Unit II

Bipolar Junction Transistor (BJT)

Coverage

BJT structure and its operation with biasing, Transistor characteristic and its operation, DC operating point, Transistor amplifier, Transistor as switch, Enhancement type MOSFET.

Bipolar Junction Transistor (BJT) is a Semiconductor device constructed with three doped Semiconductor Regions (Base, Collector and Emitter) separated by two p-n Junctions. The p-n Junction between the Base and the Emitter has a Barrier Voltage (V0) of about 0.6 V, which is an important parameter of a BJT.

N-P-N transistor

P-N-P type

Bipolar junction transistors come in two major types, NPN and PNP. A NPN transistor is one in which the majority current carrier are electrons. Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The further types of charge, holes, are a minority.

construction can be either germanium or silicon, but silicon is preferred over germanium because it possesses smaller cutoff current.

Emitter: Emitter terminal is the heavily doped region as compared two base and collector. This is because the work of the emitter is to supply charge carrier to the collector via the base. The size of the emitter is more than base but less than the collector.

Base: The size of the base region is extremely small, it is less than emitter as well as the collector. The size of the base is always kept small so that charge carriers coming from the emitter and entering base will not recombine in the base region and will be directed towards the collector region. The doping intensity of base is also less than emitter and collector for the same reason mentioned above.

Collector: The collector terminal is moderately doped, and the size of the collector region is slightly more than emitter region because all the charge carriers coming from the emitter recombine at base and heat is released in this process. Thus, it is necessary for the collector terminal to be large enough so that it can dissipate the heat and the device may not burn out.

Working of Transistor

The transistor as its names suggests transfer resistance from one channel to other channels. Thus, as there are three terminals of the transistor, i.e. base, emitter and collector. Thus, there are two junctions of the transistors. One is the Emitter-base junction, and the other is Collector- base junction. With the help of these parameters working of transistor is explained.

Consider an NPN transistor which is unbiased. Unbiased means it is not provided with any external voltage source. In this condition, the majority charge carriers in emitter region will move towards the base region.

Due to moderate doping and small size of the base terminal, only 5-10% of the charge carriers entering base will recombine. Please note that we have considered NPN transistor so majority charge carriers in emitter will be electrons.

Thus, only a few electrons will recombine at the base and the remaining will start moving towards collector. Thus, 90-95% of the electrons emitted by an emitter will get recombined with holes in collector region. This movement of electron and holes in the circuit result in the generation of current.

Mainly, the transistors work in three regions that are as follows:

1Active region: This region is utilized for the operation of the amplifier.

2Saturation Region: In this region, the transistor is operated when we need switching operation. In this region, the transistor acts as ON switc.

3)Cut Off Region: In this transistor works as a closed switch.

Advantages of using Transistors

1. Compact Size: These small size transistors have ushered to design compact processors. We need not to work anymore with that large size vacuum tubes based computers. All thanks to the inventors of transistors.

2. Light Weight: The entire arrangement of the transistor is packed in a single case with heat sink and three terminals. This entire casing is extremely lightweight which adds to the advantage of the transistor and make it a portable device.

3. High Operating Efficiency: Transistors possesses high operating efficiency no matter whether we are using it as an amplifier or oscillator or switch.

4. Long Life: It also possesses a long life which makes it reliable for various applications as it has minimized ageing effects.

Disadvantages of using Transistors1) Low operating Frequency: It possesses operating frequency only up to certain MHz. This makes it out of the league when it comes to high-frequency applications.2) Low operating Temperature: There is a threshold limit of temperature above which if the transistor is operated it may get dilapidated. The threshold limit is 75ᵒC. Thus, we cannot operate it above this temperature range.

Transistor configuration

The three different transistor configurations are:

· Common base:   This transistor configuration provides a low input impedance while offering a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other transistor configurations available. The other salient feature of this configuration is that the input and output are in phase.

Common emitter:   This transistor configuration is probably the most widely used. The circuit provides a medium input and output impedance levels. Both current and voltage gain can be described as medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good overall performance and as such it is often thought of as the most widely used configuration. 

Common collector:   This transistor configuration is also known as the emitter follower because the emitter voltage follows that of the base. Offering a high input impedance and a low output impedance it is widely used as a buffer. The voltage gain is unity, although current gain is high. The input and output signals are in phase. In view of these characteristics, the emitter follower configuration is used as a buffer circuit providing a high input impedance to prevent loading of the previous stage, and a low output impedance to drive following stages.

