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Physics Project Report on Transistor


Junction diodes are formed by placing a p-type crystal in contact with p-type crystal and subjecting to high pressure so that it becomes a single piece. The assembly so obtained is called junction diode or crystal diode or p-n junction. The surface of contact of p and n-type crystal is called junction. During the formation of a junction diode, holes from p-region diffuses into n-region and electron from n-region diffuses into p-region. In both cases when an electron meets a hole, they cancel the effect of each other and as a result a thin layer at the junction becomes free from any of the charge carriers. This is called depletion layer. The thickness of depletion layer is of the order of 10-6 m.


A junction diode which is used for amplification is know as transistor.

There are three section of transistor.

i) Emitter (F)

ii) Base (B)

iii) Collector (c)

The base (B) of a transistor is made thin and it is doped lightly. The emitter (E) supplies the majority carriers for current flow and collector (C) collect them.

When a transistor is used in a circuit the base-emitter junction is always forward biased and base-collector junction is reverse biased.

Types of Transistor

Transistors are of two types

i) p-n-p transistor

ii) n-p-n transistor

i) p-n-p transistor is formed when n-section is switched between two p-section.

ii) n-p-n transistor is formed when p-section is switched between two n-section.

Action of Transistor

Action of npn transistor : Fig. shows the proper biasing of an npn transistor. The n-type emitter is forward biased by connecting it to negative pole of battery Vee and n-type collector is reverse biased by connecting it to positive pole of the battery Vcc.

The majority carriers i.e. electrons in the emitter are repelled towards the base due the forward bias. The base contains a small no. of holes, due to this electron hole combination in base region is very small. Most of electrons (95%) swept by collector and enters the positive pole of collector base battery Vcc, an electron enter into emitter from negative pole of the emitter-base battery Vcc Thus current is carried inside the transistor as well as external circuit by electrons.

If Ie, Ib and Ic are respectively the emitter current, base current and collector current then

Ie = Ib + Ic

Action of pnp transistor : The action of both the types of transistors i.e. npn and pnp is similar except that the majority and minority carriers in the two cases are of opposite nature.

Transistor as an Amplifier

An amplifier is a device which is used for increasing the amplitude of variation of alternating voltage or current or power. The amplifier thus produces an enlarged version of input signal.

These are two input terminals for the signal to be amplified and two output terminal for connecting the load and a mean of supplying power to the amplifier.

Types of Amplifier :

i) Common base amplifier

ii) Common emitter amplifier

Common Base Amplifier

In common base amplifier, base of the transistor is common to emitter and collector.

a) Amplifier circuit using n-p-n transistor - 1

The common base amplifier circuit using npn transistor. The base is made common to both input and the output circuits.

The emitter is forward biased by using emitter base battery Vce and due to this resistance of input circuit is small. The collector is reverse biased by using collector base battery Vce and as a result, the resistance of output is quite large. The input voltage is applied across emitter base circuit and amplified output is obtained across the collector and base.

If Ie, Ib, Ic be emitter, base and collector current then

Ie = Ib + Ic According to Kirchhaffis law

If Ic current in collector circuit, Ic RL potential drop then.

Vcb = Vcc - ICRL

b) Amplifier circuit using pnp transistor Fig. shows the common base amplifier circuit using pnp circuit. The basic theory of this circuit is same as in npn transistor. Output voltage obtained across collector is in phase with input voltage.

The half cycle of the input voltage increase forward bias of the emitter. This will increase emitter current and collector current. Increase collector current and collector current. Increase collector current will increase the potential drop across RL hence collector voltage decrease. Decrease means that it become less negative i.e. positive output signal voltage will be produced. Similarly during negative half cycle negative output signal voltage will produced.

Various Gains in Common Base Amplifier

Current Gain :-

It is the ratio of output current to the input current

Let Ic  = Output current

       Ie = Input current

Current gain = = = a

VOLTAGE GAIN :- It is the ratio of output voltage to the input voltage.

       Let Vc = Output voltage

             Ve = Input voltage

Voltage gain = = = b


It is the ratio of the output power to the input power.

Power gain =


                 = Current gain x Voltage gain

                 = a x b

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