OTA problem

Discussion in 'Cadence' started by Kuan Zhou, Nov 12, 2003.

  1. Kuan Zhou

    Kuan Zhou Guest

    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, Nov 12, 2003
    #1
  2. Kuan Zhou

    Toctory Yang Guest

    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.
     
    Toctory Yang, Nov 13, 2003
    #2
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