A ballast circuit for controlling preheating, ignition or performing dimming of a gas discharge lamp such as a compact fluorescent lamp is disclosed. The ballast circuit has an inverter connected to a pair of input terminals for receiving a supply voltage, a base drive transformer connected to the switching transistor inverter to provide a drive signal, a resonant circuit connected to the switching transistor inverter, and a loading circuit connected to the base drive transformer. The base drive transformer includes a primary winding and a secondary winding set. The loading circuit is adapted for at least temporarily saturating the base drive transformer and thus effecting in the resonant circuit an oscillating frequency different from a natural resonant frequency of the resonant circuit.