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. Amplifiers may be classified by the input (source) they are designed to amplify (such as a guitar amplifier to perform with an electric guitar), or named for the device they are intended to drive (such as a headphone amplifier), or by the frequency range of the signals (Audio, IF, RF and VHF amplifiers for example), or grouped by whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or by the types of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).