Common-mode feedback in the full differential opamp?

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Common-Mode feedback tutorial
 

Common mode feedback is in general needed since the PMOS current source
and the NMOS input devices in a diff amp always have an error between
themselves.

This error when multiplied by the vast output resistance of the opamp creates
the common mode imbalance.
 

daisyinfall said:
In my opinion,the R and C generated a zero,not a pole.
Click to expand...
I think the R and C generate both a pole 1/R(C+Cg) and a zero 1/RC, where Cg is the gate capacitance of the input transistor of the CMFB amplifier.
 

The resistors for CM signal detector is offern N-well res.so the parasitic cap from n-well to sub is unavoidable, and woe may call it Cp. Cp effects as Cgs(in out cap of cm amplifier), will create a pole 1/[R(Cp+Cgs)],and this pole will degreation the stability of cmfb loop.
after parallel cap Cc has been added,we will seen a pole/zero pair.
sp= 1/[R(Cc+Cp+Cgs)]
sz= 1/(R*Cc)
If Cc much bigger than Cp+Cgs,so this pz pair can compensation each other. So, this cap is added to insuation the stability of cmfb loop.


hope helpful!
 

Try this paper, it is really nice
 

the gain is low for such cmfb
 

yyliang said:
I am designing a op amp,the first stage is differential output and the second stage is single-end output,need I add a CMFB circuits to the op amp for the first stage?
Click to expand...

no
 

the capacitor is used to generate a zero which stop the CMFB from osccilation
 

how does the common mode DC voltage change? stable or?
 


I want to know also, i have to do a project.
 

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