An amplifier is a device that uses a small amount of energy to control a larger amount. The relationship of the input to the output--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. Amplifiers are typically utilized over a specific range of frequencies and are generally of maximal utility if the gain is constant in that range.
See also: low noise amplifier.

The most 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.

Another type of amplifier is the fluidic amplifier , based on the fluidic triode. There are also mechanical amplifiers, such as the automotive servo used in braking.

The critical component of an electronic amplifier is an active device, such as a vacuum tube or transistor, typically a bipolar junction transistor (BJT) but occasionally a metal oxide semiconductor field-effect transistor (MOSFET). The essential role of the BJT is to dramatically magnify an alternating current (AC) base current to yield significantly larger AC collector current and emitter current . The amount of magnification (the "AC forward gain") is determined by interconnecting the BJT within a bias circuit that establishes a fixed ("quiescent") operating point, that is, a specific set of direct current (DC) values for the base, collector, and emitter currents. While the AC input signal to the amplifier (say, the tiny sound evidenced when a mechanical needle rides in the groove of a vinyl LP record) is typically the transistor base current, the AC output signal (say, used to drive a pair of eight-ohm stereo loudspeakers) of the amplifier could be a collector or emitter voltage (with respect to ground) or even some other value that varies with the collector current in a rigidly predeterminate manner.

See also valve sound (tube sound) car tube amp.