A solid state cathode driver circuit for controllably sinking a current substantially independent of applied voltage includes a main current carrying transistor and first and second field effect transistors connected to a controlled electrode of the current carrying transistor for setting the potential on the control electrode substantially independent of the applied voltages to the controlled terminals. The magnitude of the sunk current is essentially determined by the geometry of the device utilized and is, therefore, essentially constant. Selective activation of the current sink is provided by grounding the control electrode to turn off the device.