TRANSISTOR CONFIGURATION SUMMARY TABLE  

TRANSISTOR CONFIGURATION

COMMON BASE

COMMON COLLECTOR(EMITTER FOLLOWER)

COMMON EMITTER

  Voltage gain

High

Low

Medium

  Current gain

Low

High

Medium

  Power gain

Low

Medium

High

  Input / output phase relationship

0°

0°

180°

  Input resistance

Low

High

Medium

  Output resistance

High

Low

Medium

The most commonly used circuit configuration is the common emitter - this is used for many amplifier stages providing voltage gain. The emitter follower or common collector is also widely used. Providing a high input impedance and low output impedance it acts as a buffer and provides only current gain - its voltage gain is unity. The common base is used in more specialist applications and is seen considerably less.

Transistor Characteristic (Common Base)

Explain all three regions

Current gain in transistors.(Amplification Factor α, β, ϒ)

The amplification factor, also called gain, is the extent to which an analog amplifier boosts the strength of a signal . Amplification factors are usually expressed in terms of power.

The decibel (dB), a logarithmic unit, is the most common way of quantifying the gain of an amplifier. For power, doubling the signal strength (an output-to-input power ratio of 2:1) translates into a gain of 3 dB; a tenfold increase in power (output-to-input ratio of 10:1) equals a gain of 10 dB; a hundredfold increase in power (output-to-input ratio of 100:1) represents 20 dB gain. If the output power is less than the input power, the amplification factor in decibels is negative. If the output-to-input signal power ratio is 1:1, then the amplification factor is 0 dB.

Power amplifiers typically have gain figures from a few decibels up to about 20 dB. Sensitive amplifiers used in wireless communications equipment can show gain of up to about 30 dB. If higher gain is needed, amplifiers can be cascaded, that is, hooked up one after another. But there is a limit to the amplification that can be attained this way. When amplifiers are cascaded, the later circuits receive noise at their inputs along with the signals. This noise can cause distortion. Also, if the amplification factor is too high, the slightest feedback can trigger oscillation, rendering an amplifier system inoperative.

Current Components of BJT Transistor

When BJT transistor is not biased, that is, there is no voltage drop across its junctions and thus, no current flows through it. If Emitter-Base junction is forward biased and Collector-Base junction is reverse biased, the voltage across the device causes electrons from emitter to flow to collector. In this, electrons pass through P type lightly doped base region and some of the electrons recombine with holes. Therefore, collector current is less than that of emitter current. Emitter current, Base current and Collector current can be related by.

Emitter current = Base current + Collector current

Mainly three parameters are used to define BJT transistor performance. Current Amplification factor, Base Transport Factor, Emitter Injection Efficiency parameters shows the performance of NPN transistor and PNP transistor.

(a). Current Amplification Factor

Current amplification factor in a BJT transistor is defined as the ratio of output current to its input current. In a common base configuration, current amplification factor is the ratio of collector current to the emitter current.

α = Ic / Ie

Common Base (CB) Configuration

The name itself implies that the Base terminal is taken as common terminal for both input and output of the transistor. The common base connection for both NPN and PNP transistors is as shown in the following figure.

For the sake of understanding, let us consider NPN transistor in CB configuration. When the emitter voltage is applied, as it is forward biased, the electrons from the negative terminal repel the emitter electrons and current flows through the emitter and base to the collector to contribute collector current. The collector voltage VCB is kept constant throughout this.

In the CB configuration, the input current is the emitter current IE and the output current is the collector current IC.

The ratio of change in collector current (ΔIC) to the change in emitter current (ΔIE) when collector voltage VCB is kept constant is called as Current amplification factor. It is denoted by α.

α=ΔIC/ΔIE α=ΔIC/ΔIE at constant VCB

Along with the emitter current flowing, there is some amount of base current IB which flows through the base terminal due to electron hole recombination. As collector-base junction is reverse biased, there is another current which is flown due to minority charge carriers. This is the leakage current which can be understood as Ileakage. This is due to minority charge carriers and hence very small.

The emitter current that reaches the collector terminal is

αIE

Total collector current

IC=αIE+Ileakage

If the emitter-base voltage VEB = 0, even then, there flows a small leakage current, which can be termed as ICBO (collector-base current with output open).

