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Gain Margin, Open or Closed Loop?

  1. Jun 1, 2009 #1
    1. The problem statement, all variables and given/known data
    I'm given a closed loop transfer function with a gain, K=1 and a plant H(s). I need to find the gain margin of the system.

    2. Relevant equations
    I know how to solve this problem:

    a) I find the frequency where the phase is -180 degrees.
    b) I find the gain at that frequency, lets called that a.
    c) the gain margin is 1/a.

    What I'm confused about is, to find the gain margin, do I use the open loop transfer function K.H or the closed loop transfer function KH/(1+KH)?
     
  2. jcsd
  3. Jun 2, 2009 #2
    You use the open loop TF.
     
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