Phase margin of opamp after post layout simulation

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mordak

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Hi,

I have designed a class AB opamp and then drawn its layout. After extraction I run post layout simulation but saw that phase margin was degraded to 40 degrees. I was wondering if there is anyway I can trim the circuit to improve the phase margin, so I don't have to redraw the whole layout, or I need to start from scratch and design another opamp with higher phase margin so after layout I can get the desired PM.

Tnx
 

Yes - post the circuit and tell us how did you determine the margin?
 

Tnx guys for commenting. Below are the OTA structure and post layout frequency responce.

 

mordak said:
... I was wondering if there is anyway I can trim the circuit to improve the phase margin, so I don't have to redraw the whole layout
Click to expand...

Can you afford to reduce the UGB - say to 100MHz - by enlarging the CCs ? This would probably upgrade the PM to about 60°.
 

erikl said:
Can you afford to reduce the UGB - say to 100MHz - by enlarging the CCs ? This would probably upgrade the PM to about 60°.
Click to expand...
Unfortunately no, UGBW is critical for me so I can't sacrifice that.
 

Then I'd try the following changes:

1. reduce Cload
2. reduce Cbat & C'bat
3. play with the Rcs
 

erikl said:
Then I'd try the following changes:

1. reduce Cload
2. reduce Cbat & C'bat
3. play with the Rcs
Click to expand...
I already tested them, thanks anyway. I guess there is no way but to take one step back and redesign it!
 

Probably this is tedious, but perhaps by comparison of the pre- and post-layout netlists you could try and find out, which layout influence (parasitic capacitance) shifts your non-dominant pole to a lower frequency (and so reduces your PM). This might even be helpful for the redesign.
 
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