Class-D amplifier
Note: For clarity, signal periods are not shown to scale.
A class-D amplifier, or switching amplifier, is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs, increasingly GaN FETs) operate as electronic switches, and not as linear gain devices as in other amplifier classes. Class-D amplifiers operate by rapidly switching back and forth between the power supply rails, using pulse-width modulation, pulse-density modulation, or related techniques to produce a pulse train output. A low-pass filter is typically used to attenuate the pulse train output high-frequency content to reduce quiescent current and/or EMI; resulting in the final analog output current and voltage to be sent to the speaker. Little energy is dissipated in the amplifying transistors because they are always either fully on or fully off, so efficiency can exceed 90%.