CMFB of operational amplifier

Thread Starter


Joined Feb 9, 2019

the following image, shows a two-stage class AB operational amplifier ( from "THE DESIGN OF LOW-VOLTAGE, LOW-POWER SIGMA-DELTA MODULATORS"):

Screenshot (264).png

this amplifier has two common-mode feedback circuit, and so, here is the proposed schematics:

Screenshot (278).png

Screenshot (322).png

CMFB of the first stage is clear, and I know how it works. but I think CMFB of the second stage is vague. Voutp and Voutn are the output voltage. Vmid is the desired output voltage. in CMFB of the first stage, Vcmfb1, is connected to the tail current source. so, we know how feedback returns to our circuit. But, in the second stage CMFB, where and how does the CMFB circuit returns to the circuit? how can I determine Vcs2 and Vcs1?

thank you