There is provided a method and apparatus for maintaining a bias current that flows through two transistors at a target level. The two transistors are both connected to form a series network between positive and negative voltage supply terminals. The bias current flows through the two transistors when the circuit is at equilibrium, and the threshold voltage of the transistors is controlled by controlling the voltage that is applied to the transistors bulk terminals. In addition to the two transistors, there is provided a control circuit that measures a circuit parameter that is indicative of the level of bias current flowing through the two transistors. In response to the measured parameter, the control circuit adjusts the bulk voltage levels of the two transistors so as to alter the transistors threshold voltages and maintain the level of bias current at a target level.