> From: Stuart Allman <studio@HALCYON.COM>
>
> I am working on an amplifier design, and I was thinking about using a
> class D,G,or H design.  Could someone please give me a hint on how to
> implement the pulse width modulation that is necessary for these
> topologies?  I was thinking that I could just use a sample and hold
> device off the output and compare that value with the current input at set
> intervals.  Is this how PWM works, or is it more complicated?
>
> Just so I'm not chastised for asking this question...there is a PIC
> involved.

For the benefit of us all, perhaps you could give a brief exposition of
what classes D, G and H are (my education stopped at A, B, C).  Anyhow,
PWM as applied to audio is a digital-to-analogue technique.  If you
have an output which can only take one of 2 values (say 0 and +5V) then
you can create a low bandwidth DAC by switching between the 2 output
values at a frequency much greater than the audio frequency of
interest.  The analogue output range is precisely 0-5V.  To obtain a
value of, say, 1V then the digital output should be low 4/5 of the time
and high 1/5 of the time.  The digital output should be switched at a
rate of, say, 44KHz and fed into a 20KHz low pass filter (the
brick-wall type used in CD players etc.)

There's no need for a SH or comparator if you are talking about PWM
pure and simple.  However, since you want to convert an analogue input
into a PWM output yes you do need a comparator and a sawtooth
generator.  The SH would be better replaced by an input bandwidth
limiter.  A PWM output is obtained from the comparator by feeding the
sawtooth into the inverting input and the analogue signal into the
other.

As a matter of interest, the PIC16C73/4 contains a 10-bit resolution
PWM output.  Supposing you run it at top speed, 20MHz, then over 2**10
= 1024 clock cycles, you can adjust the output duty cycle over every
fraction (0..1024)/1024.  The output frequency will be 20MHz/1024 =
19.53125KHz.  You can trade off output precision vs output frequency
e.g. 8-bit precision at 78.125KHz.

Not exactly CD quality, but not too bad for telecom applications.  I
have successfully made a PIC-powered tone generator by feeding the PIC
PWM output directly into a 1K:8ohm transformer with the 8ohm side
connected to a mini speaker.  It's quite loud, and has the advantage of
zero power usage when off, unlike most amps which have a substantial
quiescent current.

Regards,
SJH
Canberra, Australia