci) Collector

CurrentFlows for entire cycle of input

Flows for half cycle of input

Flows for more than half cycle of input

Flows for half of the half cycle of input

ii) Location of operating point on DC load line

At centre of load line

On X-axisIe in Cutoff region

Between centre and X-axis

Below X-axis

iii) Distortion Minimum More but less than class C

Moderate Maximum

iv) Collector efficiency(η)

≅35% ≅60% ≅55% ≅75%

Question Bank For students.1. Define Transistor Audio Power Amplifier.2. Compare voltage and power Amplifier.3. State and explain specifications of Power Amplifier. 4. Draw Block diagram of practical Audio Amplifier. 5. Classify and define different Power Amplifier.6. Compare different classes of power Amplifier.7. Draw and explain working of Driver stage also explain its frequency response.8. Draw and explain working of single ended or class A power Amplifier. Also explain its

frequency response.9. Draw and explain working of class B push pull power Amplifier. Also draw its input and

output wave forms.10. Draw and explain working of class A push pull power Amplifier. Also draw its input and

output waveforms.11. Draw and explain working of class AB push pull power Amplifier. Also draw its input

and output waveforms.12. Draw and explain working of complementary symmetry class B power Amplifier. Also

draw its input and output waveforms.13. Write short note on heat sink and draw any one type of it.14. Compare voltage and power amplifier. 15. Compare the performance of class A, class B and class C amplifiers.16. Draw the circuit diagram of a single ended transformer coupled class A power amplifier

and explain its operating principle. 17. Name the different class of operations in power amplifier.18. Draw circuit for class B push pull Amplifier.19. What is crossover distortion? How it is removed. 20. Draw circuit for complementary symmetry power Amplifier.21. Define percentage efficiency for power Amplifier.[Define collector-efficiency]22. Give two differences between voltage amplifier and power amplifier. 23. Draw and explain with waveforms working of class A push pull Amplifier.24. What is thermal runaway? How it can be avoided in power amplifier? 25. Draw and explain complementary symmetry Power Amplifier. 26. Draw and explain the working of class B push-pull amplifier.27. Why heat sink is required in power amplifier?28. Draw circuit of single ended power amplifier. 29. Classify power amplifiers on the basis of operating point. 30. Draw and explain Class A push-pull amplifier.31. What is cross over distortion? 32. List the applications of power amplifier.33. Draw and explain working of push pull power amplifier. 34. State the function of heat sink. 35. List the advantages and disadvantages of push pull power amplifier. 36. Give the functions of heat sink. 37. Draw the circuit of complementary symmetry push pull amplifier and

explain. 38. Compare class A, class B, class AB, class C amplifiers. Give their

applications. 39. Draw complementary symmetry push pull amplifier and writs its

working. What are its advantages?

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)40. How efficiency in class B amplifier is more than class A amplifier? 41. List the applications of power amplifier. 42. What is cross over distortion? 43. What is heat sink? 44. What are class A, class B, class AB and class C amplifiers? 45. Define efficiency of a power amplifier. 46. State the function of heat sink. 47. Draw and explain working of push pull power amplifier.

Feedback-AmplifierWhen output signal is fed back to the input either directly or via another network is called

as feedback.

If the signal fed back is of opposite polarity or out of phase by 1800(or odd multiples of 1800) with respect to input signal the feedback is called as degenerative feedback or negative feedback. Negative feedback has a self correcting ability against any change in output voltage caused by any change in environmental condition (e.g. temperature).

Negative is also called as degenerative feedback because when used it degenerate(reduces) the output amplitude and in turn reduces the voltage gain. Negative feedback is used in amplifiers.

If the signal feed back is in phase with input signal, then the feedback is called as positive feedback.

In positive feedback, the feedback signal adds the input signal. For this reason it is also called as regenerative feedback. Positive feedback is used in oscillators.

Feedback

Negative Feedback Positive FeedbackEffect of negative feedback

1. Stabilized gain2. Increase bandwidth3. Changes the input and output resistances or Input and Output Resistance are

modified.4. Reduce voltage gain5. Decrease harmonic distortion or nonlinear distortion and Phase distortion6. Reduction in effect of input offset voltage at output7. Reduces the effect of variation in temperature and supply voltage at output8. Reduces the noise

An op-amp that uses feedback is called a feedback amplifier. A feedback amplifier is sometimes referred to as a closed-loop amplifier because the feedback forms a closed loop between the input the output.

A feedback amplifier essentially consists of two parts1. an op-amp2. a feedback circuit

Feedback circuit may be made up either passive component, active components or combination of both.

Depending on the quantity feedback, feedback is classified as1. Voltage Feedback2. Current Feedback

Feedback

Voltage Feedback Current Feedback Depending on how quantity is feedback, feedback is classified as

1. Series Feedback 2. Shunt Feedback

Feedback

Series Feedback Shunt FeedbackDepending on above two classification four way to connect feedback amplifier are

1. Voltage Series Feedback2. Voltage Shunt Feedback3. Current Series Feedback

11

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)4. Current Shunt Feedback

The voltage across load resistor RL is the input voltage to the feedback circuit.The feedback quantity (either voltage or current) is the output of feedback circuit and is

proportional to the output voltage.In current feedback circuit current iL flows into the feedback circuit. The output of the

feedback circuit (either voltage or current) is proportional to current.

Negative feedbackFeedback amplifier

Vid = Vin-Vf where Vin = input voltage

Vf = feedback voltage Vid = difference input voltage

The circuit amplifies the difference input voltage Vid. This difference is equal to the input voltage Vin minus feedback voltage Vf. In other words the

feedback voltage always opposes the input voltage (or is out of phase by 1800 with respect to the input voltage) hence the feedback is said to be negative.

Vo = AvVin – AvVf

= AvVin - AvβVo

Vo + AvβVo= AvVin

Vo(1+Aβ)= AvVin

Vo = Av .Vin = 1+AβAvf= Av .

1+AvβGain with feedback

Avf = Av 1+βAv

Gain with feedback is reduced by factor 1+βAv. Equation shows that feedback (-ve) reduces gain of amplifier.Effect of the Negative Feed Back1. Increased stability: Let us consider the equation If β is negative and βAv >> 1, then Thus, the gain of the amplifier will be dependent only upon the feedback network and changes in parameters, such as ‘ hfe ‘ of a transistor, will affect the gain of the amplifier without feedback; but will not affect the overall amplifier gain with feedback. Thus, the gain of the amplifier with feedback has been stabilised against such problems as a transistor being replaced by a transistor of a far different value of “hfe” or temperature variation.

2. Increased bandwidth : We shall see that the effect of negative feedback is to increase the bandwidth of the amplifier at the. cost of reduction in gain. Let BW be the bandwidth of an amplifier without feedback. By introducing the negative feedback there is increase in bandwidth. Let it be denoted by BWf. The relation between the two bandwidths is given by

BWf = BW(1+ βAv )

It should be noted that the gain is reduced by same factor (1+ βAv )and that the bandwidth is increased. Let us consider a typical example, say an amplifier has a BW = 20KHz and a gain of Av = 40. Consider the effect of a 1% negative feedback (i.e. β =0.01 0.01) bandwidth and voltage gain.

Avf = 40 / ( 1+ 0.01x40 40 )

= 1.4 / 40 = 28.6

BW = BW * ( 1+βAv) = 2OKHz(1+0.01* 40) = 28KHz

Actually f is increased and f is decreased as shown in Fig.Now, the gain and bandwidth product without feedback = 40 x 20 = 800 kHz and with feedback it is 28.6 x 28 KHz = 800 KHz. That is at the cost of sacrificing the

reduction in gain there is an increase in bandwidth. But the gain bandwidth product remains the same.

Fig. showing how negative feedback increases bandwidth but reduces midband gain, Curve A is for the amplifier with no feedback. Curve B is with negative feedback.

3. Decreased distortion: If an amplifier has a voltage gain A and a total harmonic distortion ‘D’, then the application of negative feedback β will reduce the distortion to

Df = D / ( 1 + β Av)

12

β = Vf

Vo

Av = Vo

Vid

Avf = Vo

Vin

AmpiliferA

v

Feedbackβ

ΣV

i

n

Vf

Vo

V

o

Vi

d

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297) for example, D = 10% when A = 40 then β = 0.01 will reduce the distortion.

Df = 10 / ( 1+0.01x 40) = 10/1.4 = 7.14%

Avf = 28.6

If 10% negative feedback is used, then the distortion with negative feedback will beDf = 10 / ( 1+0.1x 40) = 10/5 =2%

However the gain with the feed back would be reduced to = 40/(1+0.1x40 )= 40/5 = 8

Thus, if reduction of distortion is of primary interest and gain is cheap, then a large negative feedback factor would be used.4. Decreased noise : There are number of sources of noise in an amplifier depending upon whether a tube or transistor is used. The noise N can be reduced factor 1 /(1+βAv) , in a similar manner to non-linear distortion, so that the noise with feedback is given by

Nf = N / ( 1+ βAv) However, if it is necessary to increase the gain to its original level by the addition of another stage, it is quite possible that the overall system will be noisier than it was at the start. If the increase in gain can be accomplished merely by adjustment of circuit parameters, a definite reduction in noise will result from the use of negative feedback.

5. Input resistance : The increase in input resistance is generally accepted in a voltage amplifier because it reduces loading on the previous stage and increases the open circuit voltage gain.

The effect of negative feedback on input resistance and output resistance depends on the way in which the output is fed back to the input.

Rif = Ri * ( 1+ β Av)

If the output voltage (or current) is fed back in series with the input then the input current decreases and the effective input resistance increases.

If the output signal (current or voltage) is feed back to the input in parallel, the input resistance decreases, by an amount depending upon the circuit.

6.Output resistance : As with the input resistance, the effect of negative feedback on output resistance is dependent upon the way the output is feed, back, specifically upon whether output current or output voltage is feedback. If output voltage is returned ‘to the input (either in series or shunt) the output resistance decreases.

Rof= Ro /( 1+β Av)

whereas, if the signal proportional to output current is used (in series or shunt), the output resistance increases.

Voltage feedback Voltage feedback – voltage across load resistor RL is input voltage to feedback circuit. The quantity feedback may be current or voltage proportional to the output voltage.

Voltage series feedback: Here output voltage is fed back in series with input.

Input resistance with feedback is increased.Rif = Ri (1+Avβ)

Output resistance with voltage series feedback is decreased.

Rof = Ro

1+ Avβ

Voltage shunt feedbackIn voltage shunt feedback output voltage is feedback in shunt with input voltage. In put voltage in voltage shunt feedback is required.

Rif = RI . 1 + AVβ

Output resistance in voltage shunt feedback is ROF = RO . 1 + AVβ

Current feedback In current feedback load current IL flows into the

feedback circuit. The output of the feedback circuit either (voltage or current) is proportional to the load current IL.Current series feedback

13

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)In this feedback output current is feedback in series with input Vin.In current series feedback output resistance R0 with feedback increases.

Rof = Ro (1+Avβ)

Also input resistance increases.Rif = Ri (1+Avβ)

Current shunt feedbackIn current shunt feedback load current IL is feedback in shunt with input.

