Circuit means for efficiently driving an electrodeless discharge lamp

In an electrodeless discharge lamp of the type including an ionizable medium within a sealed envelope capable of emitting radiant energy when subjected to a radio frequency field, an oscillator for generating an output signal at a given radio frequency, an RF amplifier responsive to said oscillator output signal, and an induction coil responsive to the output of said amplifier, said coil being positioned in close physical proximity to said medium in said envelope for coupling to said medium an electric field having a magnitude sufficient to initiate ionization of said medium and a magnetic field for maintaining said ionization, the improvement wherein:

• said amplifier is a Class D amplifier including first and second transistors connected in series and driven alternately on and off by said oscillator output signal, and further comprising:
• a filter network positioned between the junction between said transistors and said induction coil for presenting a low impedance to said given radio frequency and a higher impedance to all other frequencies.

In an electrodeless discharge lamp of the type including an ionizable medium within a sealed envelope capable of emitting radiant energy when subjected to a radio frequency field, an oscillator for generating an output signal at a given radio frequency, an RF amplifier responsive to said oscillator output signal, and an induction coil responsive to the output of said amplifier, said coil being positioned in close physical proximity to said medium in said envelope for coupling to said medium an electric field having a magnitude sufficient to initiate ionization of said medium and a magnetic field for maintaining said ionization, the improvement wherein said amplifier is a Class D amplifier including first and second transistors connected in series and driven alternately on and off by said oscillator output signal, and further comprising:

• a combination filter and matching network operatively coupled between the junction between said transistors and said induction coil for presenting a low impedance to said given radio frequency and a higher impedance to all other frequencies and for matching the impedance of the output of said RF amplifier to the reflected resistance of said discharge in said envelope.

A filter network comprising:

• a tuned circuit including an inductor and a capacitor tuned to a given frequency so as to present a low impedance to said given frequency and a higher impedance to all other frequencies, said inductor comprising a first coil wound on a toroidal magnetic core;
• and
• a second coil wound on said core in out-of-phase relationship with said first coil, a first end only of said second coil being connected to one end of said first coil, the length of said second coil being adjusted to form a parallel resonant circuit at any frequency less than the self resonant frequency of said first coil.