Gain Margin, Open or Closed Loop?

[saad87]

Homework Statement

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.

Homework 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, let's 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)?

 

[overt26]

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)?

You use the open loop TF.