Post by pdirac on Aug 8th, 2019, 9:56am
I didn't find the answer to my question, neither here nor everywhere else. Here's my question: Consider the following FD folded cascode op amp where I used a SC-CMFB to control the output common mode voltage. Everything working fine. It settles to 0.9V after a few microseconds. The load capacitor is 0.5pF, the switched capacitor as part of the CMFB network is 1.25pF, and the non-switched is 0.25pF.
I followed the following diagram to measure the SC-CMFB open loop gain/phase.
The input voltages Vin and Vip are connected to a bias voltage of 1.1V. I set the PAC magnitude for this bias voltage source to 1.
Here are two questions:
(1) Is above the right configuration to measure the CMFB loop gain/phase? I did that and got a dc gain of 64dB and a phase margin of 87 deg. The theoretical value I computed for the dc gain matches the simulation. But I'm not sure how to find the unity gain frequency (see question 2). [Also FYI the main op amp has a dc gain of 51dB and a phase margin of about 88 deg.]
(2) How to measure the UGF of the CMFB? I'm a bit confused as to how calculate that. It should be given like gm/2pi*CT, where gm is the transconductance of M9,10, and is measured roughly 260uS, and CT is the total output capacitance. It should be about 0.59pF in order to match the simulation result. So maybe CT is just CL? Anyway, that's my understanding. Hope someone help me clear things up.
Thanks