Let us consider the scheme shown in Figure 5.1. It is known as a two-port network (or four-terminal network or quadripole or,
simply, 2-port), a sort of expansion with respect to the two-terminal component previously detailed in Chapter 4. A 2-port is simply a network with four terminals arranged into pairs called ports, which serve respectively as input and output for voltages (vi and v0) and currents (ii and i0). Similarly to the aforementioned twoterminal component, the two-port network can be made of a single device or by a set of electrical devices, but also by part of a circuit or even by a complete circuit. It is sometimes useful to consider a complete circuit, or a part of it, as made of an ensemble of two or more two-port networks, so that the problem of solving the complete circuit can be broken into a set of manageable sub-problems, to solve them separately and then to link the sub-problem solutions together.