https://www.diyaudio.com/forums/solid-state/356157-comparative-vas-cascode-stage-post6459222.html

Hi Minek, Q313 / Q315 are folded cascode transistors. Essentially current conveyors and provide no voltage amplification. Therefore, they are an extension of the input stage.

The best way to see it, is to count high impedance nodes. In this amplifier there is only one. 




https://www.diyaudio.com/forums/solid-state/356157-comparative-vas-cascode-stage-post6459377.html
Hi Stanislav,

Circuit 1 is a folded cascode VAS preceded by an over-complicated multi-stage level-shifting buffer. It doesn't seem like VT2, Q15, and Q16 will turn on, so this one is probably dead in the water.

Circuit 2 is not a VAS, but a current-boosted buffer stage. At least it wants to be.
R6 and R7 should be tied to the rails.
R8 - R10 are actually clever, separating the setting of the quiescent bias from the large signal current.
There's no reason for Q4, Q5, and Q7 - Q12 to exist. Their only function is to double the current of Q3 and Q6. A function better handled by doubling R11 and R15.
I agree with chip that there's no reason for R12 - R14 to exist. Just short them all. 




The common emitter is not only a voltage amplifier, but also a current amplifier.In contrast to a common base . 






Hi Stanislav, thanks for sharing the whole circuit. Now I can comment.
To me this looks like a single gain stage amplifier.
VT1,VT2,Q15, Q16 and R45 (which was missing in your original picture) is a GM cell with GM ~1/R45 = 1/20.

VT5 is a folded cascode that takes the output current of the GM cell (collector of Q16) and conveys it to the HIZ 'node', the collector of VT5.

So, this circuit has no second gain stage or VAS as many here call it. 





https://www.diyaudio.com/forums/solid-state/356157-comparative-vas-cascode-stage-post6460557.html

The gain of this amplifier is as follows:
~ GM_INPUT_STAGE X GAIN_LOSS AT FOLDED CASCODE X R_HIZ
~ 2 x 1/(re_319 + 18) x 1/2 x 680 / (680 + re_315) x beta_315/(1+beta_315) x R_HIZ

The first factor of 2 is due to the complementary IPS. The 1/(re_319+18) is the effective GM of the diff-pair where 18 is the value of R319. 1/2 is because only one side of the diff-pairs is used. 680 / (680 + re_315) is the output current loss of the diff-pair due to small signal current getting shunted away from Q315's emitter by R327. R_HIZ is an abstraction of the impedance at the collector of Q315.

Now, simplifying a bit, Beta/(1+beta) = ~1, 680 / (680 + re_315) = ~1 if this thing is properly designed.

Thus the gain simplifies to:

G = 1/(re_319 + 18) * R_HiZ.

As you can see, Q315 offers no voltage amplification. It is the GM of the IPS x the impedance of the HiZ node.

I hope this is clear.

Regarding the SQUARE LAW amp, there is an LT1056. This is a 2 gain stage op-amp, so the so-called VAS in inside the op-amp.

BTW, can we stop using the term 'VAS'? This is the ridiculuous term that Self coined. All non-audio literature uses the name 'second gain stage'.