1. What is electronics?

The word electronics is derived from electron mechanics, which means to study the behavior of an electron under different conditions of applied electric field.

Electronics definition:

The branch of engineering in which the flow and control of electrons in vacuum or semiconductor are studied is called electronics. Electronics can also be defined as the branch of engineering in which the electronic devices and their utilization are studied.

The motion of electrons through a conductor gives us electric current. This electric current can be produced with the help of batteries and generators.

The device which controls the flow of electrons is called electronic device. These devices are the main building blocks of electronic circuits.

Electronics have various branches include, digital electronics, analog electronics, micro electronics, nanoelectronics, optoelectronics, integrated circuit and semiconductor device.History of electronics

Diode vacuum tube was the first electronic component invented by J.A. Fleming. Later, Lee De Forest developed the triode, a three element vacuum tube capable of voltage amplification. Vacuum tubes played a major role in the field of microwave and high power transmission as well as television receivers. 

In 1947, Bell laboratories developed the first transistor based on the research of Shockley, Bardeen and Brattain. However, transistor radios are not developed until the late 1950’s due to the existing huge stock of vacuum tubes.

In 1959, Jack Kilby of Texas Instruments developed the first integrated circuit. Integrated circuits contain large number of semiconductor devices such as diodes and transistors in very small area.

Advantages of electronics:

Electronic devices are playing a major role in everyday life. The various electronic devices we use in everyday life include 

Computers, Mobile phones, ATM, Pen drive, Television and Digital camera.

2. Introduction to semiconductor

The material which has electrical conductivity between that of a conductor and that of an insulator is called as semiconductor.

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Silicon, germanium and graphite are some examples of semiconductors. Semiconductors are the foundation of modern electronics, including transistors, Light-Emitting diodes, solar cells etc.

In semiconductors, the forbidden gap between valence band and conduction band is very small. It has a forbidden gap of about 1 electron volt (eV).

At low temperature, the valence band is completely occupied with electrons and conduction band is empty because the electron in the valence band does not have enough energy to move in to conduction band. Therefore, semiconductor behaves as an insulator at low temperature.

However, at room temperature some of the electrons in valence band gains enough energy in the form of heat and moves in to conduction band. When the valence electrons moves in to conduction band they becomes free electrons. These electrons are not attached to the nucleus of a atom, So they moves freely.

The conduction band electrons are responsible for electrical conductivity. The measure of ability to conduct electric current is called as electrical conductivity.

When the temperature is goes on increasing, the number of valence band electrons moving in to conduction band is also increases. This shows that electrical conductivity of the semiconductor increases with increase in temperature. i.e. a semiconductor has negative temperature co-efficient of resistance. The resistance of semiconductor decreases with increase in temperature. 

In semiconductors, electric current is carried by two types of charge carriers they are electrons and holes.

Hole:

The absence of electron in a particular place in an atom is called as hole. Hole is a electric charge carrier which has positive charge. The electric charge of hole is equal to electric charge of electron but have opposite polarity.

When a small amount of external energy is applied, then the electrons in the valence band moves in to conduction band and leaves a vacancy in valence band. This vacancy is called as hole.

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3. Intrinsic semiconductor

A. Introduction:

Pure semiconductors are called intrinsic semiconductors. Silicon and germanium are the most common examples of intrinsic semiconductors.

Intrinsic semiconductor is also called as undoped semiconductor or I-type semiconductor.

In intrinsic semiconductor the number of electrons in the conduction band is equal to the number of holes in the valence band. Therefore the overall electric charge of a atom is neutral.

B. Atomic structure of silicon and germanium:

The atomic structure of intrinsic semiconductor materials like silicon and germanium is as follows.

Atomic structure of silicon

Silicon is a substance consisting of atoms which all have the same number of protons. The atomic number of silicon is 14 i.e. 14 protons. 

The number of protons in the nucleus of an atom is called atomic number. Silicon atom has 14 electrons (two electrons in first orbit, eight electrons in second orbit and 4 electrons in the outermost orbit). 

Atomic structure of germanium

Germanium is a substance consisting of atoms which all have the same number of protons. The atomic number of germanium is 32 i.e. 32 protons.

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The number of protons in the nucleus of atom is called atomic number. Germanium has 32 electrons ( 2 electrons in first orbit, 8 electrons in second orbit, 18 electrons in third orbit and 4 electrons in the outermost orbit).

C. Electron and hole current

In conductors current is caused by only motion of electrons but in semiconductors current is caused by both electrons in conduction band and holes in valence band. 

Current that is caused by electron motion is called electron current and current that is caused by hole motion is called hole current. Electron is a negative charge carrier whereas hole is a positive charge carrier.

At absolute zero temperature intrinsic semiconductor behaves as insulator. However, at room temperature the electrons present in the outermost orbit absorb thermal energy. When the outermost orbit electrons get enough energy then they will break bonding with the nucleus of atom and jumps in to conduction band.

The electrons present in conduction band are not attached to the nucleus of an atom so they are free to move. When the valence electron moves from valence band to the conduction band a vacancy is created in the valence band where electron left. Such vacancy is called hole.

Let’s take an example, as shown in fig there are three atoms atom A, atom B and atom C. At room temperature valence electron in an atom A gains enough energy and jumps in to conduction band as show in fig (1).

When it jumps in to conduction band a hole (vacancy) is created in the valence band at atom A as shown in fig (2). Then the neighboring electron from atom B moves to atom A to fill the hole at atom A. This creates a hole at atom B as shown in fig (3).

Similarly neighboring electron from atom C moves to atom B to fill the hole at atom B. This creates a hole at atom C as shown in fig (4). Likewise electrons moves from left side to right side and holes moves from right to left side.

