An electronic device or circuit that converts a small amount of energy into a larger amount by increasing the current, power and voltage of an ac signal source is called amplifier. Three basic properties of Amplifiers are Input impedance, Output impedance and Gain. The inside of an amplifier contains a mass of wires and circuitry components.

The core component of an amplifier is transistor and the main elements in a transistor are semiconductors.

Most amplifiers can be classified based on their function and frequency response. Voltage and Power amplifiers are based on the function and they amplify voltage and power respectively. Amplifiers based on frequency response are audio, rf and video amplifiers. Amplifiers find applications in wireless communications, in audio equipment of all kinds, broadcasting etc. Different types of amplifiers are used widely in electronic equipments as radio and television receivers, computers, high-fidelity audio equipment etc.