The input voltage of the Wilson current mirror is formula_86. The input node is a low impedance node so its voltage remains approximately constant during operation at formula_87 volts. The equivalent voltage for the standard two-transistor mirror is only one base-emitter drop, formula_26, or half that of the Wilson mirror. The headroom (the potential difference between the opposite power rail and the input of the mirror) available to the circuitry that generates the input current to the mirror is the difference of the power supply voltage and the mirror input voltage. The higher input voltage and higher minimum output voltage of the Wilson current mirror configuration may become problematic for circuits with low supply voltages, particularly supply voltages less than three volts as are sometimes found in battery powered devices.