There is disclosed a driver circuit for a power amplifier of class D type having a segmented architecture with at least one current branch which can be powered down in a low power mode of operation of the circuit. The branch comprising a switch with a cascode MOS transistor, the circuit further comprises a bias circuitry adapted for dynamically generating a dynamic bias control signal so as to cause the cascode MOS transistor of the switch to be ‘Off’ in the low power mode.