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I'm still missing something.
If I use a test current of 1A at output, I measure 508,7 V (from where I get Zo = 508.7V / 1A = 508.7 ohm).
BUT... I think that 508.7 V from collector to emitter is something crazy, unless maybe you are trying to destroy the transistor. At 508V output, this amplifier just isn't working. Am I wrong?
If I use a test current of 1mA, I measure 508 mV (this seems reasonable), which leads to the same Zo = 508 ohms.
Is it OK to use a 1A test current to find Zo in any circuit, even though that would make an impossible output?