D. Conduction in intrinsic semiconductorThe process of conduction in intrinsic semiconductor is shown in below fig.

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Here, positive terminal of battery is connected to one side and negative terminal of the battery is connected to other side. As we know like charges repel each other and opposite charges attract each other.

In the similar way negative charge carriers (electrons) are attracted towards the positive terminal of battery and positive charge carriers (holes) attracted towards the negative terminal of battery.

Electrons will experience a attractive force from the positive terminal, so they move towards the positive terminal of the battery by carrying the electric current. Similarly holes will experience a attractive force from the negative terminal, so they moves towards the negative terminal of the battery by carrying the electric current. Thus, in a semiconductor electric current is carried by both electrons and holes.

In intrinsic semiconductor the number of free electrons in conduction band is equal to the number of holes in valence band. The current caused by electrons and holes is equal in magnitude.

The total current in intrinsic semiconductor is the sum of hole and electron current.   

Total current = Electron current + Hole current

I = Ihole+ Ielectron

E. Conventional current

The electric current that flows from positive terminal of battery to the negative terminal of battery is called conventional current. The conventional current direction is in the same direction of flow of holes but opposite to the direction of flow of free electrons.

When Ben Franklin started experimenting with the electricity, he assumed that electric current (positive charge carriers) flow from positive to negative. But later world realized that it was wrong.  Actually electric current flows from negative terminal to positive terminal of battery.

The flow of charge carriers is called current. Here charge carriers are protons or electrons. But specifically current is carried by electrons not protons because protons are strongly bound to the nucleus of an atom because of strong nuclear force.

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So electrons those are loosely bounded to the nucleus of an atom break bonding with the parent atom and become free. The electrons that are not bound to atoms flow freely and constitute current. Electrons flow from negative terminal to positive terminal of battery. 

F. Intrinsic carrier concentration

In intrinsic semiconductor, when the valence electrons broke the covalent bond and jumps into the conduction band, two types of charge carriers gets generated. They are free electrons and holes.

The number of electrons per unit volume in the conduction band or the number of holes per unit volume in the valence band is called intrinsic carrier concentration. The number of electrons per unit volume in the conduction band is called electron-carrier concentration and the number of holes per unit volume in the valence band is called as hole-carrier concentration.

In an intrinsic semiconductor, the number of electrons generated in the conduction band is equal to the number of holes generated in the valence band. Hence the electron-carrier concentration is equal to the hole-carrier concentration.

It can be written as,                                     ni = n = p 

Where, n = electron-carrier concentration                P = hole-carrier concentration

ni = intrinsic carrier concentration

The hole concentration in the valence band is given as       

The electron concentration in the conduction band is given as

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   Where KB is the Boltzmann constant   T is the absolute temperature of intrinsic semiconductor   Nc is the effective density of states in conduction band.   Nv is the effective density of states in valence band.

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7. Diode Switching Times

While changing the bias conditions, the diode undergoes a transient response. The response of a system to any sudden change from an equilibrium position is called as transient response.

The sudden change from forward to reverse and from reverse to forward bias, affects the circuit. The time taken to respond to such sudden changes is the important criterion to define the effectiveness of an electrical switch.

The time taken before the diode recovers its steady state is called as Recovery Time. The time interval taken by the diode to switch from reverse biased state to forward biased

state is called as Forward Recovery Time.(tfr) The time interval taken by the diode to switch from forward biased state to reverse biased

state is called as Reverse Recovery Time. (tfr)To understand this more clearly, let us try to analyze what happens once the voltage is applied to a switching PN diode.

Carrier ConcentrationMinority charge carrier concentration reduces exponentially as seen away from the

junction. When the voltage is applied, due to the forward biased condition, the majority carriers of one side move towards the other. They become minority carriers of the other side. This concentration will be more at the junction.

For example, if N-type is considered, the excess of holes that enter into N-type after applying forward bias, adds to the already present minority carriers of N-type material.

Let us consider few notations. The majority carriers in P-type (holes) = Ppo The majority carriers in N-type (electrons) = Nno The minority carriers in P-type (electrons) = Npo The majority carriers in N-type (holes) = Pno

During Forward biased Condition − The minority carriers are more near junction and less far away from the junction. The graph below explains this.

Excess minority carrier charge in P-type = Pn−Pno with pno (steady state value)Excess minority carrier charge in N-type = Np−Npo with Npo (steady state value)

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During reverse bias condition − Majority carriers doesn’t conduct the current through the junction and hence don’t participate in current condition. The switching diode behaves as a short circuited for an instance in reverse direction.

The minority carriers will cross the junction and conduct the current, which is called as Reverse Saturation Current. The following graph represents the condition during reverse bias.

In the above figure, the dotted line represents equilibrium values and solid lines represent actual values. As the current due to minority charge carriers is large enough to conduct, the circuit will be ON until this excess charge is removed.

The time required for the diode to change from forward bias to reverse bias is called Reverse recovery time (trr). The following graphs explain the diode switching times in detail.

From the above figure, let us consider the diode current graph.

At t1 the diode is suddenly brought to OFF state from ON state; it is known as Storage time. Storage time is the time required to remove the excess minority carrier charge. The

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negative current flowing from N to P type material is of a considerable amount during the Storage time. This negative current is,

−IR = −VR / R

The next time period is the transition time” (from t2 to t3)Transition time is the time taken for the diode to get completely to open circuit condition. After t3 diode will be in steady state reverse bias condition. Before t1 diode is under steady state forward bias condition.So, the time taken to get completely to open circuit condition is

Reverserecoverytime(trr) = Storagetime(Ts) + Transitiontime(Tt)

Whereas to get to ON condition from OFF, it takes less time called as Forward recovery time. Reverse recovery time is greater than Forward recovery time. A diode works as a better switch if this Reverse recovery time is made less.

DefinitionsLet us just go through the definitions of the time periods discussed.

