explain transistor as an amplifier and a switch with derivation?

Transistor as an Amplifier

An amplifier is an electronic device to increase the strength of a weak signal.

The voltage gain of amplifier is the ratio of output voltage to input voltage

Voltage gain upto 1000 have been obtained in transistors.

As a transistor has three terminals (emitter, base and collector). One of three leads is kept common to both the input and output circuits. Accordingly, we have three basic transistor circuits in which either the base or the emitter or the collector lead is kept common. They are termed as common-base, common-emitter and common-collector amplifier. Here we shall discuss only common emitter transistor amplifier.

Common-Emitter Transistor Amplifier

Common-emitter transistor amplifier gives the highest gain and hence it is the most commonly employed circuit. Fig. depicts the circuit for a transistor. In this circuit, the emitter lead is common to both the input (emitter-base) and output (collector-emitter) circuits and is grounded. The emitter-base circuit is forward biased and the base-collector circuit is reverse biased.

In a common-emitter circuit, the collector-current is controlled by the base-current rather than the emitter-current. Since in a transistor, a large collector-current corresponds to a very small base-current, therefore, when input signal is applied to base, a very small change in base-current provides a much larger change in collector-current and thus extremely large current gains are possible.

Referring to fig., when positive half cycle is fed to the input circuit, it opposes the forward bias of the circuit which causes the collector current to decrease. It decreases the voltage drop across load  and thus makes collector voltage more negative. Thus when input cycle varies through a positive half cycle, the output voltage developed at the collector varies through a negative half cycle and vice versa. Thus the output voltage in common-emitter amplifier is in antiphase with the input signal or the output and input voltages are 180° out of phase.

Current Gain. The ratio of change in collector current  to the change in base current  is defined as the alternating current gain denoted by 

Voltage Gain. The ratio of change in output voltage to the corresponding change in input voltage is called the voltage gain. It is denoted by  The voltage gain of common-emitter transistor amplifier is given by

which is also higher than that in case of common base amplifier.

For large voltage gain from the current gain  and the ratio of load resistance to internal resistance should be high.

A switch is a device which can turn ON and OFF current in an electrical circuit. A p-n junction diode and a transistor can act as a switch. We are familiar that when a junction diode is forward biased, it conducts current but when junction diode is reverse biased, it does not conduct current. If we treat the junction diode as a switch, then during forward biasing; the switch is ON and during reverse biasing, the switch is OFF.

In a similar manner a transistor can also be used to turn current ON or OFF rapidly in electrical circuits.

Operation : The circuit diagram of n-p-n transistor in CE configuration working as a switch is shown in fig. and are two dc supplies which bias base-emitter and emitter collector junctions respectively.

Let  be the input supply voltage. This is also input dc voltage  The dc output voltage is taken across collector-emitter terminals,  is the load resistance in output circuit.

Applying Kirchhoff’s second law to input and output meshes (1) and (2), we get

Beyond  V, the change in collector current and hence in output voltage  is non-linear and the transistor goes into saturation. With further increase in  the output voltage further decrease towards zero (though it never becomes zero).

If we plot  we get the graph as shown in fig. [This characteristics curve is also called transfer characteristic curve of base biased transistor.]

The curve shows that there are non-linear regions.

(i) between cut off state and active state and (ii) between active state and saturation state; thus showing that the transitions (i) from cut off to active state and from active to saturation state are not sharply defined.

Now we are in the position to explain the action of transistor as a switch. When transistor is non-conducting , it is said to be ‘switched off’ but when it is conducting ( is not zero); it is said to be ‘switched ON’.

As long as input voltage  is low and unable to overcome the barrier voltage of the emitter base junction, is high  so the transistor is ‘switched OFF’ and if it is high enough to derive the transistor into saturation  is low, very near to zero, so the transistor is ‘switched ON’. Thus we can say low input switches the transistor is OFF state and high input switches it ON.

The switching circuits are designed in such a way that the transistor does not remain in active state.


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