# Port (electric circuit theory)

In electrical circuit theory, a port is a pair of terminals (of an electrical element) connecting an electrical network or circuit to an external circuit, a point of entry or exit for electrical energy. A port consists of two nodes (terminals) connected to an outside circuit, that meets the port condition; the currents flowing into the two nodes must be equal and opposite.

Basic circuit elements such as resistors, capacitors, inductors and independent sources form the most elementary kind of one-port circuits. More generally, a one-port circuit can have any number of passive elements, independent and dependent sources, and nodes. Often, once one of these circuits has been designed, we are no longer interested in how it works, and are happy to represent it by its behavior at its port. By using techniques we have already learned, we can reduce it to a Thévenin or Norton equivalent circuit.

Some devices have more than two terminals, for example, BJTs with three and MOSFETs with three or four. In these cases, it is impossible to represent them as one-port circuits, and two-port circuits are the only option. Two-port circuits have the same application as one-port circuits, in that, they allow us to consider only the behavior of a circuit and not unnecessary internal detail.

There are four variables $$v_1$$, $$v_2$$, $$i_1$$ and $$i_2$$ in a two-port network as shown in the figure. Out of this, we can choose two variables as independent and another two variables as a dependent. So, we will get six possible pairs of equations. These equations represent the dependent variables in terms of independent variables. The coefficients of independent variables are called parameters. So, each pair of equations will give a set of four parameters.