Class-D amplifier
A simple low-pass filter may be used to attenuate their high-frequency content to provide analog output current and voltage.
The Sinclair X-20 in 1966 produced 20 watts but suffered from the inconsistencies and limitations of the germanium-based bipolar junction transistors available at the time.
Practical class-D amplifiers were enabled by the development of silicon-based MOSFET (metal–oxide–semiconductor field-effect transistor) technology.
[2] Class-D amplifiers work by generating a train of rectangular pulses of fixed amplitude but varying width and separation.
A simple low-pass filter consisting of an inductor and a capacitor provides a path for the low frequencies of the audio signal, leaving the high-frequency pulses behind.
This is because an ideal switch in its on state would encounter no resistance and conduct all the current with no voltage drop across it, hence no power would be dissipated as heat.
Real-world power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common for class-D amplifiers.
By contrast, linear AB-class amplifiers are always operated with both current flowing through and voltage standing across the power devices.
The output filter removes the high-frequency switching components of the PWM signal and reconstructs audio information that the speaker can use.
In either case, the time resolution afforded by practical clock frequencies is only a few hundredths of a switching period, which is not enough to ensure low noise.
Two significant design challenges for MOSFET driver circuits in class-D amplifiers are keeping dead times and linear mode operation as short as possible.
[6] With fixed-frequency PWM modulation, as the (peak) output voltage approaches either of the supply rails, the pulse width can get so narrow as to challenge the ability of the driver circuit and the MOSFET to respond.
Class-D amplifiers place an additional requirement on their power supply, namely that it be able to sink energy returning from the load.
The need to feed the actual output signal back into the modulator makes the direct generation of PWM from a SPDIF source unattractive.
Note: For clarity, signal periods are not shown to scale.
