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From: firstname.lastname@example.org (Ken Smith)
Subject: Re: Choosing the right transformer
Date: Mon, 20 Jan 2003 01:07:03 +0000 (UTC)
Organization: a2i network
References: <3E2A4186.BF5DE7BC@webaccess.net> <4srW9.27782$jM5.email@example.com>
NNTP-Posting-Date: Mon, 20 Jan 2003 01:07:03 +0000 (UTC)
X-Newsreader: trn 4.0-test76 (Apr 2, 2001)
Originator: firstname.lastname@example.org (Ken Smith)
In article ,
Winfield Hill wrote:
> supply, whether linear or switching, rapidly exceeds that
> of a high-tech modulator and class-D output stage. But the
> alternate choice to use output feedback with an unregulated
> supply can spoil the high-quality class-D output that's
> otherwise possible with no feedback.
I have made a few class-D amplifiers. The trick with feedback is to not
try to convert a sow's ear into a silk purse with it. The product that I
designed last using class-D amplifier has feedback that converts a sow's
ear into a sow's ear with a pretty pink bow in it.
Without feedback, the gain of the class-D amplifier stage depends on the
power supply voltage. With feedback, the gain can be known exactly at the
cost of needing a higher switching frequency or a more complex circuit or
The forward path in the circuit always has 2 poles in it due to the output
filter. You need to add at least one zero to this if you want things
to come out ok. Adding this zero tends to saturate the switching stage at
a lower amplitude, high frequency input signal. For voice and most music
this isn't a big problem because the amplitude decreases as frequency
 the pole that goes with this zero is above the band of interest.
In the audio class-D amplifier I made, I added two zeros and a pole to the
forward path. This allow the circuit to act like a "real servo loop" for
the low frequencies. At high frequencies, the loop gain is very small so
the distortion gets quite bad. Since most of the resulting distortion is
removed by the LC output filter and the human ear, it doesn't sound too
If you want a class-D amplifier to get past the FCC with no problem, you
should consider not having a fixed switching frequency. If the switching
is random looking chaos that just happens to make the right low frequency
content, you can get much lower RF peaks in the radiated noise.
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