From: Paul Burridge
Newsgroups: sci.electronics.misc,sci.electronics.design
Subject: Help! Diode biasing of Class B amplifiers
Distribution: World
Reply-To: pb@notthisbit.osiris1.co.uk
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Date: Sat, 05 Oct 2002 01:13:02 +0100
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Hi,
I'm having serious difficulties getting my head around this
conundrum.
I've seen several circuit diagrams illustrating the use of
compensating diodes to establish the correct operating point for the
output transistors; just bringing them to the verge of conduction to
minimize x-over distortion. All fine, except that I can't see how the
input signal is supposed to ignore the 'one-way-only' property of
these diodes. For instance, across the +20v and -20v rails of the twin
supply, there is shown, in series, a 4k resistor, two diodes and
another 4k resistor. Both diodes point the same way with arrows
towards the neg. rail and are thus forward biased. The input is shown
as being applied in the middle between them. The bases of the
complimentary output pair are connected to the point between each
diode and its resistor.
A positive-going part of the input cycle can only reach the base of
the lower transistor, which is a PNP and therefore 'expects' a
negative signal to function; whereas a negative-going input signal
portion can only reach the top transistor (an NPN) which 'expects' a
positive signal to its base. I was tempted to think the publishers had
mixed up their polarities, but this circuit is so prevalent in so many
different books both US and UK that seems impossible. Any ideas,
anyone?
--
Noverint universi per presentes et futuri...