Return ratio

[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.

Figure 1: Collector-to-base biased bipolar amplifier
Figure 2: Left - small-signal circuit corresponding to Figure 1; center - inserting independent source and marking leads to be cut; right - cutting the dependent source free and short-circuiting broken leads