I have some doubts about the Scattering Matrix of a transistor seen as a 3 port devices. Precisely, I read (for instance here) that we do not have 9 independent S parameters since there is this relation (6 equations):
So there are only 4 independent S parameters, and this is correct because generally transistors are described as 2 port networks. But I do not understand the proof of these relations. It is shown in these pictures:
First question: is the fact that the sum of current = 0 always verified? Is it a sort of Kirchoff Current Law?
Second question: we are applying equal voltages to the ports, why should this procedure be general, valid for each case?
Third question: why applying equal voltages = each current is 0 ?