The Wilson current mirror achieves the high output impedance of equation (6) by negative feedback rather than by emitter degeneration as cascoded mirrors or sources with resistor degeneration do. The node impedance of the only internal node of the mirror, the node at the emitter of Q3 and the collector of Q2, is quite low. At low frequency, that impedance is given by formula_72. For a device biased at 1 mA having a current gain of 100, this evaluates to 0.26 ohms at 25 deg. C. Any change in output current with output voltage results in a change in the emitter current of Q3 but very little change in the emitter node voltage. The change in formula_3 is fed back through Q2 and Q1 to the input node where it changes the base current of Q3 in a way that reduces the net change in output current, thus closing the feedback loop.