Because the devices spend very little time in a partially on state, they present very little resistance and, subsequently, consume very little energy. The output devices switch all the way on and all the way off very quickly. The Class D driver analyzes the audio signal at extremely high speeds (some as fast as 800kHz) and creates a relatively low-voltage PWM signal that feeds the output devices. Using a square wave signal with different duty cycles is called pulse width modulation (or PWM for short).

When the on-vs.-off time is equal, that’s a 50% duty cycle.

This variation of on-versus-off time is called duty cycle. However, if we turn the switch on and off very quickly, and the switch is on for as long as it’s off, the bulb will glow at half of its possible brightness. If we turn the switch on, the light illuminates to full brightness. If we leave the switch off, the light stays off. We hook the bulb up to a power source and insert a computer-controlled switch in series with the circuit. Imagine, if you will, an incandescent lightbulb. In a modern Class D amp, the MOSFET output devices are fed by a driver IC that handles the conversion of the analog signal into a pulse width modulated signal. Once the input stage has handled any signal processing needs, the audio signal is passed to the output stage so that it increases in voltage and the MOSFETs can provide adequate amounts of current to drive our relatively low impedance speakers. Do their drawbacks outweigh their benefits? Let’s have a look! How Does a Class D Output Stage Work? In audiophile realms, Class D designs often carry an unfavorable reputation. As we continue our look at how at how car audio amplifiers work, we need to discuss what has become the most popular design on the market today: amplifiers that use a Class D output stage.

Class d stereo amplifier