ULPAnalog,
Thank you for your reply. Yes! your understanding is absolutely right.
To answer your first question, I am using a differential difference amplifier (DDA) for the firs OTA (gm1) and a so-called 5-transistor amplifier for others (gmp2 and gmp3). Please see the screenshot in CMFB123.pdf.
And about your second question, as a matter of fact, that's exactly what I was asking in my previous posts. I want to check the stability of each gm-cell (OTA) with its corresponding CMFB individually. BUT I don't know how to set up a test bench to do that :'(. That's my problem.
I attached my top-level schematic of the op-amp with "diffstbprobe" to check the stability of each loop. Can you please verify if my test bench to check the stability of the CMFB is right?
Thank you
ULPAnalog wrote on Apr 17th, 2017, 2:26am:I took a chance to check the content of the zip. My understanding is that the op-amp itself is feedforward compensated with local feedback around gm stages to set the common mode. I have a couple of questions to polyam.
1. Your CMFB has a structure by name single OTA but I do not see what is inside it. Is it a single stage OTA?
2. Did you verify that each OTA with its CMFB is stable? Instead of attacking the 3 stage opamp directly, you may want to first strip it down to a single OTA with CMFB and check if it stays stable. I wonder if the CMFB loop, itself being a two pole system (one from single OTA and other from the pmos device, the gate of which is being controlled by CMFB loop), is adequately compensated.
Thanks and regards