Amplifier is an electronic circuit which produces a large electrical output of similar characteristic that of the input parameters. Amplifier is an electronic device which increases the power of a signal. Amplifier takes the power from a power supply and control the output to match the input signal shape with larger amplitude. The amplifier increases the magnitude of the voltage or current input by means of an energy dawn from an external source. Amplifier in which the output signal is directly proportional to the corresponding input signal is called as linear amplifier and amplifier in which the output is not directly proportional to the input is called as non linear amplifier.
The block diagram of amplifier is shown in above figure. The source VS correspond to the input A.C signal to be amplified and it offers the input resistance RI. The amplifier has the voltage gain AV. At load resistance RL the amplified signal is appears.