Storage time − The time period for which the diode remains in the conduction state even in the reverse biased state, is called as Storage time.

Transition time − The time elapsed in returning back to the state of non-conduction, i.e. steady state reverse bias, is called Transition time.

Reverse recovery time − The time required for the diode to change from forward bias to reverse bias is called as Reverse recovery time.

Forward recovery time − The time required for the diode to change from reverse bias to forward bias is called as Forward recovery time.

Factors that affect diode switching times

There are few factors that affect the diode switching times, such as Diode Capacitance − The PN junction capacitance changes depending upon the bias

conditions. Diode Resistance − The resistance offered by the diode to change its state. Doping Concentration − The level of doping of the diode, affects the diode switching

times. Depletion Width − The narrower the width of the depletion layer, the faster the

switching will be. A Zener diode has narrow depletion region than an avalanche diode, which makes the former a better switch.

ApplicationsThere are many applications in which diode switching circuits are used, such as −

High speed rectifying circuits High speed switching circuits RF receivers General purpose applications Consumer applications Automotive applications Telecom applications etc.

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Breakdown Mechanisms in Semiconductor diodes:

When a diode is reverse biased the depletion layer widens to setup a large potential barrier which prevents diffusion of majority carriers from on side to other. Thus there is no current due to the majority carriers. Under normal reverse voltages only very small reverse current due to minority carriers exists. It is in the order of micro amps and is further temperature dependent. As reverse bias voltage is further increased it reaches point where the reverse current suddenly shoots up.

This occurs due to the junction breakdown. This is of two types:1. Zener breakdown2. Avalanche breakdown

Zener Breakdown: This occurs primarily in heavily doped diodes. In these diodes the depletion region is very small.

When reverse biasing a diode a very strong electric field exists across the depletion region at near breakdown voltage levels. For an applied reverse bias voltage of 6 volts or less the electric field is in the order of 2 x 107 v/m. This very high electric field breaks covalent bond and creates new electron hole pairs which increases the reverse current dramatically thus a large reverse current flows.

For lightly doped diode the Zener breakdown voltage is quite high and so breakdown predominantly occurs through avalanche mechanism.

Avalanche Breakdown: This occurs in lightly doped diodes where the depletion layer is very wide and electric field is very low.

Here the reverse voltage applied import high energy to the minority carriers. The minority carriers with sufficient kinetic energy disrupt covalent bands in the crystal thus releasing valence electron. This process is called “Impact Ionisation”. The nearly released valence electrons gain enough energy to disrupt other covalent bonds. The process is like a uncontrolled chain reaction and is a cumulative process and is known as “Avalanche multiplication”. It leads to avalanche or flood of charge carriers thus increasing the reverse current dramatically.

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Clipper Circuits

Electronic devices are very sensitive to voltage. If a large amplitude voltage is applied, it may permanently destroy the device. So, it is essential to protect the electronics devices.The protection of the electronic devices can be achieved by using the clipper circuits.A clipper is a device that removes either the positive half (top half) or negative half (bottom half), or both positive and negative halves of the input AC signal. In other words, a clipper is a device that limits the positive amplitude or negative amplitude or both positive and negative amplitudes of the input AC signal. In some cases, a clipper removes a small portion of the positive half cycle or negative half cycle or both positive and negative half cycles.

Types of clippers

The clipper circuits are generally categorized into three types: series clippers, shunt clippers and dual (combination) clippers. In series clippers, the diode is connected in series with the output load resistance. In shunt clippers, the diode is connected in parallel with the output load resistance.

The series clippers are again classified into four types: series positive clipper, series positive clipper with bias, series negative clipper and series negative clipper with bias. The shunt (parallel) clippers are again classified into four types: shunt positive clipper, shunt positive clipper with bias, shunt negative clipper, and shunt negative clipper with bias. 

The various types of clippers are as follows: 

Series positive clipper Series positive clipper with bias Series negative clipper Series negative clipper with bias Shunt positive clipper Shunt positive clipper with bias Shunt negative clipper Shunt negative clipper with bias

Dual (combination) clipper

i) Series positive clipper:

In series positive clipper, the positive half cycles of the input AC signal is removed. If the diode is arranged in such a way that the arrowhead of the diode points towards the input and the diode is in series with the output load resistance, then the clipper is said to be a series positive clipper.

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In the circuit diagram, the diode D is connected in series with the output load resistance RL and the arrowhead of the diode is pointing towards the input. So the circuit is said to be a series positive clipper.

The vertical line in the diode symbol represents the cathode (n-side) and the opposite end represents the anode (p-side). 

During positive half cycle: During the positive half cycle, terminal A is positive and terminal B is negative. That means the positive terminal A is connected to n-side and the negative terminal B is connected to p-side of the diode. Therefore, the diode D is reverse biased during the positive half cycle.

During reverse biased condition, no current flows through the diode. So the positive half cycle is blocked or removed at the output.

During negative half cycle: During the negative half cycle, terminal A is negative and terminal B is positive. That means the negative terminal A is connected to n-side and the positive terminal B is connected to p-side of the diode. Therefore, the diode D is forward biased during the negative half cycle. 

During forward biased condition, electric current flows through the diode. So the negative half cycle is allowed at the output.

Thus, a series of positive half cycles are completely removed at the output. We know that a clipper either clips a portion of half cycle or clips a complete half cycle. In this case, complete half cycles are removed.

ii) Series positive clipper with bias

Sometimes it is desired to remove a small portion of positive or negative half cycles. In such cases, the biased clippers are used.

The construction of the series positive clipper with bias is almost similar to the series positive clipper. The only difference is an extra element called battery is used in series positive clipper with bias.

A. Series positive clipper with positive bias

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During positive half cycle: During the positive half cycle, terminal A is positive and terminal B is negative. That means the positive terminal is connected to n-side and the negative terminal is connected to p-side. Therefore, the diode is reverse biased by the input supply voltage Vi. 