In current shunt feedback output resistance increases by

Rof =Ro (1+ AVβ)Also input resistance decreases in current shunt feedback.

Rif = Ri . 1+ AVβ

Comparative study of different FeedbacksType of feedback Gain Rif Rof Bandwidth

Voltage series Decreases Increases Decreases Increases Voltage shunt Decreases Decreases Decreases IncreasesCurrent series Decreases Increases Increases IncreasesCurrent shunt Decreases Decreases Increases Increases

Other effect of negative feedback:1. Stabilized gain.2. Increased bandwidth.3. Decrease in harmonic and non- linear distortion.4. Reduced input offset voltage.5. Reduces effect of variation in temperature and supply voltage on output.

Circuit With Negative Feed BackCurrent Series

Fig Shows the Circuit with current series feedback. Fig is RC coupled CE Amplifier. Re the feedback resistor .Feed back voltage is developed across resistor Re is proportional to output current.

He the feedback quantity is Voltage in terms of Current ie Ve α Ic.

In the figure Re provides Dc bias stablisation but no AC feedback because resistor Re is bypassed with the capacitor Ce. From fig

Vbe = Vb – Vbe The voltage Ve serves to reduce the input voltage Vbe between base and emitter. So that output voltage drops if its is increased.

Voltage Series Fig Shows the Circuit with Voltage series feedback. Fig. is common Collector amplifier were output is taken across the resistor Re and Re the feedback resistorand feedback voltage is developed across resistor Re is proportional to output voltage.

He the feedback quantity is Voltage in terms of Current ie Ve α Vo.

In the figure Re provides Dc bias stablisation and also the output voltage. From fig

Vbe = Vb – Vbe The voltage

Ve serves to reduce the input voltage Vbe between base and emitter. So that output voltage drops if its is increased

14

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Voltage Shunt Feed Back

Fig Shows the Circuit with Voltage Shunt feedback. Fig. is common emitter amplifier with collector to base bias were output is taken across the Collector of the transistor and R1 the feedback resistor which feed back the portion of Vc in terms of bias current Ib i.e feedback is voltage in terms of current

He the feedback quantity is current proportional to output voltage Ib α Vo.

In the figure R1 provides feedback . From figI1 = -Vc /R1

The current I1 is depended on voltage Vc serves to reduce if Collector current increases

because of which Ic Rc increases and Vc decreases further decreasing the I1.

Current Shunt Feed BackFig shows circuit diagrams of current shunt feedback. Here also the output developed across resistor Re is feedback in shunt with the input. Here Voltage Ve developed across Re is proportional to Collector ie output current Ic.

Ve α Ic andI1 α veThe voltage across is feed back to input with the help of resistor R1 ie it is converted to

current I1 which is proportional to Ve and Ve is in turn proportional to Ic ie output current.

Questions 1. Compare Negative feed back and Positive Feed back2. Discuss the effects of negative feedback an amplifier performance.3. Compare Series feedback and shunt Feed back4. Derive an expression for the gain of an amplifier with negative feedback in terms of its

gain without feedback.5. Compare voltage and current feedback.6. Explain with circuit diagram the working of an emitter follower and give its applications.7. An amplifier has a voltage gain of 1000. In order to reduce instability, a negative feedback

circuit is connected that feeds 0.5% of the output to the input. Find out the gain. Explain how negative feedback in an amplifier is useful in improving gain stability and bandwidth and reducing the distortion.

8. Draw a block diagram of a single Loop feedback amplifier and describe the function of each block.

9. Define : (i) negative feedback, and (ii) positive feedback.10.List five characteristics of an amplifier that are modified by negative feedback. What are

the different types of negative feedback?11.What is the general equation for the voltage gain of an amplifier with feedback (Avf) ? Is

Avf necessarily smaller than A? Explain.12.Explain how negative feedback can increase the value of bandwidth in an amplifier13.Explain how negative feedback can decrease the value of noise in an amplifier14.Explain how negative feedback can decrease the value of harmonic distortion an

amplifier15.What types of negative feedback are there?16.Draw the circuit diagram of all type of feedback and explain what is the input quantity

feed abck and how?17.Which ones always produce an increase in input resistance ?18.Which type increases input resistance and decreases output resistance?19.How is current series feedback introduced in a CE amplifier ? What is an approximate

expression for the feedback factor?20.What happens to the voltage gain, input resistance and output resistance in a CE

amplifier with current series negative feedback ?21.An amplifier with 1 K ohm input resistance and 50 k ohm output resistance has a voltage

gain of 40. The amplifier is now modified to provide 10% negative voltage feedback in series with input. Calculate

15

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)(i) the voltage gain with feedback?(ii) the input resistance with feedback.(iii) the output resistance with feedback.

FET BiasingQ. State applications of FETAns. Applications:

1. Used as buffer amplifier.2. Used in high impedance, wide band amplifier.3. Used as input stage amplifier for CRO, Electronic voltmeters, bio-medical instruments

etc.4. Used in analog multiplexers.5. Used as switch.6. Used in FM and TV receiver as a mixer stage.7. Used in large scale integration (LSI) circuit, computer memories because of smaller in

size.8. Used in oscillators and choppers.9. FET is used as voltage variable resistance.10.It is used in logic circuits.

Q. List specification of FET:Ans 1) Gate to Source breakdown voltage.

1) Gate reverse voltage.2) Gate to Source cutoff voltage.3) Zero Gate voltage Drain current.

Q. Explain different biasing circuit for FET.Ans. For FET to be used as an amplifier for an amplifier the basic need is faithful amplification. For faithful amplification proper biasing is needed. In order to use FET as an amplifier.

1) Proper Q point to be selected.2) Q point should remain fixed.

For biasing the FET the important parameter are Id, Vgs, Vds should be set.The Gate should be reverse biased, by proper Vgs, appropriate Vds should be set by giving

Vds and select proper value of Id. By selecting proper value of Id, Vds, Vgs Q point is selected.To use JFET in any application it is necessary, to bias the device. The reason for this

biasing is to turn the device ON and to place it in a region where it operates properly and provides a constant amount of voltage gain.There are several methods of biasing. These are

1. Self bias2. Voltage divider bias3. Source bias

1.Self-bias : Fig.shows the self bias circuit fo1r an N-channel junction field effect transistor. There is only one drain supply and no gate supply. The gate terminal is connected to the ground through a resistor (RG). The source terminal is connected to the ground through a resistor (Rs).

When the drain voltage is applied, a drain current flows even if there is no gate voltage. The drain current produces a voltage drop across resistor R This voltage drop produces the gate to source reverse voltage required for an FET operation. (The resistor R is called a feedback resistor).

If the drain current increases, it will increase the voltage drop across the resistor R The increased voltage drop increases the reverse gate-to-source voltage, which decreases the effective width of the channel. It reduces the value of drain current. Now, if the drain current decreases, then reverse action takes place i.e. the reduced

drain current decreases the gate to source voltage, which in turn increases the width of the channel, thereby increasing the value of drain current.

It is evident from the above discussion that, to prevent any variation in the FET drain current, The gate voltage with respect to ground i.e. VG = 0 we know that the source voltage with respect to ground.

Vs=Id.Rsand the drain voltage,

Vd = Vdd – IdRdThe drain-to-source voltage (Vds) is equal to the difference between the drain voltage (Vd) and the source voltage (Vs).

Vds = Vd - Vs= (Vdd – IdRd )- IdRs= Vdd – Id ( Rd + Rs )

16

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)The gate to source voltage is equal to the difference between the gate voltage and the source voltage.

Vgs = Vg - Vs= 0- Id.Rs = - Id Rs

From above equation it is clear that the gate-to-source voltage is equal to the negative of the voltage across the source resistor. Greater the value of drain current, more negative will be the gate-to-source voltage.

Id = - Vgs / Rs

If we plot a graph using the value of drain current for a constant value of resistor Rs against the gate to source voltage, we get a straight line is as shown in Fig.. This straight line is called a self-bias line.

Voltage Divider Biasing Method This voltage divider biasing method is shown in the Fig. The resistor R and R form a

voltage divider on the gate side, therefore the name is given as above.The voltage across R1 considered as Vg, if we assume that ‘Ig = 0.Then, Vg = R2 * I1

Vg = R2 * ( Vdd )/(R1+R2)since I1 = Vdd/(R1+R2)

Vg = (R2 / (R1+R2)) * VddThe value of drain current

Id = Vs / Rs= (Vg – Vgs) /Rs since Vs = Vg - Vgs

and the d.c. voltage from drain to ground.Vd = Vdd – Id*Rd

Now, if the gate voltage is very large, as compared to gate-to-source voltage, then the drain current is approximately constant. In JFET, to gate-to-source voltage (Vgs) can vary several volts from one JFET to another as in BJT the base to emitter voltage (Vbe) is approximately 0.7 V with only minor variations from one transistor to another.

As a result of this, it is difficult to make gate voltage large enough than the gate to source voltage Vgs.

Hence, it is clear that in actual practice the voltage divider bias is loss effective with JFETs than bipolar transistors.

Source Bias Fig.shows the source bias method for JFET. The idea is to swamp out the variations in Vgs. Since, most of Vss appears across Rs. The drain current is given by,

Id = (Vss – Vgs )/Rs

For source bias to work well must be much greater than Vgs

Vss >> Vgs

Id = Vss / Rs

However a typical range for Vgs is from -1V to -5V. So perfect swamping is not possible with typical supply voltage.

Q. List different configuration in which FET can be used.Ans. Different configuration in which FET can be used are

1) Common Gate configuration2) Common Drain configuration3) Common Source configuration

MOSFET ( Metal Oxide Semiconductor)Q. What is MOSFET and list its different types?Ans. Metal Oxide Semiconductor-conductor field effect transistor is also called as IGFET i.e. Insulated Gate FET. It is similar to ordinary FET but the Gate is insulated from the channel. The name MOSFET is given because of its construction in which between the metal layer and semiconductor-conductor there is oxide layer.A thin oxide layer of Si02 is formed between Gate and channel which is acting as an insulator. As there is insulator between Gate and the channel the device is also called as IGFET. MOSFET are of two types

1) Depletion MOSFET2) Enhancement MOSFET.

17

Substrate

Drain (D)

Gate (G)

Source (S)

P

N+

N+

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)In depletion MOSFET operates when Gate to source voltage is negative. The enhancement MOSFET operates when Gate to Source voltage is positive.

Q. Explain the construction of MOSFET in detail with neat diagram.Ans. Fig. Shows the constructional details of enhancement MOSFET. It consist of P-type lightly

doped substrate. The substrate terminals sometimes are connected to Source and some time it is used for controlling the potential level by giving additional voltage.

Two heavily doped N-type region are diffused in the substrate forming Drain(D) and Source(S) over this a thin layer of Si02 is given which is acting as an insulator. Then through a small hole metal contacts for Drain(D), Source (S) are done.Over the oxide, layer for Gate metal is grown. The Gate is isolated from the substrate by Si02 i.e. Silicon di-oxide (Si02) [insulator]. Hence device is called as MOSFET.

