# gain of an amplifier.

Discussion in 'Homework Help' started by screen1988, Mar 9, 2013.

1. ### screen1988 Thread Starter Member

Mar 7, 2013
310
4
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?

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Last edited: Mar 10, 2013
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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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.

screen1988 likes this.
3. ### screen1988 Thread Starter Member

Mar 7, 2013
310
4
Hello Jony:
Thank for help.
I see it now. I also see it is useful to have input resistance large.

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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screen1988 likes this.