Phase margin of opamp after post layout simulation

[mordak]

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

 

[crutschow]

Post the circuit and your layout simulation. Can't tell anything from your description.

 

[LvW]

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

 

[mordak]

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

 

[erikl]

... 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

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

 

[mordak]

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

Unfortunately no, UGBW is critical for me so I can't sacrifice that.

 

[erikl]

Then I'd try the following changes:

1. reduce C_load
2. reduce C_bat & C'_bat
3. play with the R_cs

 

[mordak]

Then I'd try the following changes:

1. reduce C_load
2. reduce C_bat & C'_bat
3. play with the R_cs

I already tested them, thanks anyway. I guess there is no way but to take one step back and redesign it!

 

[erikl]

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.