High frequency generator for use with loads subject to great impedance

Abstract

A power amplifier driven in the switching mode is started into oscillation by a pulse from an auxiliary oscillator when it is first activated. Reactive currents thereby resulting in an output filter network, containing a series resonant circuit followed by a parallel resonant circuit which is connected across a load impedance, then produce the switching in of a feedback circuit, after which the amplifier oscillates in a self-excited mode, at a frequency determined by the tuning of the resonant circuits. Branch circuits connected to the output filter network determine whether the load impedance is above or below the characteristic impedance value, which is a function of the inductance in the series resonant circuit and the capacitance in the parallel resonant circuit, and control an electronic switch which enables current-driven feedback when the load impedance is below the characteristic value and enables voltage feedback when the load impedance is above the characteristic value, thus providing the more stable type of feedback consistently, even though the load may fluctuate greatly. The electronic switch preferably operates with some hysteresis. This high frequency power oscillator adjusts itself automatically to mistuning of the output filter network as well as to large fluctuations of load impedance. The loss power in the amplifying elements of the power amplifier is very low and the efficiency very high.