The return ratio is found using Figure 5. In Figure 5, the input current source is set to zero, By cutting the dependent source out of the output side of the circuit, and short-circuiting its terminals, the output side of the circuit is isolated from the input and the feedback loop is broken. A test current "it" replaces the dependent source. Then the return current generated in the dependent source by the test current is found. The return ratio is then "T" = −"ir / it". Using this method, and noticing that "R"D is in parallel with "r"O, "T" is determined as: