what is the role of this two capacitance?

[wjxcom]

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]

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]

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

 

[rfsystem]

Here are some refs:

google: differential op amp common mode feedback

selected!

**broken link removed**
**broken link removed**
http://www4.ncsu.edu/~xzhang11/PDF/711_proj.pdf

 

[wjxcom]

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

Thanx you!!!