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question in CMFB circuit?

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wael_wael

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Hi every body,

i have folded cascode OTA with P differential pair as in the figure below,
i need some one tell me is the connection between the CMFB and the OTA correct or not, because i am not sure i i have to connect the output of the CMFB to the bottom or to the TOP of the cascode.

best regards
 

i need some one tell me is the connection between the CMFB and the OTA correct or not, because i am not sure i i have to connect the output of the CMFB to the bottom or to the TOP of the cascode.
Click to expand...

Well that depends on your CMFB circuit. But this one looks good. Just beware that your output range is now limited by 2Vsat+Vth+Voverdrive from the positive supply. I do not know if you like that. And be sure that the CMFB circuit have a wide enough differential capability (i.e. make them long enough).
 

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    wael_wael

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thank u so much,
but can i use CMFB circuit with NMOS differenital, i mean, connect the output of CMFB to the TOP of the cascode, NOTE THAT THE FOLDED CASCODE IS USING PMOS DIFFERENTIAL PAIR
regds
 

Yes you can. It depends on your input and output common mode level of the application.

By building it that way your input and output will have a different common mode range. It looks weird. But I do not know what kind of application you have in mind.
 

i am using it for Trak/Hold circuit, so now i doing the circuit with different architecture as you see here and i think there no big difference, is it right.
best regards
 

The first architecture is better than the second one. You can see a comparison of CM control circuits in this paper here:

#1083776
 

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