What kind of signal do we have to at the base of a transistor? Is it a current signal or voltage signal? On the other side, what kind of output (amplified) signal do we expect? Is it a current or voltage signal?
They said the output current is current gain times base current, so it seems that they are using current as base signal, but then why do they add a voltage source (AC) to give signal?
Best Answer
(assuming small signals)
If you have a current source, you supply a current, and the base voltage will assume whatever voltage is needed to accept that current.
If you have a voltage source, you apply a voltage, and the transistor characteristics and the emitter impedance work together to form an "input impedance" which allows a certain base current depending on the input voltage.
At the output side, you get a current according to beta (from input current, A/A) or transconductance (gm, from the input voltage, A/V).
Then the load resistances on the collector (Rc) determine the voltage Vc according to that current Ic.
Vc = Supply - Ic * Rc.
There is no difference between output current = input current * beta, or output current = input voltage * transconductance : they are just front view and side view of the same face.
(Large signal, e.g. if the voltages exceed the supply rails, get messy. Ignore them until you have a handle on small signal behaviour)