Continous-time CMFB question

[skythunder]

ops for cmfb

Hi, everyone,

I designed a folded-cascode fully differential opamp, and the structure of the CMFB is two differential pairs, which is shown in the picture.
After simulation , I found that this OPAMP works well in the continous-time configuration, like a Multiple-feedback Lowpass filter. However, oscillating occurs when this filter was connected aftert a switched-capacitor filter. Then I just made a simple switched-capacitor amplifier using this fully differential opamp, the same thing happened. I guess this opamp is not suitable for SC circuit, but I want to know what the reason is .
Can anyone tell me the possible reason please ? thanks !

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here is the picture


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and the filter circuit

 

[yibinhsieh]

Folded-cascode OPs are suitable for any kinds of application including SC

I think you need to consider the stability issue in your SC application.

Please simulate the phase margin and gain margin for your application.

You also need to simulate the phase margin for CMFB loop.

 

[skythunder]

Folded-cascode OPs are suitable for any kinds of application including SC

I think you need to consider the stability issue in your SC application.

Please simulate the phase margin and gain margin for your application.

You also need to simulate the phase margin for CMFB loop.

Thank you for your attention, yibinhsieh
I can ensure that there is no stability problem with the folded-cascode OP , but I have doubt with the stability with CMFB loop. However, I am not quite sure about how to verify the stability of the CMFB loop. I have two more questions:
1. The kind of CMFB I used is two differential pairs, but I want to know how to set up those transistors' size, what's relationship between the CMFB and main OP in terms of transistor W/L ratios;

2. How to break up the CMFB loop and simulate the phase margin of the CMFB loop

 

[filtershaoyong]

about the method of simulating the phase margin of the CMFB loop, pls go to the last chapter of P. Gary's book.

 

[jecyhale]

May be you need improve the current of CMFB.

 

[spring1860]

make the current of CMFB larger than that of differential pair.
or use SC-CMFB, this without stability problems

 

[dineshbabumm]

@ skythunder

The CMFB loop you have used seem to be having a positive feed back instead of negative feed back! Please verify if your CMFB is working fine!