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I have been trying to derive Av , Ai , Zi , Zo from a two port system model of a transistor. I know that the input voltage must be the voltage at the base to ground, and the output is from collector to ground. It seems that I have a confusion about which current I should consider the input and which current is the output.

In the following diagram the input current shows it is coming from an AC source. However, in the book it states that it is Ib. The output current is clearly shown that it's derived from Rc. Would it be safe to say that Ii comes from RB? enter image description here

When the transistor has a voltage divider network at the base would it change the input current? If so, to which component?

Arcadius
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    What you see in the diagram is an AC small signal equivalent circuit. Which shows the situation for the AC signal only. Thus, the AC signal source (not shown on the diagram) is the source of an input voltage and the input current. So Ib and the rest of the currents are just AC components. Do you get it? https://electronics.stackexchange.com/questions/518032/pi-model-of-common-collector/518561#518561 – G36 Apr 08 '23 at 11:02
  • @G36 Okay it seems that I understand it further now. I know when AC signal is directly connected to the base _Ii=Ib_. However in the OP from the diagram, how would I find _Ii_ to derive _Ai_? – Arcadius Apr 08 '23 at 13:51
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    No, the input current **Iin** is not equal to the base current **Ib**, because you forgot that **Rb** will also load the input signal source. Thus, **Iin = Ib + I_Rb** . And to find **Iin** you need to find **Zin** first. Take a look at this example https://electronics.stackexchange.com/questions/476659/kvl-equations-for-this-small-signal-model/476666#476666 – G36 Apr 08 '23 at 19:56

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