Introduction
Class-D is a switching power amplifier architecture that uses high-frequency Pulse-Width Modulation (PWM) to generate the output waveform. The transistors are fully on or fully off, which means this architecture can reach very high efficiency levels, resulting in significant power savings. However, the translation from the input signal to PWM and the PWM itself can cause more distortion on the output than other amplifier architectures. Figure 1 shows a very basic Class-D signal diagram.

Because the Class-D amplifier transistors are fully on or fully off, the transistors lose less energy in the form of heat. This architecture, therefore, lends itself to small, cheap MOSFETs. The Class-D amplifier can be implemented in either analog or digital form.
An analog Class-D amplifier is non-trivial and typically consists of a comparator, triangle waveform generator, and several blocks to condition the input signal before transferring to the output MOSFETs.