The Gate metal contact with Sio2 & substrate from the parallel plate capacitor. The input impedance is very very high i.e. 1010 or 1015Note: FET is Unipolar device because current is conducted due to majority charge carrier. Chance of thermal run away.

Q. Explain the operation of enhancement MOSFET.Ans. Fig. Shows N type of MOSFET in which Drain is made +ve with respect to Source and Gate is supplied with +ve voltage.

The +ve voltage on the Gate results in an electric field normal to SiO2 layer. The +ve Gate voltage induces –ve charge in the channel and the small drain current flows from Drain to Source.

Now if the +ve Gate voltage is increased the induced channel in the semiconductor increases. Accordingly the conductivity of channel increases.

18

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)

Fig. Circuit Diagram of Enhancement MOSFET

The increased concentration of electron near the surface induces the N channel which helps in the flow of current from Drain to Source by application of Gate voltage. Which is called as thermal voltage. The device is called as enhancement MOSFET.The minimum Vgs is required which increases the Drain current is called as threshold voltage.In above shown characteristics following points to be noted :-

1) The Idss i.e. short gate drain current is very low of the order of few nano amperes.2) As Vgs is made +ve the concentration of electron increases in the substrate and current Id

starts flowing. The minimum gate to source voltage that induces N-type layers and at which Id starts

flowing is called as threshold voltage. As Vgs is increased the concentration of electron increases and Id increases.

Q. Explain Construction details of depletion MOSFET.Ans. Fig. Shows the constructional details of depletion MOSFET. On the P-type substrate two N-region (heavily doped) are diffused which forms Drain(D) and Source(S).

In addition a moderately doped N material is diffused joining Drain(D) and Source(S) called channel. Drain and Source are connected by material contact through thin layer of silicon di-oxide. A gate is connected using a material contact insulated from a channel by Sio2. Hence there is no direct connection between gate & channel of MOSFET.The depletion MOSFET can be operated in depletion mode with gate to source voltage negative and in enhancement mode by making gate to source voltage positive.

Q. Explain the operation of D-MOSFET in depletion mode.Ans. Fig. Shows the arrangement for depletion MOSFET. In the fig Drain is made +ve

19

Drain (D) Gate (G) Source (S)

Substrate

P

N+ N+

Vgg

Vdd

Metal

Semiconductor

Oxide

Induced Channel

Vgs = 10vVgs = 8v

Vgs = 6v

Vgs = 2v

Vgs = 0V

Vgs = 4v

VpVds (Volts)

Id (mA)Ohmic Region

Pinch Region

Drain (D) Gate (G) Source (S)

Substrate

P

N+ N+

N

OxideMetal

Semicondcutor

Drain (D) Gate (G) Source (S)

Substrate

P

N+ N+

Vgg

Vdd

Depletion Channel

+ ++ + + + + + + + + N

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)with respect to Source and to operate this FET in depletion mode gate is made negative with respect to source.

When Gate is shorted i.e. Vgs=0 the positive potential between Drain to source forces the charge through the channel and the free electrons from source pass through channel and reach drain & source current Id flows which is called as shorted gate Drain to source current(Idss).

When gate is made negative a positive charge is induced in the channel because of which electric field is setup in oxide layer.

This positive charge is minority carrier for N channel hence conductivity of channel decreases because conduction in FET is due to majority carrier. If gate to source voltage is made more negative, this is further decreases.

The action of this FET depends on –ve voltage of gate which generates depletion region in the channel and due to the depletion region the conductivity is reduced. I.e. why this FET is called as depletion MOSFET. When gate is positive When Vgg is positive or Vgs is made +ve, negative charge is induced in the channel because electron. These free electron adds to the majority carrier present in the channel and hence the conductivity of channel increases and FET operates in enhancement mode. If further positive voltage is increased on the gate the conductivity of channel increases and Id is Increases.

Hence by applying gate voltage +ve or –ve D-MOSFET can be used in depletion or enhancement mode.

Q. Draw symbols of D-MOSFET and E-MOSFET Ans.

a) Symbol of D-MOSFET b) Symbol of E-MOSFET Q. State advantage of MOSFET over JFET.Ans.

JFET works only in depletion mode whereas MOSFET can work in depletion and enhancement mode.JFET is operated with reverse bias on the gate junction whereas MOSFET can work with –ve or +ve gate voltage hence leakage current in JFET is more as compared to MOSFET.

MOSFET has high input impedance compared to JFET. The capacitive effect are lower in MOSFET as compared to JFET. MOSFET requires less space as compared to JFET. MOSFET are easier in manufacturing as compared to JFET. MOSFET requires less space as compared to JFET. Voltage gain provided in MOSFET is higher compared JFET.

Q. State difference between MOSFET & JFET.Ans.

MOSFET JFET1) Gate is insulated from channel. 1) Gate is not insulated from

channel.2) Input impedance is very high. 2) Input impedance is high but less

than MOSFET.

20

Vgs = 8v

Vgs = 2vVgs = 0v

Vgs = -4v

Vgs = -6V

Vgs = -2v

VpVds (Volts)

Id (mA)Ohmic Region

Pinch Region

Vgs = 4v

Vgs = 6v

EnhancementMode

DepletionMode

Idss

Source ( S )

Drain ( D )

Source ( S )

Gate (G ) Substrate ( S )

Drain (D)

Source ( S )

Gate (G ) Substrate ( S )

Drain ( D )

Gate (G ) Substrate ( S )

Drain ( D )

Source ( S )

Gate (G ) Substrate ( S )

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)3) Voltage is very high 3) Voltage is high but less as

compared to MOSFET.4) Gate of MOSFET can be forward or

reverse biased. Hence no gate flows in either of the case.

4) Gate of MOSFET can be only reversed biased because in forward biasing gate current flows. Which reduces drain current.

Q. State application of MOSFET.Ans. MOSFET is used as

1) Amplifier 2) Oscillator3) Buffer amplifier 4) Voltage variable resistor in attenuator 5) Voltage variable resistor in DC chopper amplifier.6) Used in integrated circuit.

BIASING CIRCUITS OF DEPLETION MOSFET(i) Self bias and (ii) Voltage divider bias.(i) Self Bias: Fig. shows self bias circuit. The resistor RS is the bias resistor. The d.c component of drain current flowing through R produces the desired bias voltage. Voltage across RS = VS = ISRS

But IS ID VS = IDRS

∆VS = ID . RS

Since gate current is very small, the gate terminal is at d.c ground i.e. VG =0V∆VGS = VG - VS = 0 - ID RS.∆VGS= -ID.RS ...(l)

Thus bias voltage VGS keeps gate negative w.r.t source.From equation (1) of VGS we get:

ID = -VGS / RS

Since VGS is very small, So practically ID totally depends on constant RS.

Consider output section: Apply KVL on the output side of the self bias circuit. VDD = ID.RD + VDS +ISRS

VDD = IDRD + VDS + IDRS (IS ID)VDD = ID (RD + RS) + VDS

∴VDS = VDD - ID( RD + RS)From above equation of VDS (Operating point) It Is clear that it depends on all

constant of VDD, RD, RS and ID.Since operating point of self bias depends on constants, hence in some

applications this biasing circuit is used.

(ii) Voltage Divider Bias Circuit: ( Potential Divider Bias Method)Fig shows potential divider method of biasing at JFET. This circuit is identical to that

used for a transistor. R1 and R2 resistors provides biasing network (i.e. voltage divider across drain supply VDD). RD can act as load, RS provides stabilisation of operating point from temperature.

Input Sections:I1 = VDD/R1+R2 V2 is voltage across R2

V2 = I1 x R2 V2 = VDD /R1+R2 x R2 ……(1)From eq. (1) it is clear that V2 depends on all constants.

Apply KVL to the lower close loop of voltage divider bias circuit. V2 = VGS + ISRS

V2 = ISRS (VGS is very small, we can neglect it)V2 = IDRS (IS ID) ID= V2/RS

From above eq. of ID (Operating point) it is clear that ID depends on all constants i.e. V2

and RS.Output Sections: Apply KVL on output side of the circuit of voltage divider bias.

VDD = IDRD + VDS + ISRS

VDD = IDRD + VDS + IDRS (IS ID)VDD = ID (RD +RS) +VDS

∴VDS = VDD-ID(RD+RS)From above equation of VDS (operating point) it is clear that VDS depends on all constants.Similar to BJT, operating point of voltage divider bias circuit is independent of

temperature and parameters of MOSFET hence it is known as universal bias circuit.

SPECIFICATIONS OF MOSFET:(i) Forward gate terminal current.(ii) Reverse gate terminal current.(iii) Gate source cut-off voltage.(iv) Zero gate voltage drain current.

21

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)(Note: The MOSFET may be used in any of the circuits covered for the JFET, with the bulk or substrate connected to the source. Hence applications of MOSFET and JFET are same.)APPLICATIONS OF FET AND MOSFET:* Used In Amplifiers.* Used In oscillators.* Used as Buffer.* Used as VVR In Voltage-Controlled Attenuator.* Used as VVR In a Chopper DC Amplifier.Assignment for students:1. What is MOSFET?2. State the types of Depletion MOSFET.3. Draw symbol of n-channel and p-channel Depletion MOSFET.4. Draw and explain construction of n-channel Depletion MOSFET.5. Explain working of Depletion MOSFET in Depletion mode.6. Explain working of Depletion MOSFET in Enhancement mode.7. Draw and explain drain or output characteristics of Depletion type MOSFET.8. Define all the parameters of depletion MOSFET.10. What is Enhancement type of MOSFET?11. Draw and explain construction of n-channel Enhancement MOSFET.12. Draw symbol of n-channel and p-channel Enhancement MOSFET.13. Explain working of n-channel Enhancement MOSFET in Enhancement mode, also its drain characteristics.14. Write specifications of MOSFET.15. Write applications of MOSFET (or JFET).16. Compare JFET and MOSFET.17. Draw constructional details of MOSFET and label it.18. State the main difference between JFET and MOSFET. OR Compare MOSFET and JFET. 19. Draw characteristics of JFET for depletion and enhancement mode. 20. Explain with suitable diagrams the construction, working and characteristics of an n-channel depletion type MOSFET.

Question asked in exams of Advanced Syllabus:1. Give the inter relationship between µ,gm, and rd with reference to FET. (S02 Q1.j)2. Explain the construction of MOSFET with a neat diagram. (S02 Q3.c)3. Compare BJT and FET. (S02 Q4.e)4. Draw the experimental set up to study the drain characteristics of JFET and explain. (S02 Q5.c)5. Define following FET parameters-dynamic drain resistance, transconductance, amplification factor, pinch off voltage. (S03 Q1.f)6. Draw construction and write working of enhancement type MOSFET. (S03 Q3.c)7. Draw output characteristics and transfer characteristics of JFET. Explain them. (S03 Q6.b)8. Draw constructional diagram of N channel JFET and explain it. (W02 Q3.b)9. Explain why an ordinary junction transistor is called bipolar and why field effect transistor called as unipolar. (W02 Q3.d)10. Draw the symbols of BJT and FET. (W03 Q1.h)11. What is JFET? Why it is named so? (W03 Q3.d)12. Draw the drain and transfer characteristics of FET showing all regions. (W03 Q6.c)

22

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)OSCILLATOR

An oscillator is a circuit which basically acts as a generator, generating the output signal which oscillates with constant amplitude and constant desired frequency.

