Hi guys,
can anyone tell me the reason why we set Vi=0 (short circuit the input side) when we want to find the output impedance Zo but we don't do the same by setting Vo=0 when we find the input impedance Zi????
and why do we let Io=0 (open circuit the output side) to find the gain Av????

thank you

 

I don't have a full grasp of the theory behind the methodology, but here 's what I think:

The output impedance refers to the impedance the load "sees" when looking back from the output towards its driver. Since you want to test the output resistance with a test voltage source, you don't want those results skewed by an arbitrary voltage input that will alter the output resistance result.

As for the input resistance and gain, it is more practical. If you load the output of a circuit, you ask it to give current to that load, and that results in a drop of the output voltage. When you measure input impedance and gain you want the absolute best of those measurements and any load will drop those values.

It's still a bit fuzzy, but I posted so that I can be re-phrased by someone else more properly.