StevenIC: Tips 'n Lifehacks of Design of Analog IC, Mixed-Signal IC (AMS) & RFIC (©Steven)

[Welcome to citing and linking to the pages of this blog] Summary of learning and practice of analog circuit, mixed-signal circuit and RF circuit designs, focused on their integrated circuit (IC) implementations. General knowledge learned from books, papers and practices are summarized. This blog also holds Job Hunting Guide, including interview questions, written by Fuding Ge.

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Sunday, March 28, 2010

CMFB of differential op-amp

1. It is preferred to use CMFB to apply to active load, not current source; poorly designed CMFB or wide PVT variation may turn off the current source controlled by CMFB, however, it can seldom shorten the active load;

2. If CMFB has to be applied to current source, the current source shall compose of a constant current source plus a CMFB controlled current source; thus the total current source can never be turned off;

3. The loop of CMFB is preferred to be short, not through too many devices; otherwise, too many low-freq poles can make it unstable; in the example of folded-cascode (FC) op-amp, if the CMFB is from the second-stage output and applied to the current source for input diff pair in the first-stage FC, the loop may be too long.


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