Hence an oscillator is basically a generator which generate AC signal, basically it is a sine wave. An oscillator is basically an amplifier with +ve feedback, without any AC input. The amplitude and frequency of this AC output can be varied as per requirement. But the +ve feedback not always guarantee oscillations. A +ve feedback amplifier works as oscillator only if it satisfies a set of condition called as “Barkhausen criterion”. Output voltage Vi is sum of two inputs feedback voltage.

Vo =AVi Output voltage

Vi = Vs +Vf Vi is sum of two inputs

Vf = βVo

Vf = β AVi

Vi = Vs + β AVi

Vs = (1-A β )Vi

For oscillator input voltage Vs=0 and feedback signal Vf is suppose to maintain the

oscillations. Therefore, Vs = 0.

Vi(1-Aβ )=0 or Aβ =1

This condition must be satisfied in order to sustain oscillations. Along with this condition, the condition for the phase shift between Vs and Vf must be zero should also be satisfied. With an inverting amplifier introducing 1800 phase shift between Vi and Vo the feedback network must introduce 1800 phase shift to ensure that Vi and Vf are in phase. These two conditions are to be satisfied to operate the circuit as an oscillator are called as “Barkhausen criterion” for sustained oscillations.

Statement of Barkhausen criterion

1. The total phase shift around a loop, as the signal proceeds from input through amplifier, feedback network back to input again, completing a loop, is precisely 00 or 3600.

2. The magnitude of the product of open loop gain (A) and the feedback factor(β) should be unity i.e. |Aβ |>= 1. The product of Aβ is called as loop again.Satisfying these conditions, the circuit works as an oscillator, producing sustained

oscillations of constant frequency and amplitude. In practice A is made greater than 1 to start the oscillations and then circuit adjust itself to get Aβ = 1 ie why Aβ>=1. Finally resulting into self sustained oscillations.

Effect of value of |Aβ| on nature of oscillations:There are three possible ranges of | Aβ |. They are |Aβ| = 1, |Aβ|>1, |Aβ|<1.

|Aβ| > 1When the total phase shift around a loop is 00 or 3600 and |Aβ| > 1, then output oscillates but the oscillations are of growing type.

The output increases exponentially. The amplitude of oscillations will continue to increase continuously without limit.

Fig. When | AB>1 |

1. |A β| = 1When total phase shift around a loop is 3600 or 00,

ensure +ve feedback and |Aβ| = 1 then the oscillations are with constant frequency and amplitude called sustained oscillations.

Fig, When | AB = 1 |

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)

2. |A β| < 1When the total phase shift around the loop is 00 or

3600 and |Aβ| < 1 then oscillations are of decaying type ie such oscillations amplitude decrease exponentially and such oscillations finally ceases.

Thus the circuit works as amplifier without oscillations as shown in the fig.

Fig. When | AB < 1 |Conclusion

Hence it is clear that circuit will oscillate only if |Aβ| > = 1. To obtain sustained oscillation |Aβ| = 1 and ie Barkhausen criterion are |Aβ| = 1.

Aβ = 00 or 3600.

Q.How oscillator starts oscillating without any input signal?Every resistance has some free electron which move randomly under the influence of

normal room temperature. Such a random movement of the free electron generates a voltage called noise voltage across resistance. Such noise voltage present across the resistance is amplified. Hence to amplify such small noise voltage and to start the oscillation |Aβ| is kept greater than unity at start. Such voltage is amplified and it appears across the output terminal. The part of output is sufficient o drive the input of amplifier circuit. Then circuit adjusts itself to get |Aβ| = 1 with phase shift of 3600 or 00 to get sustained oscillations.Classification of oscillatorClassification on different basis1. Based on waveform generated:

a. Sinusoidal oscillator – generates pure sine waveform.b. Non – sinusoidal oscillator – generates triangular, square, saw tooth etc waveform.

2. Based on circuit component:a. RC oscillator – uses resistor (R) and capacitor (C).b. LC oscillator – uses inductor (L) and capacitor (C).c. Crystal oscillator – uses crystal.

3. Based on range of operating frequency: a. Audio frequency – low frequency oscillator – generates frequency range of 20 – 20kHz upto

200kHz.b. Radio or high frequency oscillator – this oscillator generates frequency range more than

low frequency oscillator upto GHz.Generally RC oscillator are used for low frequency range and LC oscillator are used for high

frequency range.Oscillator

1. Sinusoidal a. RC

i. Wein Bridge ii. RC Phase Shiftiii. Quadrature.

b. LC i. Hartley ii. Colpittsiii. Clapps

c. Crystal2. Non-sinusoidal

a. Square i. Astable Multivibrator ii. Bistable Multivibratoriii. Monostable Multivibrator

b. Sweep c. Triangular

Q. what is oscillator?Ans. Oscillator is define as electronic circuit which generates sinusoidal oscillations of desired frequency . Basically it converts dc energy to the ac energy of desired frequency of oscillations depends on constant of circuit. In oscillator positive feedback is used. i.e. The signal feedback is in phase with input. Q. Draw block diagram of positive feedback and derive equation for gainAns. Figure Shown above

In the block diagram feedback Network may be Consisting of Inductor , capacitor or Resistor. The overall gain with feedback is

Avf = Av1-βAv

24

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297) Where

Av = Open loop gain of the amplifier, B = feedback factor Q. What is Barkhausens criterion for maintaining oscillation?Ans. The Oscillator circuit should satisfy two Condition to sustain oscillations those are 1) Avβ>=1 i.e feedback or loop gain should be greater than equal to 1

2)The total phase shift around loop is 0o or 360o i.e phase shift be positive. These two Criteria for sustaining undamped oscillations is called as Barkhausens criteria.

Q. Classify the different types of oscillations. Ans. Oscillators are broadly classified into following two groups-

1) Sinusoidal oscillator - which produce sine waveform. 2) Non-sinusoidal oscillator(relaxation)- which produce square, rectangular or saw-tooth waveform or pulse shape.

Sinusoidal oscillator are further classified as 1) LC- Inductor and Capacitor oscillator. 2) RC- Resistor and Capacitor oscillator. 3) Crystal oscillator.

Non-sinusoidal oscillator as 1) UJT relaxation oscillator. 2) Multi-vibrator (Astable, Monostable, Bistable)

Q. Explain how oscillator are generated ?Ans Oscillations are generated by oscillatory circuit. also called as Tank circuit The circuit. which produces electrical oscillation of any desired frequency is known as an oscillatory circuit. or tank circuit. Tank circuit consist of capacitor and inductor connected in parallel.

Operation:- When switch S is at position 1, it is

connected to DC supply and capacitor C is charged from DC supply with polarity as shown.

When capacitor is charged, the switch S is connected to position 2 i.e. DC source is removed and inductor is connected to capacitor.

The charged capacitor start discharging through inductor; This current will set up the magnetic field around the coil.

Once the discharging is over the magnetic field reduces which gives out the energy stored by it and the capacitor is charged in opposite direction.

The fully charged capacitor starts discharging in opposite direction through inductor where magnetic field in inductor is build up in the opposite direction.

This sequence of charging and discharging continues and produces sine wave.Note:- During this process the electric energy of capacitor is converted into magnetic energy of coil and vice versa.

Since, there are loses in tank circuit the amplitude of oscillations goes on decreasing with time and at some point the amplitude becomes zero. Such oscillations are called as Damped oscillations.

Q. Give reason for damped oscillations?Ans. The reason for damped are 1) Some energy is lost in the form of heat produced in the resistance of the coil and connecting wires.2) Some energy is lost in the form of electromagnetic waves that are radiated out from the circuit through which an oscillatory current is passing.3)Due to above losses current decreases gradually till it becomes zero.4)Hence to get undamped oscillations , an amplifier with positive feedback is needed. The positive feedback maintains the oscillation with constant frequency and amplitude which are known as undamped oscillation. Fig. Shown above

Q. State Points which are considered for Frequency Stability of an Oscillator ?Ans . Frequency Stability of an Oscillator

The frequency stability of an oscillator is a measure of its ability to maintain a constant frequency, over a long time interval. However, it has been found that if an oscillator is set at some particular frequency, it does not maintain it for a longer period. In other words, the frequency of an oscillator changes slowly (or drifts away) from the initially set value. Sometimes, the change in frequency is uniform in one direction (i.e., either increasing or decreasing). But at

25

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)some other times, it may be changing quite erratically. The change in oscillation frequency may arise due of the following factors:1. Operating point of the active device. The operating point of the active device (i.e., bipolar transistor or PET) is selected in such a way so that its operation in non-linear region, changes the values of device parameters which, in turn, affects the frequency stability of the oscillator.2. Circuit components. The values of circuit components (i.e., resistor, inductors and capacitors) change with the variation i temperature. Since such changes take place slowly, they also cause a drift in oscillator frequency3. Supply voltage. The changes in d.c. supply voltage applied to the active device, shift the oscillator frequency. This problem can be avoided by using a highly regulated power supply.4. Output load. A change in the output load may cause a change in the Q-factor of the tank circuit, thereby causing a change in oscillator output frequency.5.Interelement capacitances and stray capacitances. Any change in the inter element capacitances of a transistor (particularly collector-to-emitter capacitance), cause changes in the oscillator output frequency and thus affect the frequency stability. Similarly, the stray capacitances also affect the frequency stability of an oscillator. The effect of change in inter element capacitances can be neutralized by putting an additional capacitor across the corresponding elements. However, it is difficult to avoid the effect of stray capacitances. [ Q.Give the requirement of LC oscillators? and list different type of LC oscillatorsAns. Essential component of LC oscillator are: 1]Frequency determining n/w generally it is LC tank circuit. 2]Amplifier to amplify the oscillations generated by tank circuit.

3]A positive feedback network which will feedback o/p energy to tank circuit in proper phase. The feedback energy should be sufficient to cover up the losses. Along with that two Barkhausen criterion are to be fulfilled those are:-

4] feedback factor or loop gain |βA| = 1 5] The net phase shift around loop is 0 degree or 360 degree i.e. feedback should be

positive.Different types of LC filters are :- 1] Tickler or Tuned base oscillator. 2] Tuned collector oscillator. Or Armstrong oscillator 3] Hartely oscillator. 4] Collppits oscillators. 5] Clapps oscillators.Q. State different Tuned Circuit OscillatorAns. These are also known as LC oscillators, resonant circuit oscillators or tank circuit oscillators. These oscillators are used to produce an output with frequencies ranging from 1 MHz to 500 MHz. Hence they are also known as R.F. oscillators. A *bipolar transistor or an FET is used as an amplifier with tuned circuit oscillators. With an amplifier and an LC tank circuit, we can feedback a signal with right amplitude and phase to maintain oscillations.

