Systems and methods for an electrodeless plasma lamp as described. A drive probe is coupled to the lamp body to provide the primary power for ignition and steady state operation of the lamp. Feedback is used to adjust frequency in response to changing conditions of the lamp during startup. A phase shifter is used to adjust the phase of the power between ignition and steady state operation. A sensor may detect a lamp operating condition that automatically triggers a shift in phase after the fill in the bulb is vaporized. The phase shift may then continue to be adjusted as the plasma heats up and the impedance continues to change. The bias conditions of an amplifier may be changed to change the operating class of the amplifier for different modes of the lamp.