[1] The steps for calculating the return ratio of a source are as follows:[2] These steps may not be feasible when the dependent sources inside the devices are not directly accessible, for example when using built-in "black box" SPICE models or when measuring the return ratio experimentally.
For SPICE simulations, one potential workaround is to manually replace non-linear devices by their small-signal equivalent model, with exposed dependent sources.
A result by Rosenstark shows that return ratio can be calculated by breaking the loop at any unilateral point in the circuit.
[6][7][8] Figure 1 (top right) shows a bipolar amplifier with feedback bias resistor Rf driven by a Norton signal source.
Figure 2 (left panel) shows the corresponding small-signal circuit obtained by replacing the transistor with its hybrid-pi model.