STAR: Sziklai Transconductance Analytical Redux

Several suggestions were posted about my initial question about the Sziklai transconductance.
mirlo's suggestion: GMsz = Afet * GMbjt + GMfet in (#4 &) #6: -> What is Afet?
MarkJohnson has a 'datapoint' in #8: GMsz = 207.6mA/V -> How to calculate this datapoint?
knutn states in #9: GMsz = beta * GMfet -> how does the transconductance and the beta relate, just 'as is'?
steveu comes in #23 with: 16mA/V with a circuit where the Rsz is 10k, driving the J-fet next near to pinch off (the most distorting area).
knutn has a RIAA circuit in #24 that performs fine, the K170/556B prooves a good amplifier stage...
jsdyson wonders in #30 about the very low Id setting (40μA) with "well constrained distortion and useful freq response".

This last 'low Id' contraption is already found in the late 70's Sony TA-E86B preamplifier, it's odd 'bass boost' circuit intended for some audience.
The J-fet (Vdsmax= 12V!) runs at some 60μA without problems. I don't use that output actually.

The redux is about the assumption of a high beta, so the current through Rsz is determining the transconductance of the bjt (aka 'voltage driven' instead of 'current driven').
In this case the Sziklai-transconductance boils down to a simple equation, alongside the fet-bjt also valid for a bjt-bjt combo.
Nevertheless is knutn's remark in #9 and steveu's circuit with a 10k still in debate: what if there's no high beta and/or a very high Rsz, pressing the fet into performance jeopardy?
Comments & corrections are welcome.
(I use "S" for "GM"; S^ = Smax)
 

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Thanks for the clean century.

It took me a full month to realize that the actual 'Sziklai Transconductance Amplification Ratio' ("STAR") is in the very second part of the formula: ... * Rsz * (Ic^/Vt), where the latter is the reciprocal of the internal emitter impedance ('re') or the 'thermal imedance'.
So the transconductance incremental secret of the Sziklai is: ... Rsz / re , alike a current mirror with external emitter resistors.

Jfet: Ssz = Sj * Rsz/re
BJT: Ssz = Sq * Rsz/re

I'm from the pre-MOS era, so I guess...

MOS: Ssz = Sm * Rsz/re

Cheerio,
dW [π°]
 

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The problem with the sziklai pair is the phase input to output at high freqencys. With 2 transistors making phase shift there is much more risk for self oscillation. You will have to decompensate the amplifier to a lower frequency cutoff. That means lower feedback and more distortion.
That is the reason you never see the sziklai pairs in the ultra low distortion amplifiers here.

 
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