The step-up transformer circuit 12 may comprise a voltage boost transformer 16 having an input 13 connected to the output 14 of the inverter 11 and a rectifier 18 downstream from the transformer 16 having an output 15 connected to the oscillator 4.

In a particular configuration of the device, designed for food processing, the voltage V_INV at the output 14 of the inverter 11 may be an AC voltage, with an value ranging from 0 V to 400 V, the output voltage V_TR of the transformer 16 will also be an AC voltage with a rms value not exceeding 9000 V, whereas the DC voltage V_CC at the output of the rectifier 18 is limited to about 12000 V.

Preferably, as best shown in FIG. 1, the oscillator 4 may comprise at least one triode 19 having two power terminals, i.e. the anode A and the cathode K respectively.

The triode 19 may also comprise a grid terminal G which is adapted to receive a control signal designed to vary the electrical parameters of the signal between the anode A and the cathode K.

FIG. 2 shows a voltage-current characteristic of a bias triode whose instantaneous operating point lies along a substantially straight segment T.

Particularly, a controlled displacement of the operating point P along such segment will cause a corresponding predetermined variation of the AC voltage V_RF between the anode A and the cathode K, which is represented on the x-axis in the diagram of FIG. 2.