For the Wilson current source of Fig. 2, the input current of the mirror is formula_25. The base-emitter voltages, formula_26, are typically between 0.5 and 0.75 volts so some authors approximate this result as formula_27. The output current is thus substantially dependent only on VCC and R1 and the circuit acts as a constant current source, that is, the current remains constant with variations in the impedance of the load. However, variations in VCC or changes in the value of R1 due to temperature will be reflected in variations in the output current. This method of direct generation of a reference current from the power supply using a resistor rarely has adequate stability for practical applications and more complex circuits are used to provide reference currents independent of temperature and supply voltages.