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.

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.

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 V_cc, an electron enter into emitter from negative pole of the emitter-base battery V_cc Thus current is carried inside the transistor as well as external circuit by electrons.

If I_e, I_b and I_c are respectively the emitter current, base current and collector current then

I_e = I_b + I_c

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.

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.

i) Common base amplifier

ii) Common emitter 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 V_ce and due to this resistance of input circuit is small. The collector is reverse biased by using collector base battery V_ce 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 I_e, I_b, I_c be emitter, base and collector current then

I_e = I_b + I_c According to Kirchhaffis law

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

V_cb = V_cc - 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.

It is the ratio of output current to the input current

Let Ic  = Output current

       Ie = Input current

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

       Let V_c = Output voltage

             V_e = Input voltage

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

Power gain =

                 = 

                 = Current gain x Voltage gain