[sdiy] Low voltage synthesis?

Mattias Rickardsson mr at analogue.org
Fri Jan 14 23:36:41 CET 2022

Aha, I didn't think it would be called Class-D if it was just shoving out
digital pulses, but maybe it can be either or. :-)

The Wikipedia article on Class-D amps has some interesting passages:

The term "class D" is sometimes misunderstood as meaning a "digital"
amplifier. While some class-D amplifiers may indeed be controlled by
digital circuits or include digital signal processing devices, the power
stage deals with voltage and current as a function of non-quantized time.
The smallest amount of noise, timing uncertainty, voltage ripple or any
other non-ideality immediately results in an irreversible change of the
output signal. The same errors in a digital system will only lead to
incorrect results when they become so large that a signal representing a
digit is distorted beyond recognition. Up to that point, non-idealities
have no impact on the transmitted signal. Generally, digital signals are
quantized in both amplitude and wavelength, while analog signals are
quantized in one (e.g. PWM) or (usually) neither quantity.


Error control
The actual output of the amplifier is not just dependent on the content of
the modulated PWM signal. The power supply voltage directly
amplitude-modulates the output voltage, dead time errors make the output
impedance non-linear and the output filter has a strongly load-dependent
frequency response. An effective way to combat errors, regardless of their
source, is negative feedback. A feedback loop including the output stage
can be made using a simple integrator. To include the output filter, a PID
controller is used, sometimes with additional integrating terms. The need
to feed the actual output signal back into the modulator makes the direct
generation of PWM from a SPDIF source unattractive. Mitigating the same
issues in an amplifier without feedback requires addressing each separately
at the source. Power supply modulation can be partially canceled by
measuring the supply voltage to adjust signal gain before calculating the
PWM and distortion can be reduced by switching faster. The output impedance
cannot be controlled other than through feedback."



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