Vladislav D wrote on Feb 2nd, 2012, 12:37pm:Yep, instead of Cf, of course should be 1/sCf
gm is a transconductance of the source follower. Cf is a compensation capacitor I derive this transfer function by putting a voltage source at the output and see what voltage I get at node 'B'.
Ahh, that makes perfect sense. I do know that the equivalent resistance looking into the source of an active device is 1/gm (neglecting body effect). This is very helpful. So the voltage at node B will be dependent on the feedback transfer function and
also on the equivalent small-signal resistance to ground at Node B, which is 1/gm, or "low."
Putting the test source at the output node makes all of this make sense. Of course in my case it will not be 1/sCf but rather Rc + 1/sCf. Should still work out similarly as just about everything at that node is strongly affected by the low impedance.
I finally got my hands on a copy of the Razavi book and chapter 10 talks about compensating a telescopic cascode OTA cascaded with a common source second gain stage, like I have here. It's very helpful so far; Simulations show that my level shifter has no real effect on the frequency response except for some weird poles and zeros that are 3 decades higher than the unity gain freq, so Razavi's analysis seems to hold true for my circuit as well. (though I am not driving a load capacitance, only a moderate resistance, so that saves me the trouble of one additional pole.