It will be interesting to know that most of the oscillators used in radio transmitters and receivers are of LC oscillators type. Depending upon the way, the feedback is used in the circuit, the LC oscillators are of the following five types:

1. Tuned-collector or Armstrong oscillator. It uses inductive feedback from the collector of a transistor to the base. The LC circuit is in the collector circuit of the transistor.2. Tuned base oscillator. It also uses inductive feedback. But the LC circuit is in the base circuit.3. Harley oscillator. It also uses inductive feedback.4. Colpitts oscillator. It uses capacitive feedback.5. Clapp oscillator. It also uses capacitive feedback.

Q.State Frequency of Oscillatory Circuit ?Ans. Formula for Frequency of Oscillatory Circuit

The frequency (f) of the oscillatory current, produced in the tank circuit, is determined by the following two factors:1. Capacitance of the capacitor. The frequency of oscillation (j) is inversely proportional to the square root of the capacitance of a capacitor Thus, larger the value of capacitancegreater is the time required for removal of the discharge current and hence lower is its frequency. Mathematically, the frequency,

f ∝ 1/√C ... (i)2. Self-inductance of the coil. The frequency of oscillation (J) is proportional to the square root of the self-inductance of the coil Thus larger the inductance greater is the inductanceeffect and hence longer the time required by the current to stop flowing during the discharge of the capacitor. Mathematically, the frequency,

f ∝ 1/√L Combining the equations (i) and (ii) we get

f ∝ 1√LC

26

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)0.159 in Hertz√LC

If L is in µH and µC in , then the frequency,0.159 in Hertz√LC

Q.Explain with neat diagram the circuit of Hartely oscillator? And give equation for frequency of oscillations. Ans. Fig. shows Hartely oscillator with CE configuration. Function of different components are1) The timing circuit consist of L1, L2 (center tapped inductor) & capacitor. Coil L1, L2 are inductively coupled.2) Resistor R1 & R2 are used to provide dc bias condition.3) Coupling capacitor Cc and CB is used to block DC & pass AC.4) Resistor RE is to provide stabilization of Q-point i.e. bias stabilization.5) CE is used as bypass capacitor for RE i.e. to provide low resistance path for AC signal.6) RFC (Radio Frequency Coil) is used in collector to provide AC oscillation from reaching battery & provide DC load for the collector.

Working:- When supply is turned ON collector current start charging the capacitor C. When capacitor is fully charged. It discharges through coil L1 & L2, setting up the oscillations of frequency given by F= 1

2 π√LC L= L1+L2+2M

For oscillations to start the voltage gain must be greater then 1/β = L1/L2 where β = L2/L1 The oscillations across L2 are feedback to the base emitter junction, which is available in the collector circuit in amplified form. The coil L1 feeds the collector circuit energy back to the tank circuit by means of mutual inductance between L1 & L2. In this way energy is being continuously supplied to tank circuit to overcome the losses occurring in it & given undamped oscillations.Barkhausens criteria are fulfilled as

1. CE amplifier provide 1800 phase shift . 2. 1800 phase shift is provide by tank circuit through L1 & L2 i.e 3600

3. gain of amplifier can be adjusted to full fill Aβ ≥ 1Q:- Explain with neat labeled diagram the operation of collpitts oscillator and write the

equation of frequency.Ans:- Fig. shows the circuit diagram of collpitts oscillator with CE amplifier.

The different between hartely and collpitts oscillator is in tank circuit i.e in collpitts oscillator there are two capacitors and one inductor. Hence the tank circuit consist of C1,C2 and L. Function of different components like R1, R2, Cc, RE, CE RFC is same as Hartely oscillator.

(Copy the Functions of the component from Above)Working:-

When the supply is turned ON capacitor C1 & C2 are charged. These capacitor discharges through coil setting up oscillations of frequency

F = 1 2 π √ LC C = C1C2

C1 +C2

For oscillations to start , the voltage gain Av must be greater than 1/ β = C2/C1 or β should be equal to C1/C2

The oscillations across C2 are applied to the base emitter junction of transistor which appears in amplified form across collector circuit over coming the losses of tank circuit . the capacitor C1

and C2 acts as simple ac voltage divider . Barkhausens criteria are fulfilled by

1) 3600 phase shift is proceed by tank circuit (1800 i.e C1 and C2 ) and 1800 by CE amplifier .2) Gain can be adjusted to full fill |AVβ| ≥1

Q. List application of LC oscillator?Ans. LC oscillator are generally used for high frequency oscillations: 1) Used as radio frequency generator 2) Used in radio and T.V receiver 3) Used in radio and T.V transmitters 4) Used in industrial heating process

27

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297) 5) Used in bio - medical instrumentsQ.Explain with neat block diagram Tunned Collector Oscillator or Armstrong OscillatorAns. Tuned Collector is also called Armstrong Oscillator. Figure shows the circuit of such an oscillator. In this circuit, the collector drives an LC tank circuit. The feedback signal is taken from a small secondary winding L1 and feedback to the base. There is a phase shift of 180° in the transformer and another 180° phase shift is provided by the transistor amplifier. It means that the total phase shift around the loop is zero and hence the feedback is positive. Ignoring the loading effect of base, the feedback fraction,Where

β = M/LM = The mutual inductance between the primary and secondary windings, andL = Self-inductance of the primary winding.

for oscillations to start, the voltage gain must be greater then 1/β.Circuit Components :The resistors R1 and RE are used to provide d.c. bias to the transistor. The capacitor C’ and CE act as bypass capacitors for the resistors R2 and RE respectively. The capacitor C’ also provides a.c. ground for the secondary transformer L1 Operation :

When the circuit is energized by switching on the supply, the collector current starts increasing. This current charges the capacitor (C). When this capacitor is fully charged, it discharges through the primary winding (L) producing magnetic field around

the primary coil. When the capacitor is fully discharged, the magnetic field collapses and charges the

capacitor in the reverse direction. Then again the capacitor discharges in the reverse direction. This process continues again and again. As a result of this, the oscillations are produced.

The frequency of oscillations produced by the tank circuit depends upon the values of primary inductance (L) and capacitance (C). Its value given by the expression,

Fo = 1 = 0.159 Hz 2π√ LC 2π√ LC

These oscillations induce some voltage in secondary winding L1 by mutual induction. The magnitude of the voltage induced depends upon the number of turns of secondary winding L1 and the coefficient of coupling (k) between the coils L and L1 However, the frequency of the induced voltage is the same as that of the oscillations produced in the tank circuit. The voltage across L1 winding appears between the base and emitter terminals of the transistor. This voltage is amplified at the collector of a transistor and is partially used for overcoming the losses, which take place in the tank circuit. The remaining voltage may be utilized for any other purpose.

Q.State the Basic Principle of RC oscillatorAns .Basic Principle of RC Oscillators

A single stage of common-emitter (CE) amplifier not only amplifies the input signal, but also introduces a phase shift of 180°. If a fraction of the output and feed it back to the input, without considering the phase shift, the negative feedback takes place. Due to the negative feedback, the output voltage of the amplifiers decreases. However, for producing oscillations, we must have positive feedback of sufficient magnitude. This occurs only when the fraction of output voltage is feedback with the same phase as that of the input signal. This condition may be achieved in any one of the following two ways:

1. A fraction of the output of single-stage amplifiers is passed through a phase-shift net work, before feeding back to the input. The phase-shift network gives another phase-shift of 180° in addition to the phase-shift of 180° introduced by the amplifier. Thus there is a total phase-shift of 360°, which is also equal to 0°. The RC oscillator, utilizing this principle, is known as phase-shift oscillator.

2. A second-stage of the amplifier is used for producing another 180° phase shift in addition to the phase-shift of 180° produced by the first stage. Thus there is a total phase shift of 360°, which is also equal to 0°. A fraction of the output from the second stage is feedback to the input of the first stage without producing any further phase-shift. The RC oscillator, utilizing this principle, is known as Wien-bridge oscillator.

Q. Explain principle of phase shift oscillator ?Ans. Tuned circuit are not an essential requirement for oscillation. The essential thing is that there should be phase shift of 3600 and loop gain should be greater than equal to unity The 1800 phase shift can be achieved by using suitable RC network consisting of three RC sections. The sine wave oscillator which use RC feedback network are called as phase shift oscillator. The 1800 phase shift remaining is provided by active device used i.e. amplifierQ. State the Operation with neat circuit diagram of Tuned Base OscillatorAns. Tuned Base is also known as tickler oscillator. Figure shows the circuit of such an oscillator. In this circuit, the tuned, circuit is put in the base circuit. The feedback signal is supplied to the base through the coil L1 also called tickler coil.

There is a phase shift of 1800 by the transistor amplifier and

28

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Another 1800 phase shift is provided by the transformer. It means that the total phase shift, around the loop, is zero and hence the feedback is

positive. The resistors R1, R3 and RE are used to provide d.c. bias to the transistor. The capacitors

C’ and CE act as coupling and bypass capacitors. Operation :

When the circuit is energized, by switching on the supply, the collector current starts increasing to a quiscient value. The increasing collector current, through the tickler coil, creates a changing magnetic field around it.

As a result of this, a voltage is induced in the tuned circuit. Because of the correct positive feedback of the coils, and sufficient gain of the amplifier, the oscillations start growing. The sustained oscillations are produced, when the transistor is driven into cut-off due to the voltage developed across the capacitor (C).

Q.Draw and explain with neat diagram operation of RC phase shift and write the equation for frequency?Ans. Fig. Shows RC phase shift oscillator with three sections of RC feedback network. The network comprising of R1C1,R2C2 and R3C3. The value of R and C is selected in such a way that it provides total phase shift of 1800 (i.e 600 per section) also CE amplifier is used which provides 1800 phase shift hence total phase is 3600 or 00 which is essential for sustaining oscillators. The frequency of oscillators can be calculated by

F = 1 2 πRC√6

and also feedback factor β = 1 29

hence to have |AVβ| ≈ 1 gain of amplifier should be equal to or more than 29.Operation:-

When the supply is turned on, the circuit is set into oscillation by any random or chance variations caused in base current by 1) The noise inherent in a transistor or 2) Minor noise in the voltage of DC source. These variations are then amplified in collector circuit and then feedback to the base through 3-section of RC network. This RC network gives phase shift 1800 at one particular frequency decided by above equation and transistor will 1800 phase

hence undamped oscillators are sustained.Function of components:- RB-R3 provides dc emitter bias . RC- control collector voltage. RECE -- provide temperature stability and reduce negative feedback due to ac signal. Cc - coupling capacitor.Advantages:-

1) It doesn't require any transformer or inductor so that the Circuit is more compact , less costly compared to LC oscillator

2) Used for low frequencies. 3) Circuit provides good frequency stability4) Pure sine wave output is possible because only one frequency fulfills Barkhausen

requirement.5) low distortion level.

Disadvantages:1) Gain of circuit is less as compared to other oscillators.2) It is difficult for a circuit to set up oscillations. Because feedback is very small. 3) Cannot generate high frequencies4) Frequency dependent components are temperature dependent hence instability of

frequency is more.Q. Explain with neat diagram Wein bridge oscillator and give equation for frequency of oscillators?Ans.

Fig. Shows circuit diagram of Wein bridge oscillator. The oscillator uses two CE connected, RC coupled amplifier and one RC bridge.

