The attachment is equivalent circuit of an amplifier.
I know that the gain of the circuit is \(-g_{m}R_{L}\). In next exercise require to compute gain if \(r_{\pi }\) (resistor in the input) approach to infinity. Then, I see no difference in gain of the circuit.
It make me curious, if the gain isn't depend on \(r_{\pi }\) then I can choose any value?

 

You don't see any difference in voltage gain because you Vin is a ideal voltage source.
Simply add a serial resistor with Vin that will be represent input signal internal resistance. And now check the gain for Rpi and when Rpi approach to infinity.

 

You don't see any difference in voltage gain because you Vin is a ideal voltage source.
Simply add a serial resistor with Vin that will be represent input signal internal resistance. And now check the gain for Rpi and when Rpi approach to infinity.

Hello Jony:
Thank for help.
I see it now. I also see it is useful to have input resistance large.

 

I'm glad to hear that. And I hope that you see a voltage divider formed by the Rpi and source internal resistance.
http://forum.allaboutcircuits.com/showpost.php?p=153702&postcount=8