However, we are supplying the voltage from another source called battery. As shown in the figure, the positive terminal of the battery is connected to p-side and the negative terminal of the battery is connected to n-side of the diode. Therefore, the diode is forward biased by the battery voltage VB.

That means the diode is reverse biased by the input supply voltage (Vi) and forward biased by the battery voltage (VB).

Initially, the input supply voltage Vi is less than the battery voltage VB (Vi < VB). So the battery voltage dominates the input supply voltage. Hence, the diode is forward biased by the battery voltage and allows electric current through it. As a result, the signal appears at the output.

When the input supply voltage Vi becomes greater than the battery voltage VB, the diode D is reverse biased. So no current flows through the diode. As a result, input signal does not appear at the output.

Thus, the clipping (removal of a signal) takes place during the positive half cycle only when the input supply voltage becomes greater than the battery voltage.

During negative half cycle: During the negative half cycle, terminal A is negative and terminal B is positive. That means the diode D is forward biased due to the input supply voltage.

Furthermore, the battery is also connected in such a way that the positive terminal is connected to p-side and the negative terminal is connected to n-side. So the diode is forward biased by both battery voltage VB and input supply voltage Vi.

That means, during the negative half cycle, it doesn’t matter whether the input supply voltage is greater or less than the battery voltage, the diode always remains forward biased. So the complete negative half cycle appears at the output.

Thus, the series positive clipper with positive bias removes a small portion of positive half cycles.

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B. Series positive clipper with negative bias

During positive half cycle: During the positive half cycle, the diode D is reverse biased by both input supply voltage Viand battery voltage VB. So no signal appears at the output during the positive half cycle. Therefore, the complete positive half cycle is removed.

During negative half cycle: During the negative half cycle, the diode is forward biased by the input supply voltage Vi and reverse biased by the battery voltage VB.

However, initially, the battery voltage VB dominates the input supply voltage Vi. So the diode remains to be reverse biased until the Vi becomes greater than VB. When the input supply voltage Vi becomes greater than the battery voltage VB, the diode is forward biased by the input supply voltage Vi. So the signal appears at the output.

iii) Series negative clipper

In series negative clipper, the negative half cycles of the input AC signal is removed at the output. The circuit construction of the series negative clipper is shown in the figure.

If the diode is arranged in such a way that the arrowhead of the diode points towards the output and the diode is in series with the output load resistance, then the clipper is said to be a series negative clipper.

In simple words, in a series negative clipper, the diode is connected in a direction opposite to that of the series positive clipper.

The vertical line in the diode symbol represents the cathode (n-side) and the opposite end represents the anode (p-side).

During positive half cycle: During the positive half cycle, terminal A is positive and terminal B is negative. That means the positive terminal A is connected to p-side and the negative terminal B is connected to n-side of the diode. Therefore, the diode D is forward biased during the positive half cycle. 

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During forward biased condition, electric current flows through the diode. So the positive half cycle is allowed at the output. Therefore, a series of positive half cycles appears at the output.

During negative half cycle: During the negative half cycle, the terminal A is negative and the terminal B is positive. That means the negative terminal A is connected to p-side and the positive terminal B is connected to n-side of the diode. Therefore, the diode D is reverse biased during the negative half cycle.

During reverse biased condition, no current flows through the diode. So the negative half cycle is completely blocked or removed at the output.

iv) Series negative clipper with bias

Sometimes it is desired to remove a small portion of positive or negative half cycles of the input AC signal. In such cases, the biased clippers are used.

The construction of the series negative clipper with bias is almost similar to the series negative clipper. The only difference is an extra element called battery is used in series negative clipper with bias.

A. Series negative clipper with positive bias

During positive half cycle:

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During the positive half cycle, terminal A is positive and terminal B is negative. That means the positive terminal A is connected to p-side and the negative terminal B is connected to n-side.

However, we are also supplying the voltage from another source called battery. As shown in the figure, the positive terminal of the battery is connected to n-side and the negative terminal of the battery is connected to p-side of the diode.

That means the diode is forward biased by input supply voltage Vi and reverse biased by battery voltage VB. Initially, the battery voltage is greater than the input supply voltage. Hence, the diode is reverse biased and does not allow electric current. Therefore, no signal appears at the output.

When the input supply voltage Vi becomes greater than the battery voltage VB, the diode is forward biased and allows electric current. As a result, the signal appears at the output.

During negative half cycle: During the negative half cycle, the diode is reverse biased by both input supply voltage Vi and battery voltage VB.

So it doesn’t matter whether the input supply voltage is greater or less than the battery voltage VB, the diode always remains reverse biased. Therefore, during the negative half cycle, no signal appears at the output.

B. Series negative clipper with negative bias

During positive half cycle: During the positive half cycle, the diode D is forward biased by both input supply voltage Viand the battery voltage VB.

So it doesn’t matter whether the input supply voltage is greate or less than battery voltage VB, the diode always remains forward biased. Therefore, during the positive half cycle, the signal appears at the output. 

During negative half cycle: During the negative half cycle, the diode D is reverse biased by the input supply voltage Viand forward biased by the battery voltage VB. Initially, the input supply voltage Vi is less than the battery voltage VB. So the diode is forward biased by the battery voltage VB. As a result, the signal appears at the output.When the input supply voltage Vi becomes greater than the battery voltage VB, the diode will become reverse biased. As a result, no signal appears at the output.

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v) Shunt positive clipper

In shunt clipper, the diode is connected in parallel with the output load resistance. The operating principles of the shunt clipper are nearly opposite to the series clipper.

The series clipper passes the input signal to the output load when the diode is forward biased and blocks the input signal when the diode is reverse biased.

The shunt clipper on the other hand passes the input signal to the output load when the diode is reverse biased and blocks the input signal when the diode is forward biased.

