How to use SC CMFB with low loop gain

[Analogworld]

Hi All,

Could someone please tell me how to design a SC CMFB circuit for the second stage of a two-stage opamp? The second stage has much lower gain (~20) compared to the first stage, so the commonly used switched capacitor CMFB doesn't work for the second stage (the first stage is fine), because the loop gain is very low. Could someone please help?

 

[philipwang]

Dear Analogworld,

Would you please adjust the capacitor value? This problem maybe come from the
incorrect bandwidth relationship between the second stage and SC CMFB.

Good luck.

Regards,
Philip

Hi All,

Could someone please tell me how to design a SC CMFB circuit for the second stage of a two-stage opamp? The second stage has much lower gain (~20) compared to the first stage, so the commonly used switched capacitor CMFB doesn't work for the second stage (the first stage is fine), because the loop gain is very low. Could someone please help?

 

[xwcwc1234]

How to simulate SC CMFB using Hspice ?

 

[cinvo]

HSPICE can not simulate SC CMFB,SPECTRE can

 

[kivvzhou]

you can add a nmos as tail current sink for CMFB in every stage,so 1/2w nmos as normal bias ,1/2 nmos as cmfb control .The two stage have the same cmfb ,which will work well ,i have design this ota for pipeline adc use .

 

[Analogworld]

Hi All,

Thanks for the reply. Let me clarify it.
First, since I am not trying to get the frequency response of the SC CMFB, the simulation done in Hspice is fine.

Secondly, an extra transistor is placed in parallel with the tail current source in both stage as the cmfb current control, this is not shown in the circuit diagram.

The problem is that I need to put a gain block in the SC CMFB circuit in order for the output voltages to converge to where I want the common mode output to be. This is because the CMFB loop gain for the second stage is very low.

So, my question is do I have to put a gain block in the SC CMFB circuit? If yes, how do you implement it? Or, is there any other simple ways to do it?

Thanks in advance.