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telescopic ota with sc-cmfb

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adix

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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

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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

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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

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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

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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

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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.
 

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