Here is the design philosophy behind the circuit. It is obviously a derivative of that topology that you find in just about any text: differential input + high gain interstage + common collector output. Having the diff amp, I wondered what would happen if it drove another diff amp instead of just disconnecting one output. This seemed to open up the possibility of a second feedback path that would allow stability without having to add a 100pF or so capacitor across the collector-base junction of the interstage transistor...
To be perfectly honest I don't see how that would get rid of the Ccomp. A LTP front end driving a LTP current mirror terminated VAS/driver is definitely nothing new and in fact it is most often seen with two Ccomp (one for each side of the LTP), though a single one connected differentially can be used as well with some care. BTW how often do unconditionally stable RF amps have AC feedback applied across 3 stages?
The real question is why use ner 100dB open loop gain to make a unity buffer and then use a single ended preamp that cannot by definition drive the output stage to maximum excursion?
Stability could have been achieved by limiting the VAS open loop gain by giving it a defined impedance to work into (instead of the variable impedance of the output stage). Ditto reducing input stage gain by degeneration in the emitters. If you wanted FET sound, using two FETs in the input LTP would work great, AND would reduce open loop gain too due to their lower gm.
Anyway, the use of a quasi-complementary output stage with the linearizing diode harks back at least 20 or 25 years, I think it came from Peter Walker, because of the unavailability of complementary power devices at that time.
More like 30-35 years
but that's beside the point. It is a good trick though
I'm not too worried about that. After all, the basic circuit itself harks back to the late 1940s
(Nothing wrong in that. "New" just for the sake of being "new" is not necessarily a Good Thing... However, I read that, when the matter was finally put to the test, the quasi-complementary actually performed better. (Probably from
Doug Self). The "Sziklai Pair", after all, is a very similar topology.
Actualy, D. Self himself (no pun intended) gave a decided
to the 'better audio from non-complements' artice (B. Ollsen, IIRC). I've read it myself and I would call it circumstantial (but of course, I'm no authority and heaven forbid I ever become one
).
Sziklai Pair is HALF of the standard quasicomplementary topology, the other being a follower (though depending on drive arrangement the follower may reduce to a current source which dispenses with the problem of the upper half of the putput having low output impedance and the lower half having high output impedance).
The issue with traditional quasi-complementary designs is that the transfer characteristic is very different for a follower and a CFB pair (a.k.a. Sziklai Pair). Since each passes one half of the output waveform, even order distortion is introduced. Wether that is good or bad seems to remain in the subjective domain
On the other hand, of course if you want to increase the gain of the LTP (not considering feedback) perhaps a current mirror might help.
Well, it's already there (though with a diode, which I call 'the Scrooge McDuck version' as it's less accurate and transistors (even diode connected
) are really cheap these days, so no real $ saver...
?
?
There's no arguing with facts, so the next iteration will use full complementary.