Here transistor T1

serves as amplifier oscillator and T2

provides phase reversal and additional amplification bridge circuit is used to control the phase of feedback signal atT1. This bridge is

29

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)balanced at one particular frequency hence oscillator gives oscillations of good frequency stability.

Operation:- Any random change in base current of Q1 starts Oscillations. If the base current of Q1 is

increased it is equivalent to applying a positive going signal to Q1 , this signal is amplified by Q1, but in phase reverse it is available across collector of Q1. Further phase reversal is applied by Q2 at signal with 3600 phase shift is available at Q1.

A part of Q2 signal is feedback to input of bridge. Also a part of this feedback signal is applied to emitter resistor R3 which produce negative feedback. Similarly a part of feedback signal is applied across base bias resistor R2 where it produce positive feedback effect.

When positive feedback is more than negative feedback at a rated frequency f, circuit set up into oscillation & oscillation are sustained.

Since negative feedback is provided this circuit is highly stable.

Fo = 1 if R1 = R2 and C1 = C2

2π√R1 R2 C1 C2

and Ratio of R3/R4 = 2 . Thus the ratio of R3 and R4 should be greater then 2 , which will provide a sufficient gain for the circuit to oscillate.Function of components:

R5 –RC1- RC2 - R7 – Control collector voltage.R3 – RE – Provide temperature stability i.e. bias stabilization. R6, R8 – Emitter to base bias resistor.CC – Coupling capacitor.C1 – R1 / C2-R2 – Lead Lag Network

Advantages:1) It has high stabilized amplitude and voltage amplitude.2) It has good frequency stability.3) Frequency of the oscillation can be changed by connecting a variable resistor in the

network.4) Easily starts oscillating.5) Output perfect sine wave.

Disadvantages: 1) It is require more number of components.2) Cannot be used for high frequencies.3) Circuit provide negative feedback. The Wein-bridge oscillator is a standard oscillator circuit for generating low frequencies in

the range of 20 Hz to about 1 MHz. It is used in all commercial audio signal generators. The Wein-bridge oscillator has an advantage that it gives an extremely pure sine wave output, good frequency stability and a highly stabilized amplitude. A greater amplitude stability may be obtained by using a thermistor (a device with negative temperature coefficient) in place of the resistor R3

Q. Explain the circuit diagram & operation of crystal oscillator.Ans.Crystal: A quartz crystal has very important property known as Piezo electric effect i.e. when an AC voltage is applied across crystal, it either expands or contracts and crystal wafer is set into vibration. The frequency of vibration is equal to the resonant frequency of crystal which determined by constructional characteristic of crystal.

Conversely when a mechanical stress is applied across its two opposite faces a potential difference is developed across them. It is called as Piezo electric effect. Such crystal are also called as “Piezo electric crystal”.Example of crystal:

1) Quartz2) Rochelle salt3) Tourmaline are material used for crystal oscillator.

Fig. Shows circuit diagram of crystal oscillator which consist of a tank circuit formed using L2C1 placed in collector and L1 acts as feedback to the base of transistor. The natural frequency of tank circuit is made equal to the vibrating frequency of the crystal. Function of components:

RE – CE, R1, R2 –RFC is same as in previous circuit. ( COPY FROM THE ABOVE NOTES)Operation:

When the supply is switched ON to the circuit capacitor C1 is charged and then it discharge through L2 setting up the oscillations due to mutual induction. The voltage across L2 is applied to base of transistor. The total 3600 phase shift is achieved by the transistor and the tank circuit.

The crystal controls the frequency of oscillation in circuit because crystal is connected in the base circuit.

30

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)The second condition i.e. AVβ = 1 is set by the amplified design. The circuit generates

constant frequency as frequency of crystal is independent of temperature.Advantages:

1) It is simple circuit as no tune circuit other than crystal itself is required.2) It gives high order of frequency stability.3) Different frequency can be obtained by simply replacing the crystal.4) Frequency of oscillations is set by crystal and is unaffected by change in transistor

parameters supply voltage and temperature. Disadvantages:

1) Crystal is fragile and should be handled with care.2) The output of crystal is very low.

3. Frequency of oscillator cannot be changed appreciably. Application:1) Crystal oscillator are used where high frequency stability is needed. Eg: Crystal oscillator are

used in communication system (Radar, Transmitter, Receiver and Computer)2) Microprocessor based application like temperature controller, data acquisition system.Q. Compare between Hartely and collpitts oscillator.Ans.

Hartely oscillator Collpitts oscillator

Hartely oscillator tank circuit consist of tapped inductance in parallel with capacitor.

/* Figure */

Collpitts oscillator tank circuit consist of tapped capacitor in parallel with inductor.

/* Figure */

Frequency of oscillation is given by

F = 1

2 π√ LC

L=L1+L2 + 2M

Frequency of oscillation is given by

F= 1

2π√ LC

C= C1C2

C1+C2

Q. Describe with the diagram and frequency response of the Crystal oscillatorAns. A quartz crystal has a very typical property known as piezoelectric effect. According to this effect, when an a.c. voltage is applied across a quartz crystal, it vibrates at the frequency of the applied voltage.

Conversely, if a mechanical force is applied to vibrate a quartz crystal, it generates an a.c. voltage. The other materials, which exhibit the piezoelectric effect are Rochelle salt and tourmaline.

The quartz material is preferred because it is inexpensive and readily available in nature. Moreover, its properties lie in between those of Rochelle salt and tourmaline.

Characteristics of Quartz CrystalThe natural shape of a quartz crystal is a hexagonal prism with pyramids at the ends as

shown in Figure (a). In order to get a usable crystal out of it, we have to cut a rectangular slab out of a natural crystal as shows in Figure (b).

There are a number of ways to cut the natural crystal namely X-cut, Y-cut, XY-cut, etc. Different cuts have different piezoelectric properties (i.e., resonant frequency, temperature coefficient etc.).

For use in electronic circuits, the slab is mounted between two metal plates and housed in a package, which is equal to the size of postage stamp. In actual practice, the package as a whole is known as crystal and its symbol is shown in Figure (c).

When a crystal is placed across an a.c. source, it starts vibrating. The amount of vibration depends upon the frequency of the applied voltage. By changing the frequency, we can find a frequency at which the crystal vibrations reach its maximum value. The frequency, at which it happens, is called resonant frequency of the crystal.

The quartz crystals are available with resonant frequencies ranging from 15 kHz to 10 MHz. For higher frequencies (i.e., frequencies up to 100 MHz), the crystal can be made to vibrate on overtones (i.e., multiples of fundamental frequency). However, for frequencies greater than 10 MHz, the crystals are not useful.

Q. Compare negative feedback and positive feedback.

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Ans.

Negative feedback Positive feedback

In negative feedback the input signal and feedback are out of phase by 1800

In positive feedback the input signal and feedback signal are in phase (00 or 3600 )

Voltage gain of negative feedback amplifier is given by

Avf = Av

1+ βAv

where Avf = gain with feedback

Av = open loop gain of amplifier.

β = feedback factor.

Voltage gain of positive feedback amplifier is given by

Avf = Av

1- βAv

where Avf = gain with feedback

Av = open loop gain of amplifier.

β = feedback factor.

Negative feedback increases stability of the amplifier

Stability of amplifier decreases.

Reduces distortion Increases distortion.

Bandwidth increased. Bandwidth decreases.

Used in amplifiers. Used in oscillators.

Q. Explain the Electrical Equivalent Circuits of a crystalAns. Fig. (a) and (b) shows the symbol and electrical equivalent circuit of a crystal respectively. It consists of a series R-L-C1 circuit in parallel with a capacitance C2 When the crystal mounted across the a.c. source is not vibrating, it is equivalent to the capacitance C

However, when the crystal is vibrating, it acts like a tuned circuit R-L-C1. Figure (c) shows the graph of a reactance (XL or Xc) versus frequency (F).It is evident from this graph that the crystal has two closely spaced resonant frequencies. The first one is the series resonant frequency

(Fs), which occurs when reactance of the inductance (L) is equal to the reactance of the capacitance C1 In that case, the impedance of the equivalent circuit is equal to the resistance R and the frequency of oscillation is given by the relation,

Fs = 1 2π√ LC1

The second one is the parallel resonant frequency (j which occurs when the reactance of R-L-C1 branch is equal to the reactance of capacitor C At this frequency, the crystal offers a very high impedance to the external circuit and the frequency of oscillation is given by the relation.

Fp = 1 2π√ LC1

Where C= C1C2

C1+C2

The Value of C2 is usually very large as compared to C1. Therefore the value of C is approximately equal to C1 and Hence the series resonant frequency is approximately equal to the parallel resonant frequency ( ie Fs = Fp )

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)

Non Sinusoidal Generators-MultivibratorsThe oscillators which generate waveforms other than sine waveform are called

nonsinusoidal oscillators. The waveforms generated by nonsinusoidal oscillators include 1. Square, 2. Triangular, 3. Rectangular and 4. Saw-tooth waveforms.

These oscillators use one or two transistors (BJTs or FETs). The transistor is used as switch which operates in two states, namely saturation and cut-off states. When the switch operates, there is a sharp transition from one state to another. Usually, a transistor remains in the cut-off state for a certain period of time which is followed by another interval of time during which it remains in saturation state.

Since the transistor alternately supplies power to the load and relaxes when it is in cut-off state, therefore, the nonsinusoidal oscillators are also known as relaxation oscillators.

The nonsinusoidal oscillator may be classified into the following three typesi. Multivibratorii. Blocking oscillator andiii. Saw tooth generator or sweep generator (time base Generator chapter )

Transistor As a SwitchThe nonsinusoidal oscillators employ one or two transistors, which

are used as a switch. The switch operates between two states namely1. saturation and 2. cut-off state.

The saturation state occurs when both the junctions (i.e., emitter-base junction and collector base junction) of a transistor are forward biased, the cut-off state occurs, when both the junctions are reverse biased.

Fig. Shows common emitter circuit. A pulse input (Vin) as shown in the figure is applied at the based of the transistor. It is used to control the voltage (or state of the switch) between collector and emitter of the transistor. Case 1 : Vin = -V1

When the input at time t < T1 the input voltage (Vin) is equal to –V1 and the emitter-base diode is reverse biased. Since the collector-base diode is also reverse biased, therefore the transistor is cut-off, and practically no

current exists anywhere in the circuit. As a result of this, the collector-to-emitter or the output voltage (Vo) is approximately equal to the supply voltage (Vcc). Thus when the collector current (Ic) is zero and Vo = Vcc, the transistor acts as an open switch.Case 2 : Vin = -V1

When the input voltage becomes equal to V for the time interval T1<t<T2, both the emitter-base and collector-base diodes are forward biased and the transistor is saturated. In this interval (i.e, T1 < t <T2 ) the output voltage, Vo = Vcesat. The value of Vcesat for silicon transistor is 0.2 V and the collector current is maximum. Its value is approximately equal to Vcc/Rc. These values indicate that the transistor acts as a closed switch and the current in the closed switch is determined by the supply voltage Vcc and the load resistor Rc.