In shunt positive clipper, during the positive half cycle the diode is forward biased and hence no output is generated. On the other hand, during the negative half cycle the diode is reverse biased and hence the entire negative half cycle appears at the output.

vi) Shunt positive clipper with bias

A. Shunt positive clipper with positive bias:

During the positive half cycle: the diode is forward biased by the input supply voltage V i and reverse biased by the battery voltage VB.

However, initially, the input supply voltage Vi is less than the battery voltage VB. Hence, the battery voltage VB makes the diode to be reversed biased. Therefore, the signal appears at the output. However, when the input supply voltage Vi becomes greater than the battery voltage VB, the diode D is forward biased by the input supply voltage V i. As a result, no signal appears at the output.

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During the negative half cycle: The diode is reverse biased by both input supply voltage and battery voltage. So it doesn’t matter whether the input supply voltage is greater or lesser than the battery voltage, the diode always remains reverse biased. As a result, a complete negative half cycle appears at the output.

B. Shunt positive clipper with negative bias

During the positive half cycle: The diode is forward biased by both input supply voltage V i and battery voltage VB. Therefore, no signal appears at the output during the positive half cycle. 

During the negative half cycle: The diode is reverse biased by the input supply voltage and forward biased by the battery voltage. However, initially, the input supply voltage Vi is less than the battery voltage VB.

So the battery voltage makes the diode to be forward biased. As a result, no signal appears at the output. However, when the input supply voltage V i becomes greater than the battery voltage VB, the diode is reverse biased by the input supply voltage Vi. As a result, the signal appears at the output.

vii) Shunt negative clipper

In shunt negative clipper, during the positive half cycle the diode is reverse biased and hence the entire positive half cycle appears at the output. On the other hand, during the negative half cycle the diode is forward biased and hence no output signal is generated.

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viii) Shunt negative clipper with bias

A. Shunt negative clipper with positive bias:

During the positive half cycle: The diode is reverse biased by the input supply voltage V i and forward biased by the battery voltage VB. However, initially, the input supply voltage is less than the battery voltage. So the diode is forward biased by the battery voltage. As a result, no signal appears at the output. However, when the input supply voltage becomes greater than the battery voltage then the diode is reverse biased by the input supply voltage. As a result, the signal appears at the output.

During the negative half cycle: The diode is forward biased by both input supply voltage Vi and battery voltage VB. So the complete negative half cycle is removed at the output.

B. Shunt negative clipper with negative bias

During the positive half cycle: The diode is reverse biased by both input supply voltage Vi and battery voltage VB. As a result, the complete positive half cycle appears at the output.

During the negative half cycle: The diode is forward biased by the input supply voltage V i and reverse biased by the battery voltage VB.

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However, initially, the input supply voltage is less than the battery voltage. So the diode is reverse biased by the battery voltage. As a result, the signal appears at the output. However, when the input supply voltage becomes greater than the battery voltage, the diode is forward biased by the input supply voltage. As a result, the signal does not appear at the output.

x) Dual (combination) clipper

Sometimes it is desired to remove a small portion of both positive and negative half cycles. In such cases, the dual clippers are used.

The dual clippers are made by combining the biased shunt positive clipper and biased shunt negative clipper.

Let us consider a dual clipper circuit in which a sinusoidal ac voltage is applied to the input terminals of the circuit.

During positive half cycle:

During the positive half cycle, the diode D1 is forward biased by the input supply voltage Viand reverse biased by the battery voltage VB1. On the other hand, the diode D2 is reverse biased by both input supply voltage Vi and battery voltage VB2.

Initially, the input supply voltage is less than the battery voltage.  So the diode D1 is reverse biased by the battery voltage VB1. Similarly, the diode D2 is reverse biased by the battery voltage VB2. As a result, the signal appears at the output. However, when the input supply voltage Vi becomes greater than the battery voltage VB1, the diode D1 is forward biased by the input supply voltage. As a result, no signal appears at the output.

During negative half cycle: During the negative half cycle, the diode D1 is reverse biased by both input supply voltage Viand battery voltage VB1. On the other hand, the diode D2 is forward biased by the input supply voltage Vi and reverse biased by the battery voltage VB2.

Initially, the battery voltage is greater than the input supply voltage. Therefore, the diode D1and diode D2 are reverse biased by the battery voltage. As a result, the signal appears at the output. When the input supply voltage becomes greater than the battery voltage VB2, the diode D2 is forward biased. As a result, no signal appears at the output.

 Applications of clippers:

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Clippers are commonly used in power supplies. Used in TV transmitters and Receivers They are employed for different wave generation such as square, rectangular, or

trapezoidal waves. Series clippers are used as noise limiters in FM transmitters.

Clamper circuits

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Clamper definition

A clamper is an electronic circuit that changes the DC level of a signal to the desired level without changing the shape of the applied signal. In other words, the clamper circuit moves the whole signal up or down to set either the positive peak or negative peak of the signal at the desired level.The dc component is simply added to the input signal or subtracted from the input signal. A clamper circuit adds the positive dc component to the input signal to push it to the positive side. Similarly, a clamper circuit adds the negative dc component to the input signal to push it to the negative side.

Types of clampers

Clamper circuits are of three types: Positive clampers Negative clampers Biased clampers

A. Positive clamper

The positive clamper is made up of a voltage source V i, capacitor C, diode D, and load resistor RL. In the below circuit diagram, the diode is connected in parallel with the output load. So the positive clamper passes the input signal to the output load when the diode is reverse biased and blocks the input signal when the diode is forward biased.

During negative half cycle:

During the negative half cycle of the input AC signal, the diode is forward biased and hence no signal appears at the output. In forward biased condition, the diode allows electric current through it. This current will flows to the capacitor and charges it to the peak value of input voltage Vm. The capacitor charged in inverse polarity (positive) with the input voltage. As input current or voltage decreases after attaining its maximum value -Vm, the capacitor holds the charge until the diode remains forward biased.

