If the two port is a bilateral network, reciprocity is present; it is easy to show that this results in the equality of z12 and z21.
Equivalent circuits may again be obtained from an inspection of (28) and (29); their construction is facilitated by adding and subtracting either z12I1 to (29) or z21I2 to (28). Each of these equivalent circuits contains a dependent voltage source.
Let us leave the derivation of such an equivalent to some leisure moment, such as during the next hourly exam, and consider next an example of an example of a rather general nature. Can we construct a general Thevenin equivalent of the two port, as viewed from the output terminals It is necessary first to assume a specific input circuit configuration, and we shall select an in dependent voltage source Vg (positive sign at top) in series with a generator impedance Zg. Thus