Loop Gain and Phase margin

Thread Starter


Joined Nov 26, 2023
Hello All,

I am trying to simulate this circuit in LTspice and I am new to circuit design and LTspice. How do I achieve the following things? How should I measure the loop gain and phase margin? I have put the ac source for the gain at the current sense feedback. Is it correct to measure the loop gain?

1. the gain and phase margin of the open loop system
2. the gain and bandwidth of the closed loop system
3. along with a transient simulation.


Joined Feb 24, 2006
You should consult the LTspice Help File for the following article on "The Method of Middlebrook". I'm including a short excerpt:

How to get a Bode Plot from a SMPS
Laboratory measurement of a SMPS loop gain is automated with commercial instrumentation pioneered by Venable Corporation and now also available from other companies. The technique of using injected test signals and Fourier analysis is called Frequency Response Analysis(FRA). While this method is routine in the lab, not everyone is aware of how to use it simulation. This article explains how to do FRA in LTspice XVII. The method uses the voltage gain part of the Middlebrook method, .measure statements to do the Fourier transform, a step statement to sweep frequency, and the feature in LTspice that allows one to plot the results of .measure statements. In reading through the steps below, you might want to refer to the working FRA examples that are part of the general LTspice XVII release typically installed in directory

LTspice 17.1.15 has additional features which simplify the method used in LTspice XVII version17.0.37