At time T = T2, the input waveform returns back to -VThis causes the transistor to switch back to cut-off state.

Hence the results is as follows:Ic = 0 and Vce = Vcc

(Open switch)Ic = Vcc/Rc and Vce = Vcesat

(Closed switch)

Transistor Switching Time By controlling the base current, the

collector current can be controlled and thus make the BJT acts like a switch. Ideally, as soon as the base current exceeds the threshold value IB(sat) the BJT should saturate. But, practically it does not happen because of the capacitive effect of transistor

The collector voltage, instead of changing instantaneously, always takes time to change from one voltage level to another. Therefore, it is observed that the application

33

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)of a pulse input at the base of a BJT is not followed directly by a change in output voltage. In other words, there is always some delay before the output changes as compared to the input.

Fig shows the waveform of the input pulse applied to the BJT and Fig shows the resulting waveform of collector current Ic along with the various time delays involved. The various-important parameters are defined as below

Delay time (td)The delay time is the finite time that, lapses between application of the base input voltage

and the start of collector current flow in the BJT. The time t is measured at values of input voltage and output current that are 10% of the maximum values. This is indicated by T The time that elapses during these delay together with the time required for the collector current to rise to 10% of its maximum, i.e. saturation value Icsat equal to Vcc/RL is called the delay time (td). The following two factors contribute to delay time:

1) The time required for carriers to pass from emitter to collector, andii) The base-emitter capacitance.

Rise time (tr)The time required for the collector current to increase from 10% to 90%of its maximum

value is called rise time t,.. The value of rise time for a transistor 2N2222 A is about 25 nanosecond.

Turn-ON time (Ton)The sum of the delay time td and the rise time tr, is called the turn-ON time.

Mathematically,Ton=td

Storage time (ts)when the input signal returns back to its initial state, Let –V1 at time t = T2 the collector

current again fails to response immediately. The time required for the collector current to decrease to 90% of its maximum value after

the input has decreased to 90% of its maximum value is called storage time or saturated delay time ts.

It arises because a large number of carriers are stored at the collector junction when the BJT is saturated. These excess carriers must first be cleared out of the junction before the BJT can cut-off.

Falltime(tf)The time required for the collector to fall from 90% to 10% of its maximum value is called

fall time tf. Like rise time, fall time is also related to the transistor cut-off frequency and external capacitances.

Turn-OFF time (Toff)The turn-off time is the sum of storage time ts and fall time tf Mathematically,

Toff = ts + tf

Types of MultiVibrator Depending upon the type of coupling network, the multivibrators are classified as:

1. Astable (or free running) multivibrator,2. Monostable (or one-shot) multivibrator, and3. Bistable (or ifip-flop) multivibrator.

Astable Multivibrator (AMV)The astable multivibrator also known as free running multivibrator which alternates

automatically between the two states (ON or OFF) and remains in each state for a time dependent upon circuit constants. Thus, it is just an oscillator since it requires no external pulse for its operation. But it requires a source of d.c. power. It generates a square wave of known period.

It uses two capacitor and Astable multivibrator uses only capacitive couplingsIt does not have any permanent stable stages, but it has two quasi-stable, i.e. temporary

states. The circuit changes the state continuously from one quasi-stable to another without any external stimulus or trigger after a predetermined length of time.

This predetermined length of time is decided by the circuit time constants and parameters. Thus, an astable multivibrator generates continuous square waveform without any external signal.

Monostable MultivibratorThe monostable multivibrator also known as one shot multivibrator or delay Multivibrator

of Univibrator, it has 1. one stable state and 2. another quasi-stable state,

i.e. half-stable state. Normally the circuit stay in a stable state. It has one energy storage element ie capactor , Monostable multivibrator uses resistive-

capacitive coupling.

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)When an external stimulus or trigger, i.e. pulse is applied, the stable state it changes

into quasi-stable state for a predetermined length of time. After this period, the circuit returns back to its initial stable state automatically, i.e. by itself.

The process is repeated upon the application of each triggering pulse. The time duration of quasi-stable state is strictly decided by the circuit time constants and parameters and it is independent of pulse duration.

Bistable MultiVibratorThe bistable-multivibrator is also known flipFlop, binary and scaler of two circuits. It doesnot use any type if charge storing element , Bistable multivibrator uses only

resistive coupling.It has two stable states and can stay in a particular state indefinitely. It requires the

application of an external triggering pulse to change the operation of circuit from either one state to the other.

At the occurrence of each triggering pulse, the circuit state changes abruptly from one stable state to another. Thus, one pulse is used to generate half-cycle of square wave and another pulse to generate the next half-cycle of square-wave. It is known as flip-flop because of the two possible states it can assume.

Application of MultivibratorSrNo MultiVibrator Description Application

1 Astable multivibrator (free running)

Two quasi-stable states with the length or time in each one controllable.

Oscillator, timing circuits, square wave generators.

2 Monostable multiyibrator(one shot or gating circuit)

One stable state and one quasi-stable state of controlled duration

Delay or gate circuits

3 Bistable multivibrator (ifip-flop or binary)

Two stable states Scalers, memory, counter, Arithmetic operations.

4 Schmitt trigger One stable state and one state that is maintained only as long as a minimum input is present.

Voltage discriminators, Analog to Digital conversion.

Astable Multi VibratorIt is also called a free-running or relaxation oscillator and is commonly used to generate

square waveform. Figure shows the circuit of a collector-coupled astable multivibrator. It uses two identical NPN transistors Q1 and Q2. It is possible to have Rc1= Rc2 = Rc, R1 = R2 = R and C1 = C2 = C. In such case, the circuit is known as symmetrical astable multivibrator. The transistor Q1

is forward biased by the Vcc supply through resistor R1. Similarly, the transistor Q2 is

forward biased by the Vcc supply through resistor R2. The output of transistor Q1 is coupled to the input of transistor Q2 through the capacitor C. Similarly, the output of transistor Q2 is coupled to the input of transistor Q1 through the capacitor C2.

Since capacitive coupling is used one of the transistor can remain permanently cut-off or saturated. Instead the circuit has two quasi-stable states (ON and OFF) and it makes periodic transition between these two states.

The output of an astable multivibrator is available at the collector terminal of either transistor (i.e, Q1 and Q2) as shown in the figure. However, the two outputs are 180° out of phase with each other. Therefore one of the output is said to be the complement of the other shown in Fig.

Working of AMVThe circuit operation of an astable multivibrator is easy to understand with the

waveforms are at the base and collector of transistors Q1 and Q2.Fig. shows the waveform for the base voltage of transistor Q (i.e vb1 and Fig. for the

collector voltage of transistor Q1 (i.e. Vc1). Similarly Figure shows the waveform for the base voltage of transistor Q2 (i.e., vb2 ) and the collector voltage of transistor Q2 (i.e., Vc2). The circuit operation may be explained as follows:

1. When the d.c. power supply (Vcc) is switched ON, (say at t = 0) one of the transistor will start conducting more than the other due to difference

35

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)in β of transistor . suppose that transistor Q1 start conducting more than that of transistor Q2.

Then because of positive feedback, the transistor Q1 will be driven into saturation and transistor Q2 to cut-off. Thus at t > 0, the transistor Q1 is ON and Q2 is Off. Thus at t > 0, Vb1 = Vbesat (i.e., 0.7 V for silicon transistor), Vc1 = Vcesat (i.e. Vcesat = 0.2. for silicon transistor)0.3 V for silicon transistor), v is negative and Vc, = Vcc.

2. During the time t > 0 (i.e. when Q1 is ON and Q2 is OFF), the capacitor C is charging towards the voltage Vcc through R1 The charging takes place exponentially with time constant τ =R1C1. Since the base of transistor Q2 is directly connected to capacitor C1 as shown in Fig Therefore the voltage Vb1 also increases exponentially towards Vcc.

3. As soon as the voltage VB2, increases, above the cut-in voltage (i.e., 0.7 V for silicon transistor), the transistor Q2 starts conducting. It occurs at t = t1 As the transistor Q2 goes into saturation, its collector voltage (Vc2) falls to Vce (sat). The fall in voltage Vc2 causes an equal fall, (i.e. Vcc — Vce(sat) = Vcc) in voltage Vb1 because the two are capacitively coupled.

The fall in voltage Vb1 cuts-off the transistor Q1 and its collector voltage (Vc1) starts rising towards Vcc with a time constant τ’ = RC2C2.

The rise in voltage Vc1 is coupled through capacitor C to the base of the transistor Q2, causing a small overshoot in voltage Vb2. Soon the voltage Vb2 settles at Vbe(sat) i.e., 0.7 V level. Thus at t >= t1, the transistor Q1 is OFF and Q2 is ON. The voltage levels at this instant are Vb1 is negative, Vc1=Vcc, Vbe2 = Vbe = Vbe(sat) and Vc2 = Vce(sat)

During the time t>t1 (i.e., when Q1 is OFF and Q2 is ON) the voltage Vb1 rises exponentially with time constant τ2 = R2.C2 towards Vcc. At t = T2, the voltage Vb1, reaches the cut-in level (i.e., 0.7 V) and a reverse transition takes place (i.e. Q1 turns ON and Q2 turns OFF). The voltage levels for t>t2 Vb1 = BE (sat),Vc1 = Vce(sat) , Vb2 is negative and Vc2 = Vcc. Thus the voltage level for t > t2 are the same as for t > 0.

Time period and Duty cycle The time period T1 represents the duration for which the transistor Q1 is ON and Q2

OFF. Similarly, time period T2 represents the duration for which the transistor Q2 is ON and

Q1 is OFF. Both the time periods T1 and T2 depend upon charge of capacitors C1 and C2 T1= 0.693 R1 C1 ... (i.e., ON time for Q1)

and T2= 0.693 R2 C2 ... (Le., ON time for Q2)Total period of the wave,

T = T1 + T2 = 0.693(R1C1 + R2C2)If R1 = R2 = R and C1 = C2 = C, then we have a symmetrical astable multivibrator, whose time periods T1 = T2 and the total time period,

T = 0.693R1 C1+0.693R2 . C2 = 1.386 R C

and the frequency of oscillation is given by the reciprocal of the period, i.e.,

F = 1 = 1 T 1.386RC

The frequency of oscillation may be varied by adjusting the values of R and C. However, it is more practicable to adjust R than C. It may be noted that if the values of resistors R1 and R2 have not been selected with d.c. current gain of the transistor (β or hfe) in mind, the oscillations may not take place. To ensure oscillations, t value of resistor

R1 ≤ hfe(min) Rc1 andR2 ≤ hfe(min) Rc2

Where hfe(min) is the minimum value of the dc current gain of the transistor Q1 and Q2. If value of the Rc1= Rc2 =Rc and R1=R2=R then

R≤ hfe(min).RcF = 1 1.386RC

Application of Astable MVSome of the important applications of an astable multivibrator are:

36

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)1. It is used as an oscillator.2. It is used in timing circuits or time delay circuits, and3. It is used as a square wave/rectangular wave generator

Monostable MultivibratorIt is also called one shot or uni vibrator and can be used to generate a gating pulse,

whose width can be controlled. The monostable multivibrator provides a

single pulse of desired duration in response to an external trigger. The external trigger causes the circuit to go to the quasi-stable state.

