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How to increase my two stage folded cascode opamp's CMRR? Help me pls!

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pz5921087

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I am designing a two stage folded cascode opamp, but when I did the testing of CMRR, I found the result is not enough for my requirement. I use monte carlo to simulate it. Process&Mismatch

I found that only select Process the result is fine. But with Mismatch, the result is not good. How to increase the CMRR? Can anybody give me some suggestions?
Thank you all!
 

Good question!
I think that increasing the numbers of poles in your design will increase the CMRR, but I am not sure in this.
 

Generally you can improve CMRR by reducing offset and by increasing of output resistance of diff. pair's tail current.
If you have simple mirror for tail current you can enhance it to cascode or ever to regulated cascode.
 

I have tried your guys method, but it increase just a little. Actually I need to increase 20dB
 

increase tail current source impedance, increase differential pair and current mirror size to reduce mismatch.
futher more you can try reduce the gain of the cascode
 

20dB looks very small. Hopefully you know how to simulate CMRR. If you have an issue with it I can refer you to Cadence's white paper on https://www.edaboard.com/threads/67434/.
It's for differential amplifier but if you understand a principle you will be able to apply it for single ended amplifier.
 

I mean increase 20dB I need a 60dB CMRR but now i can only achieve 40dB.
I know how to simulate it
Anyway, thanks all the same for the paper!
 

pz5921087 said:
I found that only select Process the result is fine. But with Mismatch, the result is not good. How to increase the CMRR?
Click to expand...

If mismatch is responsible for CMRR degradation, you should of course try and reduce mismatch causes: use repeat-symmetrical, even better: common centroid design for corresponding transistors (at least for differential stages), and, if interleaved structures are used, add side dummy structures.
 
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