Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. 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.
In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually voltage or current. In audio applications, amplifiers operate loudspeakers used in PA systems