During positive half cycle:

During the positive half cycle of the input AC signal, the diode is reverse biased and hence the signal appears at the output. In reverse biased condition, the diode does not allow electric current through it. So the input current directly flows towards the output.

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When the positive half cycle begins, the diode is in the non-conducting state and the charge stored in the capacitor is discharged (released). Therefore, the voltage appeared at the output is equal to the sum of the voltage stored in the capacitor (Vm) and the input voltage (Vm) { I.e. Vo = Vm+ Vm = 2Vm} which have the same polarity with each other. As a result, the signal shifted upwards.

The peak to peak amplitude of the input signal is 2Vm, similarly the peak to peak amplitude of the output signal is also 2Vm. Therefore, the total swing of the output is same as the total swing of the input.

The basic difference between the clipper and clamper is that the clipper removes the unwanted portion of the input signal whereas the clamper moves the input signal upwards or downwards.

B. Negative clamper

During positive half cycle:

During the positive half cycle of the input AC signal, the diode is forward biased and hence no signal appears at the output. In forward biased condition, the diode allows electric current through it. This current will flows to the capacitor and charges it to the peak value of input voltage in inverse polarity -Vm. As input current or voltage decreases after attaining its maximum value Vm, the capacitor holds the charge until the diode remains forward biased.

During negative half cycle:

During the negative half cycle of the input AC signal, the diode is reverse biased and hence the signal appears at the output. In reverse biased condition, the diode does not allow electric current through it. So the input current directly flows towards the output.

When the negative half cycle begins, the diode is in the non-conducting state and the charge stored in the capacitor is discharged (released). Therefore, the voltage appeared at the output is equal to the sum of the voltage stored in the capacitor (-Vm) and the input voltage (-Vm) {I.e. Vo = -Vm- Vm = -2Vm} which have the same polarity with each other. As a result, the signal shifted downwards.

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C. Biased clampers

Sometimes an additional shift of DC level is needed. In such cases, biased clampers are used. The working principle of the biased clampers is almost similar to the unbiased clampers. The only difference is an extra element called DC battery is introduced in biased clampers.

i) Positive clamper with positive bias:

If positive biasing is applied to the clamper then it is said to be a positive clamper with positive bias. The positive clamper with positive bias is made up of an AC voltage source, capacitor, diode, resistor, and dc battery. 

During positive half cycle:

During the positive half cycle, the battery voltage forward biases the diode when the input supply voltage is less than the battery voltage. This current or voltage will flows to the capacitor and charges it.

When the input supply voltage becomes greater than the battery voltage then the diode stops allowing electric current through it because the diode becomes reverse biased.

During negative half cycle:

During the negative half cycle, the diode is forward biased by both input supply voltage and battery voltage. So the diode allows electric current. This current will flows to the capacitor and charges it.

ii) Positive clamper with negative bias

During negative half cycle:

During the negative half cycle, the battery voltage reverse biases the diode when the input supply voltage is less than the battery voltage. As a result, the signal appears at the output.When the input supply voltage becomes greater than the battery voltage, the diode is forward biased by the input supply voltage and hence allows electric current through it. This current will flows to the capacitor and charges it.

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During positive half cycle:

During the positive half cycle, the diode is reverse biased by both input supply voltage and the battery voltage. As a result, the signal appears at the output. The signal appeared at the output is equal to the sum of the input voltage and capacitor voltage.

iii) Negative clamper with positive bias

During positive half cycle:

During the positive half cycle, the battery voltage reverse biases the diode when the input supply voltage is less than the battery voltage. When the input supply voltage becomes greater than the battery voltage, the diode is forward biased by the input supply voltage and hence allows electric current through it. This current will flows to the capacitor and charges it.

During negative half cycle:

During the negative half cycle, the diode is reverse biased by both input supply voltage and battery voltage. As a result, the signal appears at the output.

iv) Negative clamper with negative bias

During positive half cycle:

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During the positive half cycle, the diode is forward biased by both input supply voltage and battery voltage. As a result, current flows through the capacitor and charges it.

During negative half cycle:

During the negative half cycle, the battery voltage forward biases the diode when the input supply voltage is less than the battery voltage. When the input supply voltage becomes greater than the battery voltage, the diode is reverse biased by the input supply voltage and hence signal appears at the output. 

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Transistor

A Transistor is a three terminal semiconductor device that regulates current or voltage flow and acts as a switch or gate for signals.

Why Do We Need Transistors?

Suppose that you have a FM receiver which grabs the signal you want. The received signal will obviously be weak due to the disturbances it would face during its journey. Now if this signal is read as it is, you cannot get a fair output. Hence we need to amplify the signal. Amplification means increasing the signal strength.

This is just an instance. Amplification is needed wherever the signal strength has to be increased. This is done by a transistor. A transistor also acts as a switch to choose between available options. It also regulates the incoming current and voltage of the signals.

Constructional Details of a Transistor:--

The Transistor is a three terminal solid state device which is formed by connecting two diodes back to back. Hence it has got two PN junctions. Three terminals are drawn out of the three semiconductor materials present in it. This type of connection offers two types of transistors. They are PNP and NPN which means an N-type material between two Ptypes and the other is a P-type material between two N-types respectively.

The construction of transistors is as shown in the following figure which explains the idea discussed above.

The three terminals drawn from the transistor indicate Emitter, Base and Collector terminals. They have their functionality as discussed below.

Emitter The left hand side of the above shown structure can be understood as Emitter. This has a moderate size and is heavily doped as its main function is to supply a

number of majority carriers, i.e. either electrons or holes. As this emits electrons, it is called as an Emitter. This is simply indicated with the letter E.

