In an electrosurgical instrument susceptible to varying load impedance, particularly varying load reactance, a fixed frequency radio frequency generator has an output stage (Q
) consisting of at least one output power device and, coupled to the power device, an output network with a load-dependent resonant condition. To compensate for variations in the load reactance, the output network has a dynamically variable capacitor (VC
) which maintains the tuning of the network substantially constant. Variation of the capacitor (VC
) is performed by means of a phase comparator and servo device (
) which are responsive to variations in phase difference occurring in the output stage due to load reactance changes.