OTA problem

[Kuan Zhou]

Hi,
I found a problem in my OTA design. It's basically a two-stage
OTA with a common mode feedback. When I have input with 5u V and
30K Hz, the output looks fine. However, when I increase my frequency
to 1G (the cutoff frquency is 1.6G), the output has big oscillations.
Does any body know how to avoid it?
My design is fully differential. The common mode feedback is basically
four pfets. Two pfets are pre-biased to a fixed voltage. The other two
pfets' gates are connected to the outputs of the OTA. The circuit is
copied from Paul & Gray's fourth addition book.

Thank you !

sincerely
-------------
Kuan Zhou
ECSE department

 

[Toctory Yang]

Hi, Kuang,

Pleasecheck the phase margin of the CMFB loop. It sounds like that. You can
find the related material in P.Gray's textbook.

"Kuan Zhou" <zhouk@rpi.edu> ????
newsine.SOL.3.96.1031112171554.18765A-100000@vcmr-86.server.rpi.edu...

Hi,
I found a problem in my OTA design. It's basically a two-stage
OTA with a common mode feedback. When I have input with 5u V and
30K Hz, the output looks fine. However, when I increase my frequency
to 1G (the cutoff frquency is 1.6G), the output has big oscillations.
Does any body know how to avoid it?
My design is fully differential. The common mode feedback is basically
four pfets. Two pfets are pre-biased to a fixed voltage. The other two
pfets' gates are connected to the outputs of the OTA. The circuit is
copied from Paul & Gray's fourth addition book.

Thank you !

sincerely
-------------
Kuan Zhou
ECSE department