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From: "Phil Allison"
Subject: Re: Thermal runaway: please settle this dispute!
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Fri, 3 Jan 2003 16:27:46 +1100
NNTP-Posting-Date: Fri, 03 Jan 2003 16:17:43 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Paul Burridge" wrote in message
> Hi all,
> I ended up having a bit of a disagreement with a friend of mine over
> the holiday concerning thermal runaway in a BJT.
> I maintain that an increase in temperature across the emitter diode
> leads to a fall in that junction's barrier height, making the
> so-called 'energy hill' easier to overcome for the forward biased
> current flowing through it. The causes increased current flow, giving
> rise to further heating of the emitter diode, thereby lowering the
> barrier potential still further and so on until the device is
> destroyed. The fall in barrier height is around 2mV per degree C., if
> I recall correctly.
** That is the usual figure for Silicon BJTs.
> My friend, OTOH, maintains that it's nothing to do with the
> emitter/base junction at all, but solely arises due to an increase in
> temperature at the collector, causing the same cycle of
> heating/increased current until the device is destroyed.
** If your friend was right there would be no way to stabilise a BJT's
collector current with temperature.
BJT amplifier OP stage bias stability schemes correct the output
devices Vbe at the rate of - 2 mV per junction per degree C, assuming that
Vbe is in the bias determining loop (ie the OP stage is a darlington
emitter follower) That is what the "Vbe multiplier" stage in the bias cct
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