Unit – III

Transistors:

1. Transistor consists of three layers made up of p and n-type semiconductors. The transistor is an arrangement obtained by growing a narrow section of one type of semiconductor crystal between relatively wide sections of other types.
2. When the n-type crystal is grown between relatively wide sections of p-type crystals then the transistor is called PNP transistor.

3. When the p-type crystal is grown between relatively wide sections of n-type crystals then the transistor is called NPN transistor.

4. The sandwiched section is called the base, the section on one side is emitter (emits electrons) and that on the other side is called collector (collects or attracts electrons).
5. Emitter performs the function of the cathode, the collector performs the function of the plate, the base performs the function of a grid of a triode valve.
6. The transistor may be regarded as two p-n junctions attached back to back. Transistor can be used as an amplifier. When using as an amplifier the emitter-base is forward biased and collector-base are reverse biased.

Action of PNP Transistor:

The circuit diagram is as shown.



1. The emitter-base junction is forward biased with the battery VBB and the collector-base junction is reverse biased by the battery VCC.
VCC is larger than VBB.
2. At emitter-base junction due to forward biasing a large number of majority carriers i.e. holes move into base causing emitter current IE.
3. A few holes in the base are neutralized by majority carriers i.e. electrons. The number of electrons are small hence they cause a very small base current IB.
4. The remaining holes are pulled to collector due to higher negative potential due to reverse bias. It causes collector current IC.
By Kirchoff’s junction law IE = IB + IC. But IB is very small, hence IE and IC are almost equal.

Action of NPN Transistor:

The circuit diagram is as shown.

2. The emitter-base junction is forward biased with the battery VBB and the collector-base junction is reverse biased by the battery VCC. VCC is larger than VBB.
3. At emitter-base junction due to forward biasing a large number of majority carriers i.e. electrons move into base causing emitter current IE.
4. A few electrons in the base are neutralized by majority carriers i.e. holes. The number of holes are small hence they cause a very small base current IB .
5. The remaining electrons are pulled to collector due to higher negative potential due to reverse bias. It causes collector current IC.
By Kirchoff’s junction law IE = IB + IC
But IB is very small, hence IE and ICare almost equal.



Study of Transistor Characteristics:

Circuit Diagram:

Transistor

Input Characteristics:
The output voltage VCE is kept constant and P.D. between the base and emitter VBE is gradually increased. The base current IB is measured. The curves obtained by plotting base current on the y-axis and VBE on the x-axis at different values of VCE are called input characteristics of the transistor. The reciprocal of the slope of the curve in the linear region gives input resistance Ri. For CE mode is very small.

Output Characteristics:
The Base current IB is kept constant and the output Voltage between the collector and emitter VCE is gradually increased. The output current Iis measured. The curves obtained by plotting collector current Ion y-axis and VCE on x-axis
at different values of IB are called output characteristics of the transistor.

Transistor



Transfer Characteristics:
The P.D. VCE between the collector and emitter is kept constant and the base current IB is gradually increased. The output current Iis measured. The lines obtained by plotting collector current Ion y-axis and base current IB on x-axis
at different values of IB are called output characteristics of the transistor.

Use of Transistor as Amplifier:

1. The amplifier is an electronic device which converts low input signals into large output signals. The transistor which is similar to a triode can be used as an amplifier.
2. Due to the high resistance of the collector-base circuit, the transistor can be used as a voltage amplifier. Voltage gain up to 1000 is possible.
3. Transistor has only three elements hence three leads. We need four leads, two for input circuit and two for output circuit. One of the leads must be kept common to both input and output circuits, accordingly, we have three types of amplifier circuits.
a. Common base mode ( C-B mode)
b. Common emitter mode ( C-E mode)
c. Common collector mode ( C-C mode)
4. Common emitter amplifiers are widely used because they give satisfactory current amplifications.

Common Emitter Amplifier



1. The circuit diagram shows use of PNP transistor as common emitter amplifier.
2. A load resistance RL is connected between the collector and the emitter. A weak signal which is to be amplified is applied between the base and the emitter along with battery VB so that the base and emitter junction is forward biased and the collector-emitter junction is reverse biased.
3. The output voltage is obtained between the collector and emitter. A small change in base current produces large variation in collector current.

4. Working:
Let IC, IB, IE be the collector current, base current, and emitter current before the signal is applied. Now an A.C. signal VS of amplitude DVS be applied in the base-emitter circuit.

Use of Transistor as a Switch:

1. Let us study the operation of the transistor as a switch by analyzing the behavior of the base-biased transistor in CE configuration as shown.
2. Applying Kirchhoff’s voltage rule to the output sides of this circuit, we get

Thus no current flows through load resistance RL. The entire voltage develops across the transistor. This situation is similar to open switch.
3. Let us see how Vo changes as Vi increases from zero onwards. As long as input Vi is less than 0.7 V, the transistor will be in cutoff state and current Iwill be zero. When Vi becomes greater than 0.7 V the transistor is in actives state with some current Iin the output path and the output Vo decrease as the term ICRC increases. With the increase of Vi, Iincreases almost linearly and so Vo decreases linearly till its value becomes less than about 1.0 V. Beyond this, the change becomes nonlinear and transistor goes into saturation state. With further increase in Vi the output voltage is found to decrease further towards zero though it may never become zero.
4. Thus transistor acts as a switch without any make and break arrangement of the circuit. It is essential of all logic circuits. Transistor switches have the advantage of the high speed of operation and convenience of electronic control.

Transistor Oscillator:

1. The electrical oscillations in an L-C circuit get damped with the passage of time. Necessary energy has to be supplied to the circuit to sustain them. This can be done by a circuit shown in the figure which is known as an oscillator.



2. The oscillator is an electronic device which generates A.C. signal from D.C. source. The A.C. output may be sinusoidal, square or triangular waveform. Oscillators are used in radio T.V. receiver sets, radio T.V. transmitter sets, Radar, cell phones, microwave ovens etc.
3. Oscillator consists of an amplifier and feedback network with frequency determining components.
4. When part of the output from the oscillator is coupled to input amplifier, it is called feedback. When the feedback sample is out of phase with input it is called negative feedback. When the feedback sample is in phase with input it is called positive feedback. For oscillator positive feedback is required.

5. The voltage gain in complete system is given by



6. When Aβ = 1, then Af = ∞. At this condition, the circuit begins to oscillate at that frequency.
The condition Aβ = 1 is called Berkhausen criterion.
7. When the oscillator is switched on, electrical noise of wide range of frequency is generated in the circuit but the Berkhausen criterion is satisfied by particular frequency and oscillator oscillates at that frequency.

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