From: "John Jardine"
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
Date: Thu, 14 Nov 2002 18:05:52 -0000
NNTP-Posting-Date: 14 Nov 2002 17:54:38 GMT
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Paul Burridge wrote in message
> On Thu, 14 Nov 2002 17:34:21 +1100, "Phil Allison"
> opined thusly:
> > ** 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
> > Using 1 ohm resistors will reduce the available power -
> >by a large factor if the amp has "VI limiting" built in - most do.
> This one hasn't: it's just a breadboard experiment. BTW, there's no
> reactive load, just a loudspeaker. Okay, I know even a loudspeaker is
> technically a reactive load, but we're only talking low audio
> frequencies here, so I guess it'd be purely resistive, yeah?
> What I'm outlining here is class B *audio* of a few watts. Sorry if I
> should have made that clearer.
> > You may parallel connect several 1 ohms to get 0.33 or 0.25
> >ohms - that should work fine.
> AFAICS, reducing the inter-emitter resistances would only exacerbate
> the thermal problem!
> "I was going to help him up, but then I thumped him."
> Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975
Check the static DC voltage across those emitter resistors. It seems the
standing current is way too high.
Probably a problem with the biasing components that set the static
difference voltage between those two o/p transistor bases.
With 1 ohm emitter resistors 100mV across each could be unnecessarily high.