Audio Amplifi er Performance 319pulses and fi lters harmonics. Current-dumping amplifi ers can be regarded as combining
Class-A (the correcting amplifi er) with Class-C (the current-dumping devices); however,
it is hard to visualize how an audio amplifi er using devices in Class-C only could be built.
10.4.5 Class-D
These amplifi ers continuously switch the output from one rail to the other at a supersonic
frequency, controlling the mark/space ratio to give an average representing the
instantaneous level of the audio signal; this is alternatively called pulse width modulation.
Great effort and ingenuity have been devoted to this approach, for the effi ciency is, in
theory, very high, but the practical diffi culties are severe, especially so in a world of
tightening EMC legislation, where it is not at all clear that a 200-kHz high-power square
wave is a good place to start. Distortion is not inherently low^5 and the amount of global
NFB that can be applied is severely limited by the pole due to the effective sampling
frequency in the forward path. A sharp cutoff low-pass fi lter is needed between amplifi er
and speaker to remove most of the RF; this will require at least four inductors (for stereo)
and will cost money, but its worst feature is that it will only give a fl at frequency response
into one specifi c load impedance. The technique now has a whole chapter of this book to
itself. Other references to consult for further information are Goldberg and Sandler^6 and
Hancock.^7
10.4.6 Class-E
An extremely ingenious way to operate a transistor is to have either a small voltage across
it or a small current through it almost all the time; in other words, the power dissipation is
kept very low. 8 Regrettably, this is an RF technique that seems to have no sane application
to audio.
10.4.7 Class-F
There is no Class-F, as far as I know. This seems like a gap that needs fi lling.
10.4.8 Class-G
This concept was introduced by Hitachi in 1976 with the aim of reducing amplifi er power
dissipation. Musical signals have a high peak/mean ratio, spending most of this at low
levels, so internal dissipation is much reduced by running from low-voltage rails for small
outputs, switching to higher rails current for larger excursions.