From: Paul Burridge
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
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Date: Wed, 20 Nov 2002 09:22:58 +0000
NNTP-Posting-Date: Wed, 20 Nov 2002 10:51:48 GMT
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On 19 Nov 2002 15:35:20 -0800, email@example.com (N. Thornton) opined
>You say you have biasing / temp compensating there, yet its meant to
>be class B: how is that so? For real class B you don't need to apply
>any bias chain, so there should be no need for temp compensation.
You certainly have to provide *drive current* to the bases and this is
the chain through which it's provided. And with all due respect, your
claim that class B doesn't require temp.compensation is at odds with
everything I've thus far read on the subject in a number of books.
>the tr bases are connected directly and driven from an opamp, it
>should work OK, as well as Class B does at least.
These are actually driven from a discrete driver stage of one single
transistor. I'm not using op-amps at all.
Incidentally, I've just noticed that at switch-on time, the collector
of the driver is at -8v instead of the mid-point zero it should be. It
quite quickly rises to zero and slowly stabilizes there, but by that
time the output transistors are very hot indeed. Anyway, it should be
zero volts right from swich-on so this may be something to do with the
>I am wondering if your circuit actually might not be Class B. Could
>you clarify on that for us?
It's certainly *supposed* to be Class B!
[point about germanium curves duly noted]
>I feel I still don't know your particular circuit, nor do I know yet
>why you're using that one in particular.
If my ascii art were better, I'd draw it. It's simply easier to
describe than to draw.
"I was going to help him up, but then I thumped him."
Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975