Amplifier

               Generally, an Amplifier or simply amp, is a device for increasing the power of a signal by use of an external energy source.

               In popular use, the term usually describes an Electronic Amplifier , in which the input "signal" is usually a voltage or a current. In audio applications,  Amplifier  drive the loudspeakers used in PA systems to make the human voice louder or play recorded music.  Amplifier  may be classified according to the input (source) they are designed to amplify (such as a guitar  Amplifier , to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for example), whether they invert the signal (inverting amplifiers and non-inverting  Amplifiers , or the type of device used in the amplification (valve or tube  Amplifier , FET amplifiers, etc.).

               A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.