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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Help! Diode biasing of Class B amplifiers
References: <3D9EF808.B8A6ADC9@webaccess.net> <firstname.lastname@example.org>
Date: Sat, 05 Oct 2002 18:21:19 GMT
NNTP-Posting-Date: Sat, 05 Oct 2002 11:21:19 PDT
Paul Burridge wrote:
> On Sat, 05 Oct 2002 14:32:11 GMT, Chuck Simmons
> opined thusly:
> >You need to look at this differently. Keep in mind that there is a bias
> >current through the diodes.
> Okay. So is the core of the solution that the level of this forward
> bias current is chosen so that even at 'peak opposing-bias input
> voltage swing' there will still be just enough flowing to prevent the
> devices from going into cut-off? The diagrams I've seen will produce
> only about 5mA of bias current under quiescent conditions; it doesn't
> seem like enough to prevent cut-off at driver voltage output levels?
> [confusing explanation snipped]
> >Sorry, that is really confusing.
> I'll say! :-) Any clarification from yourself or anyone reading this
> would be welcome.
Think of it as current steering. The bias current is "steered" by the
driving voltage into either base. The base current will really be
determined by the load current. Still, what you are looking at is two
> >For that type of circuit to work well, there is usually a boost voltage
> >for the diode bias.
> You mean a decent *current* source, surely? Something that can cope
> with extremes of input signal swing in a way that resistors can't
> without wasting too much current at other points in the cycle?
You could use current mirrors in place of resistors. It is not uncommon
in some integrated circuit amplifiers. Current mirrors allow getting
closer to the rails than resistors do. With resistors, near the rails
the transistors become current starved.
> In AC applications, the output can be rectified and
> >filtered to produce the needed boost. Drat! Even more confusing.
> You mean "bootstrapping"?
Yes, it is sometimes called that. The technique is also used for
quasicomplementary circuits where the up drive needs boost but the down
drive is strong. Examples would be using two N-channel enhancement mode
MOSFETs for a power output stage. The up driver needs a voltage higher
than the positive rail to be "on" when the output is near the rail.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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