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The transition frequency (Ft) of a transistor is the frequency at which the current-gain BETA = unity.
You know Ft depends on the DC bias current of the collector Ic. Ft is maximum for a certain value of Ic.
Now if you want to measur Ft you have first to bias the tansistor : fix the base current and VCE, then perform an AC simulation to plot the current gain Ic/Ib (that is Beta) Vs frequency you will find Ft the frequency that gives 0dB current gain
When you perform the AC simulation
you have to use a current generator to fix the DC value of the base current. In this generator put AC value = 1
The measured AC value of the Collector current Ic gives you the current gain : Beta (because the input current (AC value) = 1)
Dont worry about this AC value of in put current if it seems you to be high, During AC simulations SPICE or any athor simulator linearize the circuit and there is no saturation possibility...
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