148 Chapter 4 Circuit Theorems

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148 Chapter 4 Circuit Theorems

i = D⁰ = −isc, where isc is the short-circuit current flowing out of terminal a, which is the same as the Norton current IN, i.e.,

$$D_0 = -I_N \tag{4.19}$$

When all the internal independent sources are turned of f, D⁰ = 0 and the circuit can be replaced by an equivalent resistance Req (or an equivalent conductance Geq = 1∕Req), which is the same as RTh or RN. Thus, Eq. (4.19) becomes

$$i = \frac{v}{R_{\text{Th}}} - I_N \tag{4.20}$$

This expresses the v oltage-current relation at terminals a-b of the cir cuit in Fig. 4.47(b), confirming that the two circuits in Fig. 4.47(a) and 4.47(b) are equivalent.