Amplifier

Generally, an amplifier is any device that will use a small amount of energy to control a larger amount of energy. In popular use, the term today usually refers to an electronic amplifier, often as in audio applications. The relationship of the input to the output of an amplifier - usually expressed as a function of the input frequency - is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

There are many types of electronic amplifiers for different applications.

One common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Its critical components are active devices, such as vacuum tubes or transistors.