The collector current therefore can be expressed as

Hence the above derived is the expression for collector current. The value of collector current depends on base current and leakage current along with the current amplification factor of that transistor in use.

Characteristics of CB configuration

· This configuration provides voltage gain but no current gain.

· Being VCB constant, with a small increase in the Emitter-base voltage VEB, Emitter current IE gets increased.

· Emitter Current IE is independent of Collector voltage VCB.

· Collector Voltage VCB can affect the collector current IC only at low voltages, when VEB is kept constant.

· The input resistance Ri is the ratio of change in emitter-base voltage (ΔVEB) to the change in emitter current (ΔIE) at constant collector base voltage VCB.

Ri=ΔVEB/ΔIE at constant VCB

· As the input resistance is of very low value, a small value of VEB is enough to produce a large current flow of emitter current IE.

· The output resistance Ro is the ratio of change in the collector base voltage (ΔVCB) to the change in collector current (ΔIC) at constant emitter current IE.

Ro=ΔVCB/ΔIC at constant IE

· As the output resistance is of very high value, a large change in VCBproduces a very little change in collector current IC.

· This Configuration provides good stability against increase in temperature.

· The CB configuration is used for high frequency applications.

Common Emitter (CE) Configuration

Base Current Amplification factor (β)

The ratio of change in collector current (ΔIC) to the change in base current (ΔIB) is known as Base Current Amplification Factor. It is denoted by β.

β=ΔIC/ΔIB

Relation between β and α

Let us try to derive the relation between base current amplification factor and emitter current amplification factor.

β=ΔIC/ΔIB

α=ΔIC/ΔIE

IE=IB+IC

ΔIE=ΔIB+ΔIC

ΔIB=ΔIE−ΔIC

We can write

β=ΔIC/ΔIE−ΔIC

Dividing by ΔIE

β= (ΔIC/ΔIE) ⁄ (ΔIE/ΔIE−ΔIC/ΔIE)

We have

α=ΔIC/ΔIE

Therefore,

β=α ⁄ 1−α

From the above equation, it is evident that, as α approach 1, β reaches infinity.

Hence, the current gain in Common Emitter connection is very high. This is the reason this circuit connection is mostly used in all transistor applications.

Relation between β and α

Let us try to derive the relation between base current amplification factor and emitter current amplification factor.

β=ΔIC/ΔIB

α=ΔIC/ΔIE

IE=IB+IC

ΔIE=ΔIB+ΔIC

ΔIB=ΔIE−ΔIC

We can write

β=ΔIC/ΔIE−ΔIC

Dividing by ΔIE

β=(ΔIC/ΔIE) ⁄ (ΔIE/ΔIE−ΔIC/ΔIE)

We have

α=ΔIC/ΔIE

Therefore,

β=α ⁄ 1−α

From the above equation, it is evident that, as α approach 1, β reaches infinity.

Hence, the current gain in Common Emitter connection is very high. This is the reason this circuit connection is mostly used in all transistor applications.

Expression for Collector Current

In the Common Emitter configuration, IB is the input current and IC is the output current.

We know

IE=IB+IC

And

IC=αIE+ICBO

=α (IB+IC)+ICBO

If base circuit is open, i.e. if IB = 0,

The collector emitter current with base open is ICEO

Substituting the value of this in the previous equation, we get

Hence the equation for collector current is obtained.

Knee Voltage

In CE configuration, by keeping the base current IB constant, if VCE is varied, IC increases nearly to 1v of VCE and stays constant thereafter. This value of VCE up to which collector current IC changes with VCE is called the Knee Voltage. The transistors while operating in CE configuration, they are operated above this knee voltage.

Characteristics of CE Configuration

· This configuration provides good current gain and voltage gain.

· Keeping VCE constant, with a small increase in VBE the base current IBincreases rapidly than in CB configurations.

· For any value of VCE above knee voltage, IC is approximately equal to βIB.

· The input resistance Ri is the ratio of change in base emitter voltage (ΔVBE) to the change in base current (ΔIB) at constant collector emitter voltage VCE.

Ri=ΔVBE/ΔIB at constant VCE

· As the input resistance is of very low value, a small value of VBE is enough to produce a large current flow of base current IB.

· The output resistance Ro is the ratio of change in collector emitter voltage (ΔVCE) to the change in collector current (ΔIC) at constant IB.

Ro=ΔVCE/ΔIC at constant IB

· As the output resistance of CE circuit is less than that of CB circuit.

