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From: "Phil Allison"
References: <firstname.lastname@example.org> <email@example.com> <4KfR9.92768$hK4.firstname.lastname@example.org> <email@example.com>
Subject: Re: Thermal runaway: please settle this dispute!
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Date: Sun, 5 Jan 2003 11:38:19 +1100
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"chris" wrote in message
> Hi Lukas, quite wrong I'm afraid, in a typical class ab amplifier the
> bias voltage accounts for just enough collector current to avoid
> crossover distortion, typicaly 50-100mA. At elevated temp this might
> rise to say 70-140mA which is still an very small amount compared to
> the collector current due to the signal.
** Your figures above are correct ONLY because a very carefully
designed bias compensation scheme is involved in the design of a class AB
A Vbe multiplier transistor (or string of diodes) is thermally
coupled to the heatsink or sometimes the actual output devices. This cct
tracks the varying Vbe of these transistors. There is always a time lag
involved in the compensation action and the actual bias can go way higher
In many cases simply lifting the Vbe multiplier tranny clear of the
heatsink is enough to send the amp into thermal runaway. Lifting the OP
devices off the heatsink guarantees it.
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