An electronic ballast for driving a gas discharge lamp comprises a measurement circuit for measuring a lamp current flowing through the lamp and a lamp voltage produced across the lamp. The ballast comprises a first winding magnetically coupled to a main transformer of an inverter circuit, and a second winding magnetically coupled to a resonant inductor of a resonant tank circuit. The first and second windings are coupled in series electrical connection to generate a voltage representative of the magnitude of the lamp voltage. The ballast further comprises a current transformer having primary windings coupled in series with the electrodes of the lamp. The measurement circuit integrates the current conducted through secondary windings of the current transformer only during every other half-cycle of the lamp voltage to generate a control signal representative of the magnitude of the lamp current that is in-phase with the lamp voltage.