Some Useful Network Theorems

Thévenin’s theorem is used to represent a part of a network by a voltage source Vt and a series impedance Zt , as shown in Fig. D.1. Figure D.1(a) shows a network divided into two parts, A and B. In Fig. D.1(b), part A of the network has been replaced by its Thévenin equivalent: a voltage source Vt and a series impedance Zt . Figure D.1(c) illustrates how Vt is to be determined: Simply open-circuit the two terminals of network A and measure (or calculate) the voltage that appears between these two terminals. To determine Zt , we reduce all external (i.e., independent) sources in networkA to zero by short-circuiting voltage sources and open-circuiting current sources. The impedance Zt will be equal to the input impedance of network A after this reduction has been performed, as illustrated in Fig. D.1(d).