A ballast circuit for operating a lamp provided with a voltage-current converter, the ballast circuit having a differential amplifier provided with a first input terminal for connection to a reference voltage source for generating a reference voltage, a second input terminal for connection of a reference resistor, and an output. A first current generator supplies a first current to the reference resistor. A current amplifier generates a second current and is provided with an input coupled to the output of the differential amplifier. The differential amplifier is provided with a low-pass filter. The current amplifier on the one hand and the current generator and the reference resistor on the other hand exclusively comprise mutually separate components. The ballast circuit is in addition provided with a current control circuit coupled to the current amplifier and to the current generator for influencing the first current dependent upon the second current. As a result, an interference signal present at the second input terminal causes no appreciable interference in the second current.