Inclusive Miller Compensation

Baxandall Papers page 30 - 35

INCLUSIVE MILLER COMPENSATION

Since typing p. 29, I regret to say that several months have elapsed, owing to the unstoppable intrusion of several other topics!

In your Article 7 you raise the question of whether to include the output stage in the C_dom feedback loop, and comment that you have not so far found “Inclusive Miller Compensation” to be workable.

I notice that Edward Cherry (a delightful character, who visited me very recently) has, however, like me, found the scheme to be workable - EW + WW, Jan 1995. He comments – and I agree – that if oscillation does occur, it is normally a purely local parasitic, and suggests trying a 50 pF collector-to-base capacitor, with very short leads, on the driver transistors. He says the frequency is usually “several MHz”, but my experience is that it is quite liable to be well over 10 MHz.

Some months ago I devoted much thought and experiment to this problem, and feel I obtained some useful enlightenment, as follows.

Certainly, in the past, like you, I have found that feeding C_dom from the amplifier output instead of from the VAS collector usually does provoke HF parasitic oscillation, at least at some output levels and/or load conditions, and I have tended, therefore, to regard it as a very questionable technique, probably better avoided in practice.

At these quite high parasite frequencies everything tends to be exasperatingly difficult to understand fully – one is never quite sure what the equivalent circuit really is, since every lead inserts inductance and the transistor equivalent circuits become more complex than can be properly represented by the normal hybrid–π equivalent circuit. And in experimenting it is so easy, for example, to try shorting out a tentatively-inserted 470 pF capacitor with a miniature-croc.-clip lead a mere 1” long, only to become aware a little later that what one has really done is to convert this capacitor into a moderate-Q parallel tuned circuit; 0.1 µH across 470 pF gives 23 MHz.