Base The middle material in the above figure is the Base. This is thin and lightly doped. Its main function is to pass the majority carriers from the emitter to the collector. This is indicated by the letter B.

Collector The right side material in the above figure can be understood as a Collector.

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Its name implies its function of collecting the carriers. This is a bit larger in size than emitter and base. It is moderately doped. This is indicated by the letter C.

The symbols of PNP and NPN transistors are as shown below.

The arrow-head in the above figures indicated the emitter of a transistor. As the collector of a transistor has to dissipate much greater power, it is made large. Due to the specific functions of emitter and collector, they are not interchangeable. Hence the terminals are always to be kept in mind while using a transistor.

In a Practical transistor, there is a notch present near the emitter lead for identification. The PNP and NPN transistors can be differentiated using a Multimeter. The following figure shows how different practical transistors look like.

Transistor Biasing

As we know that a transistor is a combination of two diodes, we have two junctions here. As one junction is between the emitter and base, that is called as Emitter-Base junction and likewise, the other is Collector-Base junction.

Biasing is controlling the operation of the circuit by providing power supply. The function of both the PN junctions is controlled by providing bias to the circuit through some dc supply. The figure below shows how a transistor is biased.

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By having a look at the above figure, it is understood that

The N-type material is provided negative supply and P-type material is given positive supply to make the circuit Forward bias.

The N-type material is provided positive supply and P-type material is given negative supply to make the circuit Reverse bias.

By applying the power, the emitter base junction is always forward biased as the emitter resistance is very small. The collector base junction is reverse biased and its resistance is a bit higher. A small forward bias is sufficient at the emitter junction whereas a high reverse bias has to be applied at the collector junction.

The direction of current indicated in the circuits above, also called as the Conventional Current, is the movement of hole current which is opposite to the electron current.

Operation PNP Transistor

The operation of a PNP transistor can be explained by having a look at the following figure, in which emitter-base junction is forward biased and collector-base junction is reverse biased.

The voltage VEE provides a positive potential at the emitter which repels the holes in the P-type material and these holes cross the emitter-base junction, to reach the base region. There a very low percent of holes recombine with free electrons of N-region. This provides very low current which constitutes the base current IB. The remaining holes cross the collector-base junction, to constitute collector current IC, which is the hole current.

As a hole reaches the collector terminal, an electron from the battery negative terminal fills the space in the collector. This flow slowly increases and the electron minority current flows through the emitter, where each electron entering the positive terminal of VEE, is replaced by a hole by moving towards the emitter junction. This constitutes emitter current IE.

Hence we can understand that − The conduction in a PNP transistor takes place through holes. The collector current is slightly less than the emitter current. The increase or decrease in the emitter current affects the collector current.

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Operation NPN Transistor

The operation of an NPN transistor can be explained by having a look at the following figure, in which emitter-base junction is forward biased and collector-base junction is reverse biased.

The voltage VEE provides a negative potential at the emitter which repels the electrons in the N-type material and these electrons cross the emitter-base junction, to reach the base region. There a very low percent of electrons recombine with free holes of P-region. This provides very low current which constitutes the base current IB. The remaining holes cross the collector-base junction, to constitute the collector current IC.

As an electron reaches out of the collector terminal, and enters the positive terminal of the battery, an electron from the negative terminal of the battery VEE enters the emitter region. This flow slowly increases and the electron current flows through the transistor.

Hence we can understand that − The conduction in a NPN transistor takes place through electrons. The collector current is higher than the emitter current. The increase or decrease in the emitter current affects the collector current.

Advantages

There are many advantages of a transistor such as –

High voltage gain. Lower supply voltage is sufficient. Most suitable for low power applications. Smaller and lighter in weight. Mechanically stronger than vacuum tubes. No external heating required like vacuum tubes. Very suitable to integrate with resistors and diodes to produce ICs.

There are few disadvantages such as they cannot be used for high power applications due to lower power dissipation. They have lower input impedance and they are temperature dependent.

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Common Base Configuration

In common base configuration, emitter is the input terminal, collector is the output terminal and base terminal is connected as a common terminal for both input and output. That means the emitter terminal and common base terminal are known as input terminals whereas the collector terminal and common base terminal are known as output terminals.

In common base configuration, the base terminal is grounded so the common base configuration is also known as grounded base configuration. Sometimes common base configuration is referred to as common base amplifier, CB amplifier, or CB configuration.

The input signal is applied between the emitter and base terminals while the corresponding output signal is taken across the collector and base terminals. Thus the base terminal of a transistor is common for both input and output terminals and hence it is named as common base configuration.The supply voltage   between base and emitter is denoted by VBE while the supply voltage between collector and base is denoted by VCB.As mentioned earlier, in every configuration, the base-emitter junction JE is always forward biased and collector-base junction JC is always reverse biased. Therefore, in common base configuration, the base-emitter junction JE is forward biased and collector-base junction JC is reverse biased.The common base configuration for both NPN   and PNP transistors is shown in the below figure.

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From the above circuit diagrams of npn and pnp transistors, it can be seen that for both npn and pnp transistors, the input is applied to the emitter and the output is taken from the collector. The common terminal for both the circuits is the base.Current flow in common base amplifierFor the sake of understanding, let us consider NPN transistor in common base configuration.The npn transistor is formed when a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers.

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The base-emitter junction JE is forward biased by the supply voltage VBE while the collector-base junction JC is reverse biased by the supply voltage VCB.Due to the forward bias voltage VBE, the free electrons (majority carriers) in the emitter region experience a repulsive force from the negative terminal of the battery similarly holes   (majority carriers) in the base region experience a repulsive force from the positive terminal of the battery. As a result, free electrons start flowing from emitter to base similarly holes start flowing from base to emitter. Thus free electrons which are flowing from emitter to base and holes which are flowing from base to emitter conducts electric current. The actual current is carried by free electrons which are flowing from emitter to base. However, we follow the conventional current direction which is from base to emitter. Thus electric current is produced at the base and emitter region.

