DC analysis gain differs from AC analysis gain

Discussion in 'Cadence' started by Amr Kassem, Jul 7, 2009.

  1. Amr Kassem

    Amr Kassem Guest

    When I design a simple common source amplifier to give a gain of 100
    for example. The AC analysis indicates that the gain is actually 100.
    But when I do DC analysis and plot the derivative of Vout against Vin
    I see a gain of 30. Same happens when I use cascode amplifiers and
    even aim for a gain of 5000. I see a gain of 40 from the DC analysis
    and 5000 from the AC analysis.

    I tried removing the load capacitor but nothing changes. Any idea what
    the reason for that could be?
    Amr Kassem, Jul 7, 2009
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  2. Amr Kassem

    pavan.s.pai Guest

    Hi Amr,

    Whatever you are seeing is right . It is bound to happen that way .
    Because gain given by AC analysis is at the Qpoint or Bias Point
    and provided signal is small compared to the bias point of ckt.

    You can see the same Gain in DC as that of AC only if you
    sweep you signal about the Q point in small range .

    For example if your Qpoint is 0.9 volt (input common mode) then in
    DC you sweep from about 0.9+/- 100uV.

    In above case you will observe same gain in both.

    With Regards
    Pavan Pai
    pavan.s.pai, Jul 8, 2009
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  3. Amr Kassem

    Amr Kassem Guest

    Does this mean that I should decrease the value of abstol and reltol
    in cadence if I want to see the same gain in both analyses?
    Amr Kassem, Jul 12, 2009
  4. Amr Kassem wrote, on 07/12/09 23:17:
    Andrew Beckett, Jul 13, 2009
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