After a certain interval of time, the circuit returns to its original stable state. Fig. shows the circuit of a monostable multivibrator using NPN transistors. Here, the output of transistor Q2 is coupled to the base of transistor Q1 through the resistance R1.

Whereas the output of transistor Q1 is coupled to the base of transistor Q2 through the capacitor C2. The capacitor C1 is known as commutating capacitor or speed up capacitor. Its function is to speed up the circuit in making

abrupt transitions between the ON and OFF states. The base of transistor Q2 is returned to the Vcc supply through a resistor R3, while the

base of transistor Q1 is connected to the negative supply through a resistor R2 The advantage of this biasing is that it keeps the transistor Q1 OFF and Q2 ON. This state is known as a stable state of the monostable multivibrator.

The output of a monostable multivibrator is available at the collector terminal of either transistor (i.e. Q1 or Q2) as shown in the fig. However, the two outputs are the complement of each other i.e., when one of the output is at Vcc level, the other is at Vce (sat) level. Operation of Monostable MultiVibrator

1) In the Stable state i.e. Q1 is OFF and Q2 is ON. when the trigger is applied:When a positive trigger pulse of sufficient amplitude is applied to the base of transistor Q1 it overrides the reverse bias provided by the Vbb supply and gives it a forward bias. Because of this, the transistor Q1 starts conducting.

2) As the transistor Q1 conducts, its collector voltage falls due to the voltage drop across resistor This fall in voltage is coupled through capacitor C which decreases the forward bias of transistor Q2.

3) The reduced forward bias, the collector current of transistor Q2 starts decreasing and its collector voltage rises exponentially towards Vcc.R1/(R1 + RC2) with a time constant τ2 = C1(R1||Rc2)

4) The rising collector voltage of transistor Q2 is coupled to the base of transistor Q1 through resistor R1 , where it further increases its forward bias. Because of the increased forward bias, the transistor Q1 conducts more. This action is cumulative because of the positive feedback, and the collector voltage of transistor Q1 falls to VCE (sat)

5) The capacitor C starts charging exponentially towards Vcc with a time constant τ1 = R3C2. As C1 charges, the voltage at the base of the transistor Q2 decreases. As C1 charges further, the transistor Q2 is pulled out from the cut-off and the reverse transition takes place i.e., Q2 turns ON and Q1 turn OFF.

6) When the transistor Q2 starts conducting, its collector voltage falls because of the drop across resistor Rc This drop is coupled to the base of the transistor Q1, whose collector voltage rises towards Vcc with time constant τ3 = Rc1.C2. Finally, the transistor Q1 turns fully ON and the transistor Q2 goes OFF. The circuit remains in this stable state till another pulse is applied.

Fig. shows the waveforms at the base and collector of the transistor Q1 and Q2 of a monostable multivibrator. The width or duration of the pulse obtained at the collectormonostable multivibrator is given by the expression,

τ = 0.693 R3 C3Application of MMV

37

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)The monostable multivibrators are used for generation of well defined pulses, the logic design of pulse delay, variable pulse width etc.

Bistable MultiVibratorIt is also called flip-flop, Eccles-Jordan circuit, trigger-circuit or binary. The bistable

multivibrator is used for counting and storing of binary information in computer circuits. It also used in the generation and processing of pulse type waveforms.

The bistable multivibrator has two stable states and can stay in any one of these two states, indefinitely, as long as the power is supplied.

It changes to other state only when it receives a trigger (a short direction pulse) from outside.

Fig. shows the circuit of a bistable multivibrator using two NPN transistors. Here, the output of a transistor Q2 is coupled to the base of transistor Q1 through a resistor R1

Similarly, the output of a transistor Q1 is coupled to the base of transistor Q2 through a resistor R2. The capacitors C1 and C2 are known as speed up capacitors. Theft function is to increase the speed of the circuit in making abrupt transition from one stable state to another stable state.

The base resistors R3 and R4 of both the transistors are connected to a common source -VBB. The output of a bistable multivibrator is available at

the collector terminal of both the transistors Q1 and Q2. However, the two outputs are the complements of each other.

Operation Monostable Multivibrator When Vcc supply is switched on one of the transistor will start conducting more than the other. Then because of the feedback action, this transistor will be driven into s4turation and the other to cut-off.

Let us assume that the transistor Q1 is in saturation (i.e ON) and Q2 is cut-off (i.e., OFF). It is a stable state of the circuit and will remain in this state, till a trigger pulse is applied from outside. A negative pulse applied to the set input will turn OFF the transistor Q1 and Q2 to ON. A similar action can also be achieved by applying a positive pulse at reset input.

Suppose a positive pulse is applied at the reset input. It will cause the transistor Q2 to conduct. As the collector voltage of Q2 falls, it cuts-off the transistor Q1 Thus the circuit switches to the other stable state i.e., a state where Q1 is OFF and Q2 is ON.

Now if a positive pulse is applied at the set input, it will switch the circuit back to its original stable state i.e. Q1 ON and Q2 OFF.Fig shows the waveforms at the collector of transistor Q1 and Q2 of the bistable multivibrator in response to the trigger pulses applied at the set and reset input. It is seen from the waveforms, that the output waveforms are the complement of each other.Notes: There are two methods of triggering bistable multivibrators namely unsymmetrical triggering and symmetrical triggering. The unsymmetrical triggering requires two trigger inputs called set and reset. These are two independent trigger sources. This method of triggering is used in computer logic circuits.

The symmetrical trigging requires a common trigger source and is used In counter circuits. It may be noted that we have explained the operation of a bistable multivibrator using a method of unsymmetrical triggering.The trigger pulses may be applied either at the base or collector of the transistors, as it does not make any difference.

Application of Bistable MVSome of the important applications of bistable multivibrators are

1) It is used as a basic memory element which stores a single bit. It is useful in digital circuits.

2) It is used for divide by two operation.3) It is used as a scaler.4) It is used as counters and registers.5) It is also used for arithmetic operations.

Application Of Bistable Multivibrator : Schmitt Trigger

38

Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)Figure shows the circuit of a Schmitt trigger (after the name of its inventor). Even

though it is multivibrator circuits, Schmitt trigger is used for wave shaping circuits. It is used for generation of square wave from a sine-wave input.

Basically, the circuit has two opposite operating states as in all other multivibrator circuits. Here the trigger signal is not, a pulse waveform but a slowly varying ac. voltage. The Schmitt trigger is level sensitive and switches the output state at two distinct trigger levels.

One of the triggering levels is called a lower-trigger level (abbreviated as L.T.L) and the other as upper-trigger level (abbreviated as U.T.L.)

Circuit DescriptionThe circuit of a Schmitt trigger consist of two

identical transistors Q1 and Q2 coupled through an emitter RE. The resistors R1 and R2 form a voltage divider across the Vcc supply and ground. These resistors provide a small forward bias on the base of transistor Q2

Operation of the circuitInitially when there is no signal at the input. and the power supply is switched on, the

transistor Q2 starts conducting. The flow of its current through resistor RE produces a voltage drop across it. This voltage drop acts as a reverse bias across the emitter-base junction of transistor Q1 due to which it cuts-off.

As a result, the voltage at its collector rises to Vcc. This rising voltage is coupled to the base of transistor Q2 through the resistor R1. It increases the forward bias at the base of transistor Q2 and therefore drives it into saturation and holds it there. At this instant, the collector voltage, levels are Vc1 = Vcc and Vc2 = vcE (sat) asshown in Figure.

When an a.c. signal is applied at the input of the Schmitt trigger (i.e., at the base of the transistor Q1, as the input voltage increases above zero, nothing will happen until it crosses the upper trigger level (U.L.T.). As the input voltage increases, above the upper-trigger level, the transistor Q1 conducts.

The point, at which it starts conducting, is known as upper-trigger point (U.T.P.).

As the transistor Q1 conducts, its collector voltage falls below Vcc. This fall is coupled through resistor R1 to the base of transistor Q2 which reduces its forward bias. This in turn reduces the current of transistors Q2 and hence the voltage drop across the resistor RE. As a result of this, the reverse bias of transistor Q1 is reduced, and it conducts more.

As the transistor Q1 conducts more heavily, its collector current further reduces due to which the transistor Q2 conducts near cut-off. This process continues till the transistor Q1 is driven into saturation and Q2 into cut-off

At this instant, the collector voltage levels are Vc1 =Vcecsat and Vc2= Vcc as shown in the figure

The transistor Q1 continue to conduct till the input voltage falls below the lower-trigger level (L.T.L.). When the input voltage becomes equal to the lower-trigger level, the emitter-base junction of transistor Q1 becomes reverse biased.

As a result of this, its collector voltages starts rising towards Vcc. This rising voltage increases the forward bias across transistor Q2, due to which it conducts. The points, at which transistor Q2 starts conducting is called lower-trigger point (L.T.P.).

Soon the transistor Q2 is driven into saturation and Q1 to cut-off. This completes one cycle. The collector voltage levels at this instant are Vc1=Vcc and Vc2 = Vce (sat). No change in state will occur during the negative half cycle of the input voltage.

It is clear that the output of a Schmitt trigger is a positive going pulse, whose width depends upon the time during which transistor Q1 is conducting. The conduction time is set by the upper- and lower-trigger levels.

Application of Schmitt Trigger

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Applied Electronics-Sem – IV (M.S.Kavedia-9324258878,9860174297)The application of a Schmitt trigger are as given below:

1) It is used as a comparator, i.e. voltage discriminator.2) It is used as a pulse counter.3) It is used as asynchronization driven multivibrator.4) It is used as a rectangular waveform generator for squaring sinusoidal, sawtooth or other

wave-shapes.5) It is used as analog to digital converter.

Comparision Between MultivibratorsSr.No

Astable MV Monostable MV Bistable MV

1 It has two quasi-stable states with the length of time in each one controllable.

It has one stable state and one quasi-stable state of controlled duration.

It has two stable states.

2 It is self-triggered. It is externally triggered. It is externally triggered.3 It requires no external

trigger pulse to generate one square wave,

It requires one external trigger pulse to generate square wave.

It requires two external trigger pulses to generate one square wave.

4 It uses capacitive coupling.

It uses resistive capacitive. It uses resistivecoupling.

Assignment Question1) What is a nonsinusoidal oscillator? Explain it briefly.2) Explain the switching action of a transistor3) What is a multivibrator? Explain the difference between the three types of multivibrators.4) With a neat sketch, explain the working of an astable multivibrator.5) Draw the switching waveforms for the astable multivibrator.6) What is monostable multivibrator? Explain its working with the help of waveforms.7) With a neat sketch, explain the operation of a bistable multivibrator.8) How a Schmitt trigger is different from a multivibrator?9) Draw any one type of a Schmitt trigger circuit and explain with waveforms.10)Compare AMV , MMV, BMV11)What is Hysteresis ? Define UTP , LTP .12)Define the terms delay time , Rise Time , Turn –On time , Fall tine , turn oFF Time ,

Storage time.13)Explain the operation of the transistor as the switch14)State the device which are used multivibrator.

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