5.5 Summary

The class D amplifier boasts very high efficiency, theoretically approaching 100%. The amplifier operates its output devices as switches; they are either fully on or fully off. The power losses are mostly relegated to switching edge losses so it is important to not switch the output devices at too high of a frequency.

The input signal is encoded as a series of pulses, typically via pulse width modulation. The pulse frequency is much higher than the highest input signal frequency, typically by an order of magnitude. The width of the pulses is a function of the amplitude of the input signal. That is, the higher the input signal, the greater the pulse width. The pulses are amplified by applying them to the output devices which then act as switches to alternately connect and disconnect the power supplies to the output terminal. The sequence of much larger amplitude pulses are then fed to a low-pass filter, typically, an 𝐿𝐶 filter, to remove the high frequency components of the pulse train. The reconstituted input signal is what remains, but at a much higher amplitude.

The output can be configured using either two-device half-bridge or four-device fullbridge arrangements. The full-bridge is preferred for higher performance. The input capacitance of the output devices can be relatively high, so care must be taken to ensure that sufficient capacity is available from the driver circuit.