From: Watson 'Atto Parsec' Name
Subject: Re: Help! Diode biasing of Class B amplifiers
X-Newsreader: MicroPlanet Gravity v2.60
Date: Sat, 5 Oct 2002 11:32:54 -0700
NNTP-Posting-Date: Sat, 05 Oct 2002 11:24:39 PDT
Organization: InReach Internet
In article ,
firstname.lastname@example.org talked about...
> I'm having serious difficulties getting my head around this
> 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,
You would be right if there were no 4k pullup and pulldown resistors.
These furnish current so that if the diode is not conducting, the base is
pulled down or up by the resistor.
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you put NOSPAM in the
Subject: line. alondra101 hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html