Why should the bias servo transistor not be placed on the main heat sink for a Sziklai pair?

Question

The Wikipedia page on Sziklai pairs contains the following statement (emphasis mine)

Sziklai pairs can also have the benefit of superior thermal stability. [...]. [Q]uiescent current is much more stable with respect to changes in the temperature of the higher power output transistors vs the lower power drivers. This means that a Sziklai output stage in a class AB amplifier requires only that the bias servo transistor or diodes be thermally matched to the lower power driver transistors; they need not (and should not) be placed on the main heatsink

I'm specifically interested in the "should not" part of that statement. I suppose that placing the bias transistor on the main heatsink may be overcompensating, which could create distortion at normal operating temperature? Or is there something else at play? Does this mean that I should make a separate heatsink for the lower power transistors so that they can be thermally connected to the bias servo transistor?

Answer 1

The Sziklai topology places the output BJTs within a local NFB loop. It's enough that the quiescent current is approximately 20 times less sensitive to the output BJT temperature changes. Partly because of this, you do NOT need to include the power output BJTs on a shared and monitored heat sink.

It's the driver BJTs that need to be monitored for temperature and used to adjust the \$V_\text{BE}\$ multiplier. And that's handy because their heat sink can be quite small and therefore also the thermal time constants will be quite short (which is a positive thing.)

You can (mostly) ignore the output BJT temperatures (though you still need to allow them to dissipate properly, of course.) It would be detrimental to allow their dissipation to affect the \$V_\text{BE}\$ multiplier. It's the base-emitter junctions of the driver BJTs that need to be tracked and not that of the output BJTs.

The Sziklai design arrangement allows the driver BJTs to undergo somewhat wider temperature variations as the output power demands change (than is the case with the Darlington arrangement.) So it's a little more important to do good thermal tracking with the \$V_\text{BE}\$ multiplier. But it should only be observing the driver BJTs (often by putting the \$V_\text{BE}\$ multiplier BJT(s) on the same heat sink.) Keeping the heat sink small (thermal mass is "light"), the "system" should respond more quickly to changes. You don't want the output BJTs' dissipation messing that up. Just allow the output BJTs to have their own dissipation heat sinks and keep them away from the driver BJTs, where possible.

Answer 2

they need not (and should not) be placed on the main heatsink

Beware of broad statements like this. In some cases, a high-power amplifier mounts both transistors of the pair onto the same heatsink.
In another case, the first transistor of a compound pair (be it Darlington or Sziklai pair) is a printed circuit board mounted transistor with no (or small) heatsink - it may run hotter or cooler than the big transistor it drives.

In any case, an amplifier should be tested for two thermal situations:

• thermal bias stability for ambient temperature changes (done at low output power).

• thermal bias stability at high power output (does AB bias change as those transistor heat?)

The broad statement addresses the latter case. No matter what the topology used, the placement of bias servo temperature compensating components are subject to these tests. In some cases, thermal coupling is tight to maintain bias current. In other cases, temperature coupling is far looser, with bias servo parts merely nearby.