Amplifier is an electronic device. The device which increases the value of a signal is called amplifier. In input a weak signal is applied, in output a strong, same shape signal is gained. Amplify means to increase the value of something.
Transistor is used in amplifier. It can amplify a signal many times. In two ways a transistor can amplify a signal
(i) by using high value of output resistance compared to the input resistance,
(ii) by controlling the collector current with the help of base current.
The collector is reverse biased and the emitter is forward biased while using transistor as an amplifier. Each electronic circuit has input and output. Every input has two terminals and output has two terminals also. But a transistor has three terminals. One terminal remains common in input and output in transistor circuit. In this way a transistor has three different configurations in electronic circuit.
The configurations are
(i) Common emitter(CE),
(ii) Common base(CB),
(iii) Common collector(CC).
Common emitter configuration has a large amplification. So, it is used mostly.
Common emitter amplifier:
Circuit diagram shows common emitter amplifier for n-p-n and p-n-p transistor. In circuit input signal is applied between emitter and base and the output signal taken between emitter and collector. So, we can say emitter is common for input and output both. Hence it is called common emitter amplifier. In input circuit a battery (VBB) is used as addition with the A.C. signal. The D.C. voltage of (VBB) is called bias voltage. Its value is such that during negative half cycle of the applied a.c. Signal the emitter base junction remains forward biased. If the D.C. voltage is not applied the emitter-base junction will be reverse biased and we cannot obtain any current in the output circuit. Consequently the amplifier will lose faithfulness. For the application of (VBB) in input IC current flows in the output.
Working principle of amplifier
Emitter-base junction becomes forward biased for the positive half cycle of the input signal applied. For this more electrons flow from the emitter through the base to the collector and hence the collector current increases. The increasing of this collector current IC makes more voltage drop across to the load resistance (RL). The emitter–base junction becomes reverse biased during the negative half cycle input signal. In this situation forward bias decreases. So the collector current also decreases the output signal will be gained small in the reverse direction. In this way we get output signal in transistor circuit which is similarly to input signal. An amplified output signal is shown variation of current with time.
Current amplification factor
In common emitter mode the ration of change in collector current to the change in base current is called current amplification factor or current gain. It is denoted by β. In common emitter amplifier input current IB and the output current IC. Mathematical representation of current amplification factor is
Here β is always greater than 1. Different values transistors have in market depending on the value of β. Available ranges about 30 to 900. In the case of common base configuration the ratio of the change in collector current to the change in emitter current is called current amplification factor or current gain. It is denoted by α.
Relation between α and β
We know that from transistor equation,
IE = IB + IC
This is the relation between α and β.