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what is the role of this two capacitance?

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wjxcom

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Hi, all: Please look at the full differential OPA. The CMFB circuit includes capacitance C3, C4 and the resistance R3 and R4. I think R3 and R4 is enough for the CMFB circuit, and I do not know why there have capacitance C3 and C4.

or maybe somebody can introduce some papers about these two capacitances to me.

Regards!!!
 

rfsystem

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R3||R4 and the MOS cap of M17 create an parasitic pole impacting the stability of the CMFB loop. So C3 and C4 made a feedforward compensation.
 

wjxcom

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Hi, rfsystem: thanx you to help me. and would you give some papers about these tow capacitance?
 

wjxcom

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Hi, rfsystem: I think these papers is useful for me to learn differential op amp common mode feedback.

Thanx you!!!
 

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