When the circuit is biased at higher currents that maximize the frequency response of the transistor current gain, it is possible to operate a Wilson current mirror with satisfactory results at frequencies up to approximately one tenth of the transition frequency of the transistors. The transition frequency of a bipolar transistor, formula_67, is the frequency at which the short-circuit common-emitter current gain falls to unity. It is effectively the highest frequency for which a transistor may supply useful gain in an amplifier. The transition frequency is a function of the collector current, increasing with increasing current until a broad maximum at a collector current slightly less than what causes the onset of high injection. In simple models of the bipolar transistor when the collector is grounded, formula_68 shows a single-pole frequency response so formula_67 is also the current gain-bandwidth product. Crudely this implies that at formula_70, formula_71. By equation (4) one might expect the magnitude of the ratio of output to input current at that frequency to differ from unity by about 2%.