The input impedance of a current mirror is the ratio of the change in input voltage (the potential from the input terminal to ground in Figures 1 and 2) to the change in input current that causes it. Since the change in output current is very nearly equal to any change in input current, the change in the base-emitter voltage of Q3 is formula_55. Equation (3) shows that the collector of Q2 changes by nearly the same amount, so formula_56. The input voltage is the sum of the base-emitter voltages of Q2 and Q3; the collector currents of Q2 and Q3 are nearly equal implying that formula_57. The input impedance is formula_58. Using the standard formula for formula_59 leads to: