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I want to plot the AC current gain versus the collector current in common base configuration. Ac current gain "alpha ac" for CB configuration is defined as the change occurring in collector current IC for a given change in emitter current IE when Vcb is held constant.

But if I apply AC signal to the input the current will change, and so will the collector-base voltage Vcb will change.

So the only way to hold Vcb constant I shorted the load as shown below and increased IE from 1mA to 100mA in transient analysis in one second, and plotted the expression:

AC current gain D(-Ic(Q1))/D(Ie(Q1)) versus collector current.

enter image description here

Is this way of doing it in transient analysis correct?

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A better way would be to insert a voltage or current source with an AC source and use an AC simulation. This is an example (unrelated to your circuit) below:

enter image description here

Above I have plotted the current gain of the circuit, the circuit needs an AC source to run an AC analysis instead of running a transient analysis. One could find the AC current gain with a transient analysis but only at one frequency (unless they ran a source with a frequency sweep)

Use the small section of the voltage parameters to set the AC amplitude of the source used in simulations.

enter image description here

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