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From: Paul Burridge
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
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Date: Tue, 19 Nov 2002 09:15:03 +0000
NNTP-Posting-Date: Tue, 19 Nov 2002 10:43:50 GMT
Organization: Virgin Net Usenet Service
On Thu, 14 Nov 2002 20:26:46 +0000, John Woodgate
>I read in sci.electronics.design that Paul Burridge
> wrote (in firstname.lastname@example.org>) about 'Class B amplifiers: what are the large low-value
>resistors for?', on Thu, 14 Nov 2002:
>>On Thu, 14 Nov 2002 17:34:21 +1100, "Phil Allison"
>>> ** They *are* necessary, with no emitter resistors the amp would
>>>probably self destruct when it got hot by a process called "thermal
>>>runaway" - kind of a vicious cycle that overheats the silicon.
>>But I'm getting thermal runaway anyway even with 2 * one ohm resistors
>Can you give more details of what you are doing? What output devices?
>What supply voltage(s)? What load impedance? Where did you get the
>schematic? What are you using for a heat sink?
This isn't an attempt to build a hi-fi amp at all. The circuit is
published in a book called the Art of Electronics by some guy from
Harvard. It uses three compensating diodes to separate the transistor
bases by 3 diode drops, although I found I had to take a couple out or
there was far too much q. current. The thing actually worked best of
all when *all* the diodes were shorted, but then of course there was
no check at all on temperature increase in the output devices. I have
to say that for the sake of expediency, the output pair were *not*
matched (75W and 90W) nor was it possible to select diodes with the
appropriate characteristic curves to match those of the output
trannies. The load was a 4ohm speaker. There was also a large
bootstrap cap from o/put back to the middle of a split resistance that
formed the driving stage's collector resistance. Supply was 30 - 0
-30v. and the heat sink was just a sheet of alloy.
"I was going to help him up, but then I thumped him."
Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975
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