Hi.

For the attached inverting amplifier config using fully diff folded cascode opamp, should the loop gain found using STB analysis (CMDM = -1) equal to [ (open-loop gain of opamp) * (beta) ] ?
- or is the loop gain not so easy to be calculated here?
- because, my simulation result is not matching with such calculation!

(here, I used beta = feedback factor = 1/11, and the open loop gain was found from open loop simulation = 933, thus, loop-gain calculated=933/11=84.8, whereas STB analysis gave a gain of 70 at dc)

Any help in this regard will be very helpful. I have searched the internet but haven't found any answer matching my query

Thanks Ken for the reply to my query  :)

I indeed didn't use any feedback while simulating the open-loop gain:
just connected a sine source with ac mag=1 between + and - inputs, and measured differential o/p (also providing each input terminal with CM bias=Vdd/2).

Is there any testbench for getting open-loop response while also maintaining the feedback, for fully-diff opamps?

Thanks petitidiot for your concern

Yes, that's what I subsequently figured out!
On simulating the open-loop gain with a load resistance of 10 M Ohm connected at the output, I found that the open loop-gain decreased, and the [ (open-loop gain of opamp) * (beta) ] approx. matched the loop gain found using STB analysis (CMDM = -1).

BUT, correlating the Phase-margin between the two methods seems to be difficult. Like, the phase margin calculated from STB analysis (for closed loop with beta=1/11) should be better (i.e., more) than the P.M. from the open-loop gain Bode plot (as it is like the plot of A*beta with beta = 1)? - but, I am seeing the opposite!

...maybe the pole due to large 1 MOhm is affecting the P.M. when the f/b is used?...any idea on this?

Many thanks Raja for pointing that out..  :)