some question about CMFB capacitor

[lhlbluesky]

for a fully differential folded cascode opamp, the sc-cmfb circuit is used. i want to know how to calculate the CM-GBW and how to calculate the CM feedback factor FCM?

the following figure is the simplified sc-cmfb half circuit, Cg is the NMOS tail transistor gate parasitic capacitor,
FCM=C2/(C2+Cg) or FCM=(C1+C2)/(C1+C2+Cg), then which is right? if none, what is the correct value?

pls give me some clear explanations, thanks all.

besides, in spectre op points, for a MOS transistor, there are many capacitor name, such as Cgs\Cgd\Cdg\Cdd\Cgg\Cdb\Csb etc, then what are their seperate meaning? if i want to estimate the MOS transistor's gate capacitance, then what value should be used, i often use Cgg to estamate the gate capacitance, is that right? if not, how to estimate the gate capacitance correctly?
similarly, how to estimate the node capacitance of drain correctly?

thanks all for reply.

 

[lhlbluesky]

can anyone help me, pls?

 

[lhlbluesky]

someone says CMFB loop is just an integrator,
but how to understand? why?

 

[yxo]

I think, It wil be helpful

IEEE Trans. on Circuit and Systems -II: Analog and Digital Signal Processing Vol.50 no.12 December 2003. ¨Analys of Switched Capacitor Common-Mode Feedback Circuit¨. Unfortunately, I don´t have pdf now but you can ask here or try to google. Cgg, Cdd - the sum of all capactinace from corresponding node to ground.

 

[lhlbluesky]

thanks all.

 

[arunkumar446]

can any one tell me when this SC-CMFB becomes unstable and has ringing and how to avoid it.