Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The "signal" is usually voltage or current. 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 refers to an electronic amplifier, often as in audio applications to operate a loudspeaker that is being used in a PA system to make the human voice louder or play recorded music; or in a guitar amplifier to perform with an electric guitar. 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. Figures of merit

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