The two-port network has a pair of input terminals, shown as PQ in Figure below, & a pair of output terminals, shown as RS in the diagram below. When a voltage V1 is supplied to terminals PQ, the input & output currents,I1 and I2, flow & an output voltage V2 is produced.There are therefore 4 variable quantities V1, I1,V2 and I2 for a two-port network. If the elements are supposed to be linear, then there are a number of ways in which we can write the relationships. One such way is termed ABCD parameters which we consider in this tutorial; there are others few parameters, such as z-, y-, h- and g- parameters that are not discussed here.
Since A & D are ratios, they possess no units, while B is an impedance and is measured in ohms, and C an admittance in siemens. These parameters are often known as the general circuit or transmission parameters & these are in general used for the analyzing the two-port networks which are heavy current circuits or power frequency transmission lines.