A circuit is a current source only to the extent that its output current is independent of its output voltage. In the circuits of Figures 1 and 2, the output voltage of importance is the potential from the collector of Q3 to ground. The measure of that independence is the output impedance of the circuit, the ratio of a change in output voltage to the change in current it causes. Figure 3 shows a small signal model of a Wilson current mirror drawn with a test voltage source, formula_41, attached to the output. The output impedance is the ratio: formula_42. At low frequency this ratio is real and represents an output resistance.