In message  <Pine.SOL.3.91.961206235629.28717F-100000@clarion.cec.wustl.edu>
 PICLIST@MITVMA.MIT.EDU writes:


>         I've been thinking about this for a week now and I'm not sure I
> understand.  I thought resistance always increased with increasing
> temperature.  The basic idea being that as you heat something up the
> mocules move around faster making it that much harder for the electrons
> to move through the conductor which is in effect increasing the resitance.
> Take super conductivity for example, you decrease the temperature to the
> point where the mocules are moving very slowly which makes the resistance
> very low.
>
>         Now I can believe that increasing the temperature affects the
> transitors Beta, but I find it hard to believe that it decreases the
> resitance.
>
>         The total emitter current is given by:
>
>         Ie=Ieo(e^(Vbe/nVt)-1)
>
>         Vt=kT/q
>
>         As T increase, the (Vbe/nVt) term will decrease.  Raising the
> exponent to a smaller power results in a smaller Ie.  So it looks to me
> like increasing the temperature results in smaller current flow through
> the eimitter which will cause a smaller current in the collector.
>
>         The one thing I may be overlooking is that if the Beta does change
> with temperature in a real part.  Does anyone have some insight for me?

From a purely practical point of view I can assure you this happens, amongst
other things I sometimes repair audio amplifiers. These are almost always
class AB amplifiers, and these have a tendency to 'thermal runaway'.

In class AB amplifiers the output transistors are biased on slightly, to
avoid (reduce!) crossover distortion, this makes them prone to 'thermal
runaway'. To avoid this the bias setting arrangement usually includes some
kind of temperature compensation, in very old amplifiers thermistors were
often used, but almost all modern amplifiers use a transistor connected as
a VBE multiplier to do this. A VBE multiplier is simply a transistor with
a preset connected to all three terminals, the slider feeds the base, and the
outer connections connect to collector and emmitter. This basically gives a
simple variable zener diode, and this is then connected between the bases of
the two output transistors, and adjusted to give the required bias current.

This transistor is then fastened to the same heatsink as the output
transistors so it monitors the temperature. As the output transistors heat
up, they pass more current, then they get hotter, and pass more current still,
this is called 'thermal runaway' and will lead to distruction of the amp.
However as the heatsink gets warmer, so does the VBE multiplier, and this
also passes more current. By doing this it reduces the bias current for the
output transistors and stops the runaway.

Nigel.

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