Tacking the diodes on to the case of the transistor works better than
attaching them to the heat sink due to the relatively slow thermal response,
the heat sink changes temperature slower that the cap on the transistor. If
you want as fast a response as possible then the diodes are placed about as
well as you can get. I've also seen small diodes clipped on to the flange of
TO-3 style cases for the same reason. I've seen some high power transistors
used for motor drive and high frequency inverter use that has the
temperature sensing diode, or thermistor, built inside of the package.

http://pdf1.alldatasheet.co.kr/datas...50HM75STG.html

As the temperature of the base-emitter junction goes up the required forward
voltage for a given base current decreases. If nothing was done, the
base-emitter bias voltage held fixed, the base current for increases and
thus the collector current too. Of course increased collector current
results in more power dissipation, thus more heat and higher resulting
temperature, i.e. thermal runaway.

The diode used for bias temperature compensation has to very closely match
the Ib-Vbe curve of the transistor to work effectively . You can't use just
any diode and expect it to work very well. The problem with mounting the
diode on the heat sink is the large thermal mass. If the transistor starts
to go in to thermal runaway the heat sink will take a significant time to
heat up, thus reducing the bias to the base-emitter junction, long after the
runaway has started. You want to get the compensation right as soon as
possible, reduce the base-emitter bias, before the junction gets too hot.
The small cap on the transistor will heat up much faster that anything else
giving the designer a chance to design a fast responding bias circuit which
could avoid transistor destruction.

Diode bias circuits are on their way out in favor of more advance bias
circuits. The chief problem with diode bias compensation is either too much,
the transistor gets starved for needed bias current, or not enough, tendency
to go in to thermal runaway.

The reason the transistor can get starved for bias current is simple. The
Beta, current gain, of a bipolar transistor is not constant over the
operating range, it varies based on the instantaneous operating point. At
some point the gain is so low that the current flowing through the bias
compensation diode may drop to a low value, or go to zero, limiting the bias
current forcing the operation to go non-linear over part of the cycle.
That's why some of the simple diode bias circuits draw so much standing
current. It has to supply the worst case bias current. Newer active bias
circuits use a voltage source type design. The bias voltage tracks the
transistor temperature and since it is a "voltage source" there is no
practical limitation on the bias current. The transistor will draw what it
needs without being limited by the current limiting resistors in the diode
bias type of circuit.

I'm sure somebody is going to nit-pick this post. They're welcomed.

--
Regards,
Leland C. Scott
KC8LDO

"Telstar Electronics" wrote in message
ups.com...
wrote:
It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

That's what it's supposed to do... I claim it can't work as shown. It's
not quite that simple as tacking a few diodes on the bias circuit and
laying them on top of the transistors... LOL

www.telstar-electronics.com