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)?