[NLRS] Transistor thermal runaway?

[Dr. Gerald N. Johnson]

On Wed, 2007-05-30 at 02:08 +0000, Scott wrote:
> 
> Can't exactly remember from tech school...what does heat in a transistor 
> do that causes thermal runaway?  As the PN junction heats, does its 
> "resistance" decrease, causing more current to flow in the base circuit 
> thus causing more heat, thus causing lower resistance, etc.???

Its voltage drop goes down making the base current from a stiff source
rise.

Ideally current feed back from the collector or emitter helps control
the base current. Unfortunately in an RF power situation, the current
sample resistors hurt stage efficiency excessively. An emitter resistor
(or individual emitter resistors help multiple transistors in parallel
share current) helps the bias stability and the linearity of the stage a
the cost of gain and power output.

Some RF power transistors actually are made of many small transistors on
a chip with individual emitter resistor to try to force current sharing.

> Being 
> that transistors are current controlled devices, would using a current 
> source such as an LM334 to drive the base, help alleviate thermal 
> runaway by supplying a constant current? 

It would except that base current varies with drive.

> I tried the diode (1N4001) 
> from base to emitter of the PA transistor (with the two bodies touching 
> through heatsink compound), but it isn't working as well as I'd like...
> 

Have you hunted the Motorola articles on linear amplifier biasing that I
believe I've mentioned before? The best ones use a LM723 to supply base
current and include the diode as part of the control. Like AN1035 or
EB27A. They worked it out and those circuit works. No need to invent new
ones.

http://lists.contesting.com/_tentec/1998-05/msg00006.html

http://64.233.179.104/translate_c?
hl=en&u=http://www.hellocq.net/forum/attachment.php%3Fattachmentid%
3D216936&prev=/search%3Fq%3D%2B%2522eb27a%2522%2B%26hl%3Den%26lr%3D%26sa
%3DG%26as_qdr%3Dall

You may have to run the last link together, but I was successful in
downloading EB-27 from it. So I'll attach it. The list won't pass it
thouogh.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer