Loop gain measurements spice set-up

One of the most important parameter in feedback circuit characterization is the loop gain. Loop gain direct measurement is not an easy task for the large majority of practical situations because the signals that appear in a system may form various and complicated loops. The classic approach is to break the loop and calculate the transfer around the loop. The loop breaking is not difficult to perform by manual analysis but can be impossible in computer analysis, using a circuit simulator. This paper presents a method, based on signal-flow graph, for loop gain determination without breaking any loop and therefore without circuit determinant or other circuit operating conditions like biasing modification.