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The free electrons which are flowing from emitter to base will combine with the holes in the base region similarly the holes which are flowing from base to emitter will combine with the electrons in the emitter region.  From the above figure, it is seen that the width of the base region is very thin. Therefore, only a small percentage of free electrons from emitter region will combine with the holes in the base region and the remaining large number of free electrons cross the base region and enters into the collector region. A large number of free electrons which entered into the collector region will experience an attractive force from the positive terminal of the battery. Therefore, the free electrons in the collector region will flow towards the positive terminal of the battery. Thus, electric current is produced in the collector region.The electric current produced at the collector region is primarily due to the free electrons from the emitter region similarly the electric current produced at the base region is also primarily due to the free electrons from emitter region. Therefore, the emitter current is greater than the base current and collector current. The emitter current is the sum of base current and collector current.IE = IB + IC

We know that emitter current is the input current and collector current is the output current.The output collector current is less than the input emitter current, so the current gain of this amplifier is actually less than 1. In other words, the common base amplifier attenuates the electric current rather than amplifying it.The base-emitter junction JE at input side acts as a forward biased diode. So the common base amplifier has a low input impedance (low opposition to incoming current). On the other hand,

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the collector-base junction JC at output side acts somewhat like a reverse biased diode. So the common base amplifier has high output impedance.Therefore, the common base amplifier provides a low input impedance and high output impedance.Transistors   with low input impedance and high output impedance provide a high voltage gain. Even though the voltage gain is high, the current gain is very low and the overall power gain of the common base amplifier is low as compared to the other transistor amplifier configurations.The common base transistor amplifiers are primarily used in the applications where low input impedance is required.The common base amplifier is mainly used as a voltage amplifier or current buffer.This type of transistor arrangement is not very common and is not as widely used as the other two transistor configurations.The working principle of pnp transistor with CB configuration is same as the npn transistor with CB configuration. The only difference is in npn transistor free electrons conduct most of the current whereas in pnp transistor the holes conduct most of the current.To fully describe the behavior of a transistor with CB configuration, we need two set of characteristics: they are

Input characteristics  Output characteristics.

Input characteristicsThe input characteristics describe the relationship between input current (IE) and the input voltage (VBE).First, draw a vertical line and horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The input current or emitter current (IE) is taken along the y-axis (vertical line) and the input voltage (VBE) is taken along the x-axis (horizontal line).To determine the input characteristics, the output voltage VCB (collector-base voltage) is kept constant at zero volts and the input voltage VBE is increased from zero volts to different voltage levels. For each voltage level of the input voltage (VBE), the input current (IE) is recorded on a paper or in any other form.A curve is then drawn between input current IE and input voltage VBE at constant output voltage VCB (0 volts).

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Next, the output voltage (VCB) is increased from zero volts to a certain voltage level (8 volts) and kept constant at 8 volts. While increasing the output voltage (VCB), the input voltage (VBE) is kept constant at zero volts. After we kept the output voltage (VCB) constant at 8 volts, the input voltage VBE is increased from zero volts to different voltage levels. For each voltage level of the input voltage (VBE), the input current (IE) is recorded on a paper or in any other form.A curve is then drawn between input current IE and input voltage VBE at constant output voltage VCB (8 volts).This is repeated for higher fixed values of the output voltage (VCB).When output voltage (VCB) is at zero volts and emitter-base junction JE is forward biased by the input voltage (VBE), the emitter-base junction acts like a normal p-n junction diode. So the input characteristics are same as the forward characteristics of a normal pn junction diode.The cut in voltage of a silicon transistor is 0.7 volts and germanium transistor is 0.3 volts. In our case, it is a silicon transistor. So from the above graph, we can see that after 0.7 volts, a small increase in input voltage (VBE) will rapidly increase the input current (IE).When the output voltage (VCB) is increased from zero volts to a certain voltage level (8 volts), the emitter current flow will be increased which in turn reduces the depletion region width at emitter-base junction. As a result, the cut in voltage will be reduced. Therefore, the curves shifted towards the left side for higher values of output voltage VCB.Output characteristicsThe output characteristics describe the relationship between output current (IC) and the output voltage (VCB).First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The output current or collector current (IC) is taken along the y-axis (vertical line) and the output voltage (VCB) is taken along the x-axis (horizontal line).

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To determine the output characteristics, the input current or emitter current IE is kept constant at zero mA and the output voltage VCB is increased from zero volts to different voltage levels. For each voltage level of the output voltage VCB, the output current (IC) is recorded.A curve is then drawn between output current IC and output voltage VCB at constant input current IE (0 mA). When the emitter current or input current IE is equal to 0 mA, the transistor operates in the cut-off region.

Next, the input current (IE) is increased from 0 mA to 1 mA by adjusting the input voltage VBE and the input current IE is kept constant at 1 mA. While increasing the input current IE, the output voltage VCB is kept constant.After we kept the input current (IE) constant at 1 mA, the output voltage (VCB) is increased from zero volts to different voltage levels. For each voltage level of the output voltage (VCB), the output current (IC) is recorded.A curve is then drawn between output current IC and output voltage VCB at constant input current IE (1 mA). This region is known as the active region of a transistor.This is repeated for higher fixed values of input current IE (I.e. 2 mA, 3 mA, 4 mA and so on).From the above characteristics, we can see that for a constant input current IE, when the output voltage VCB is increased, the output current IC remains constant. At saturation region, both emitter-base junction JE and collector-base junction JCare forward biased. From the above graph, we can see that a sudden increase in the collector current when the output voltage VCB makes the collector-base junction JC forward biased.

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