telescopic ota with sc-cmfb

[adix]

Hi all,
I am designing a telescopic ota with SC-CMFB. These are my design specs:
vdd=1.8V
gbw=500MHz
load=100fF
DC gain>40db

My design works well with an ideal CMFB. My problem is the design of the SC-CMFB. I read the paper of Choksi and also several books. Once i connect the SC-CMFB to my OTA, the DC gain drops. Furthermore, the CM output is not the one I set. Any tips on designing the SC-CMFB or errors you think I may have done?

 

[dhaval4987]

i think it might drop because of the loading of extra capacitors. I am not too sure. try improving it for higher load. Also- did you check if all the transistors in saturation after connecting the SC circuit?

 

[adix]

All transistors are in saturation with or without SC-CMFB. And the gain at DC shouldnt change if I add SC-CMFB. Strange...And I also get a ripple at the output when I apply a differential signal at the input. Is that normal?

 

[dgnani]

how are you measuring your DC gain?
at what frequency are you clocking your CMFB?
can you show us a transient plot of how the cmfb is settling?

 

[adix]

Sorry for being late, I didnt have access to my design.
1) I measure the DC gain by parametric analisys by changing one of the imputs amplitude by a few mV and see the difference at the output.
2) I clock at 500MHz. The unity gain bandwidth of the opamp is now ~1GHz.
Here is the settling of the cmfb


Thanks for your answers!

 

[gkb1989]

did you do a DC analysis first? I am trying the same circuit but my common mode(vdd/2) is not setting at the output.