An amplifier is a circuit that increases the amplitude of an input signal. The frequency remains the same at the output. They form an essential part of any communication circuit and thus find wide application in radio, television circuits.

Amplifiers can be classified as follows:

According to its input,

According to the transistor configuration

According to number of stages

According to bias conditions

According to frequency response

These amplifiers have a single amplifying device, i.e., a transistor or FET in CE, CB, CC or CS or CD configuration respectively.