From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
Date: 20 Nov 2002 13:23:37 -0800
References: <email@example.com> <firstname.lastname@example.org> <3DKC9.467$Lq.email@example.com>
NNTP-Posting-Date: 20 Nov 2002 21:23:37 GMT
"Kevin Aylward" wrote in message news:<3DKC9.467$Lq.firstname.lastname@example.org>...
> Paul Burridge wrote:
> > On 19 Nov 2002 15:35:20 -0800, email@example.com (N. Thornton) opined
> > thusly:
> >> 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.
I thihk this is where the confusion comes in. A class B output device
does not need any temp comp: when required to be off it is well and
truly off, the base is at 0v, so there is nothing to compensate.
BUT class B amps are more often run with some Vbias on the op trs
simply to reduce the magnitude of the crossover error. No bias gives
1.2v of crossover, whereas putting say 0.3v on your op trs will result
in half the distortion.
What has to be compensated is not the class B output stage itself, but
the drive to the class B output, to ensure it stays in class B.
If you look at an elemental Class B circuit, there is no bias and
nothing to temp compensate.
> *Pure* class B dose not require temperature compensation. Temperature
> compensation is to compensate the *bias* current. If there is no bias
> current, there is nothing to compensate. The reason for this
> misunderstanding could be that is usually to say class B as shorthand,
> when in fact one rarely uses class B, it is always class AB, i.e. there
> is at least some standing current.
Yep, there's that too. This is why I have been asking if this is a B
or an AB amp - I still dont actually know. If its B you want, you can
probably throw out some of your bias components. But it does depend on
how you're driving it.
You still havent given us the circuit btw. Then praps we could say
something practical more than theoretical.