I'd like to calculate the gain of a common emitter transistor circuit I'm looking at that has a a constant current source as the emitter load. Now I know the gain is going to be -gm(Re||RL) but how do I calculate what the effective resistance of the current source is? The current source I'm using is one that uses a Zener diode to keep a constant voltage across base to emitter junction, and thus a constant current through the collector of and the emitter to collector of the input transistor. A tangentially related question is, if I were using a Darlington pair as the device connected to the constant current source instead of a transistor, how would I go about calculating the transconductance of the Darlington? Thanks for any assistance!