Thevenin’s Theorem

The Thevenin’s theorem,basically gives the equivalent voltage source corresponding to an active network.If a linear, active, bilateral network is considered across one of its ports, then it can be replaced by an equivalent voltage source (Thevenin’s voltage source) and an equivalent series impedance (Thevenin’s impedance). 

        Since the two sides are identical, they must be true for all conditions. Thus if we compare the voltage across the port in each case under open circuit conditions, and measure the input impedance of the network with the sources removed (voltage sources short-circuited and current sources open-circuited), then 

Ethevenin=  Voc, and 
Zthevenin=  Zin