· This configuration is usually used for bias stabilization methods and audio frequency applications.

Characteristics of CE Configuration

· This configuration provides good current gain and voltage gain.

· Keeping VCE constant, with a small increase in VBE the base current IB increases rapidly than in CB configurations.

· For any value of VCE above knee voltage, IC is approximately equal to βIB.

· The input resistance Ri is the ratio of change in base emitter voltage (ΔVBE) to the change in base current (ΔIB) at constant collector emitter voltage VCE.

Ri=ΔVBE/ΔIB at constant VCE

· As the input resistance is of very low value, a small value of VBE is enough to produce a large current flow of base current IB.

· The output resistance Ro is the ratio of change in collector emitter voltage (ΔVCE) to the change in collector current (ΔIC) at constant IB.

Ro=ΔVCE/ΔIC at constant IB

· As the output resistance of CE circuit is less than that of CB circuit.

· This configuration is usually used for bias stabilization methods and audio frequency applications.

· EXERCISE Carry out for CC

· www.tutorialspoint.com/amplifiers/transistor_configurations.htm

Transistor biasing

DC supply is needed for transistor operation. This supply is biasing. Transistor can be made to operate forward or reverse bias. Table shows biasing of transistor.

Emitter Junction

Collector Junction

Region of Operation

Forward biased

Forward biased

Saturation region

Forward biased

Reverse biased

Active region

Reverse biased

Forward biased

Inverse active region

Reverse biased

Reverse biased

Cut off region

Active Region

This is the region in which transistors have many applications. This is also called as linear region. A transistor while in this region, acts better as an Amplifier.

The following circuit diagram shows a transistor working in active region.

This region lies between saturation and cutoff. The transistor operates in active region when the emitter junction is forward biased and collector junction is reverse biased.

In the active state, collector current is β times the base current, i.e.

IC=βIB

Where IC = collector current, β = current amplification factor, and IB = base current.

Saturation Region

This is the region in which transistor tends to behave as a closed switch. The transistor has the effect of its collector and emitter being shorted. The collector and emitter currents are maximum in this mode of operation.

The following figure shows a transistor working in saturation region.

The transistor operates in saturation region when both the emitter and collector junctions are forward biased.

In saturation mode,

β

As in the saturation region the transistor tends to behave as a closed switch,

IC=IE

Where IC = collector current and IE = emitter current.

Cutoff Region

This is the region in which transistor tends to behave as an open switch. The transistor has the effect of its collector and base being opened. The collector, emitter and base currents are all zero in this mode of operation.

The figure below shows a transistor working in cutoff region.

The transistor operates in cutoff region when both the emitter and collector junctions are reverse biased.

As in cutoff region, the collector current, emitter current and base currents are nil, we can write as

IC=IE=IB=0

Where IC = collector current, IE = emitter current, and IB = base current.

Transistor biasing

A biasing is a phenomenon of getting a proper dc collector current at a certain dc voltage by setting up a proper point. Transiator biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor.

Need for biasing: 1) Inherent variation of biasing parameter.

2) Stabilization (Temp.change),Temp dependence of Ic, Individual variation.

3) Thermal runaway.

Stability Factor

Stability Factor (S):

The extent to which the collector current ICIC is stabilized with varying ICOICO is measured by a stability factor S. It is defined as the rate of change of collector current ICIC with respect to the collector base leakage current ICOICO, keeping both the current IB and the current gain β constant.

Transistor operating point

When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the saturation point. As well, when a value for the maximum possible collector emitter voltage is considered, that point will be present on the X-axis, which is the cutoff point.

When a line is drawn joining these two points, such a line can be called as Load line. This is called so as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes contact at a point called as Operating point.

This operating point is also called as quiescent point or simply Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in active region. This can be better understood through the figure below.

The load line has to be drawn in order to obtain the Q-point. A transistor acts as a good amplifier when it is in active region and when it is made to operate at Q-point, faithful amplification is achieved.

Faithful amplification is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is applied at its input. 

DC Load line

When the transistor is given the bias and no signal is applied at its input, the load line drawn at such condition, can be understood as DC condition. Here there will be no amplification as the signal is absent. The circuit will be as shown below.

The value of collector emitter voltage at any given time will be

VCE=VCC−ICRC

As VCC and RC are fixed values, the above one is a first degree equation and hence will be a straight line on the output characteristics. This line is called as D.C. Load line. The figure below shows the DC load line.

