On Sun, 24 Sep 2006 21:59:32 -0700, Frank Gilliland wrote:

On Sun, 24 Sep 2006 23:30:24 -0400, "LeIand C. Scot"
wrote in :

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


Not picking nits, just taking a different approach.....

Thermal bias compensation works to a point but neglects one important
issue: it takes time for heat to travel from the junction to the
outside of the package, and thermal runaway can happen faster than a
-thermal- compensation circuit can respond to it.

Exactly. That's why those diodes are place on the ceramic cap of the
device and not on the heat sink.

Since the heat
buildup is due to excessive EC current, it makes more sense to control
the EC current directly.

There are two solutions that use this approach. One is to include a
resistor on the emitter with a TC opposite of the transistor. Not
precision but much faster response.

This is done in many audio amps. The chief problem is the negative
feedback introduced by the emitter resistor. At auto frequencies this
resistor is bypassed by a rather large electrolytic capacitor sized such
that at the lowest frequency of interest the reactance is much smaller
that the emitter resistor value. Thus the "AC" gain isn't affected much by
the emitter resistor.

Believe it or not I've seen many of the old Motorola RF devices use
internal emitter resistors. Those took the form of many small tungsten
bonding wires from different areas of the emitter structure to the emitter
terminal. The main idea there was the many wires, resistors, in parallel
resulted in a very small overall emitter resistor. Also they found that a
problem called "second break down" would occur if they didn't do this.
What it amounted too was local hot spots, thermal runaway, in tiny areas
of the transistor's emitter structure. I think the term they used for RF
devices built this way was "emitter ballasting".

The other is to monitor the EC
voltage and the base current; send both measurements to a differential
OP amp and use the output as feedback for the bias regulator.

You would have to look at the "DC" emitter current minus the "AC"
component, which I don't think is going to be so easy to do.

Regards,
Leland C. Scott
KC8LDO