A variable voltage generator circuit is described for generating, from a substantially constant supply voltage VS, a variable high-voltage control voltage VC for a variable power capacitor (1) having a variable-permittivity dielectric. The control voltage generator circuit comprises a top-up circuit (10) for maintaining the voltage VCin on an input capacitor (12) at least at supply voltage VS, and a bidirectional DC-DC converter circuit (20) having a variable voltage conversion factor G controlled by control input signal (27). The bidirectional DC-DC converter (20) is arranged to convert voltage, at the voltage conversion factor G, between the input capacitor voltage VCin and the output voltage VC. When VC G×VCin, the DC-DC converter circuit (20) uses charge stored in the load capacitance (1) to charge the input capacitor (12).