To obtain A

When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as

VCE=VCC−ICRC

0=VCC−ICRC

IC=VCCRC

This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure.

To obtain B

When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as

VCE=VCC−ICRC

=VCC

(As IC = 0)

This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the above figure.

Hence we got both the saturation and cutoff point determined and learnt that the load line is a straight line. So, a DC load line can be drawn.

The importance of this operating point is further understood when an AC signal is given at the input. 

https://www.tutorialspoint.com/basic_electronics/basic_electronics_transistor_load_line_analysis.htm

Transistor as an Amplifier

Amplifier: An amplifier is an electronic device or circuit which is used to increase the magnitude of the signal applied to its input.

Amplifier graphical representation from load line

When operating voltage i.e. bias voliate is 0.7 v and transistor is bias at Q point 10v,15 mA,then (Explain) reason for first –ve and then positive.

Explain biasing method in stort i.e. The commonly used methods of transistor biasing are Base Resistor method, Collector to Base bias, Biasing with Collector feedback resistor, Voltage-divider bias

Refer V.K. Mehta including all numerical

TRANSISTOR AS SWITCH

Transistor switches can be used to switch a low voltage DC device (e.g. LED’s) ON or OFF by using a transistor in its saturated or cut-off state.

The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below.

The pink shaded area at the bottom of the curves represents the “Cut-off” region while the blue area to the left represents the “Saturation” region of the transistor. Both these transistor regions are defined as:

Cut-off Region

Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, VB < 0.7v and IC = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.

2. Saturation Region

Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.

The transistor operates as a “single-pole single-throw” (SPST) solid state switch. With a zero signal applied to the Base of the transistor it turns “OFF” acting like an open switch and zero collector current flows. With a positive signal applied to the Base of the transistor it turns “ON” acting like a closed switch and maximum circuit current flows through the device.

Using the transistor values from the previous tutorials of: β = 200, Ic = 4mA and Ib = 20uA, find the value of the Base resistor (Rb) required to switch the load fully “ON” when the input terminal voltage exceeds 2.5v.

Here catch diode is used to eliminate flyback, when the abrupt voltage spike is witnessed across the inductive load when the supply current abruptly reduced. It helps the circuit from damaging. It will get prevented from buying new circuit. Freewheeling diode is simplified form where voltage source is connected to an inductor with a switch.

Digital logic.

Enhancement type MOSFET.

https://www.electronics-tutorials.ws/transistor/tran_4.html

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic devices. The MOSFET is a core of integrated circuit and it can be designed and fabricated in a single chip because of these very small sizes. The MOSFET is a four terminal device with source(S), gate (G), drain (D) and body (B) terminals.The body of the MOSFET is frequently connected to the source terminal so making it a three terminal device like field effect transistor. The MOSFET is very far the most common transistor and can be used in both analog and digital circuits.

The MOSFET works by electronically varying the width of a channel along which charge carriers flow (electrons or holes).  The charge carriers enter the channel at source and exit via the drain. The width of the channel is controlled by the voltage on an electrode is called gate which is located between source and drain. It is insulated from the channel near an extremely thin layer of metal oxide. The MOS capacity present in the device is the main part.

The MOSFET can function in two ways

Depletion Mode: When there is no voltage on the gate, the channel shows its maximum conductance. As the voltage on the gate is either positive or negative,  the channel conductivity decreases. The depletion MOSFET the channel exists even for zero gate to source voltage. 

Enhancement mode: When there is no voltage on the gate the device does not conduct. More is the voltage on the gate, the better the device can conduct.

Working Principle of MOSFET:

The aim of the MOSFET is to be able to control the voltage and current flow between the source and drain. It works almost as a switch. The working of MOSFET depends upon the MOS capacitor. The MOS capacitor is the main part of MOSFET. The semiconductor surface at the below oxide layer which is located between source and drain terminal. It can be  inverted from p-type to n-type by applying a positive or negative gate voltages respectively.  When we apply the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward with the substrate. The depletion region populated by the bound negative charges which are associated with the acceptor atoms. The electrons reach channel is formed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. Instead of positive voltage if we apply negative voltage , a hole channel will be formed under the oxide layer.

https://www.elprocus.com/mosfet-as-a-switch-circuit-diagram-free-circuits/

www.youtube.com/watch?v=2eRlTYUaK_0