Generators used for industrial applications such as heating dielectric materials have been self-oscillating circuits composed of an amplifier provided with a channel bringing back part of the available output power from the said amplifier to an input of the amplifier. The stability of the output frequency of the amplified wave is achieved by a resonant circuit with a high over voltage coefficient. Considering the increasingly strict requirements about stability of the frequencies produced and tolerated variations with respect to authorized frequencies, it was necessary to design amplifiers amplifying a very stable frequency wave for example produced by a quartz oscillator. This type of generator is then capable of respecting imposed standards about the frequency band to be respected. For information, the following is a list of frequencies authorized in Europe for industrial uses such as heating of dielectric materials:
6.78 MHz within plus or minus 15 kHz,
13.56 MHz within plus or minus 7 kHz,
27.12 MHz within plus or minus 163 kHz,
40.68 MHz within plus or minus 20 kHz,
433.92 MHz within plus or minus 870 kHz.
Generators capable of respecting these frequency stability constraints comprise a quartz controlled oscillator stage supplying one or several frequency divider or multiplier circuits, and possibly a phase loop to guarantee a very stable frequency. The output from the oscillator stage is then input to an intermediate amplification stage composed of one or several amplifier stages and finally an output stage powered by the intermediate amplification stage. The invention relates to this output amplifier stage.
A known amplifier stage consists of a power electronic tube capable of amplifying the received input power by about 20 times. For example, the known stage may be composed of a power triode or a tetrode. In both cases, the electronic tube used operates in class B or C, in other words a current only passes through the tube for a time less than or equal to the half-period of the fundamental signal received as input.
The current produced by the tube excites a resonant circuit with a coaxial output. The over voltage factor of the resonant circuit is usually of the order of ten when a load supplied by the said generator has an impedance equal to the nominal impedance of the generator, usually about 50 ohms. This is the so-called “50 ohms” technique. This technique enables a particularly stable operation of the generator in terms of power and particularly the output frequency. In general, changes to the load impedance, for example due to a change in the state of the material under the effect of temperature, are compensated by providing an adaptor box between the output from the generator output stage and the load, with the function of compensating for the said variations in the impedance of the load applied to the generator.
An example embodiment of a triode amplifier part according to prior art is shown in FIG. 1. This example diagrammatically shows only the tube 2 and the intermediate amplifier 24. The tube 2 is a triode composed of a cathode 4, a grid 3 and an anode 5. An input 25 to the tube 2 connected to the cathode 4 is coupled to an output 23 from the intermediate amplifier 24. This intermediate amplifier has an input 29 coupled to an oscillator not shown that outputs a signal at the required frequency.
Operation and orders of magnitude are described below:
The tube may for example have a power gain of 20. To obtain an output power of 50 kW, a power of 2.5 kW has to be applied on its input 25. This power of 2.5 kW must be output by the intermediate amplifier 24. The power available at the output from the quartz oscillator (not shown) is 5 Watts, and the intermediate amplifier must have a very high power gain. Thus, it can be seen that according to prior art, an intermediate amplifier is required with a power gain of about 500 and outputting a power of about 2.5 kW.
This difficulty is usually avoided in prior art by using triode generators to produce output powers up to 30 kW. Tetrodes are preferred for higher powers.
As mentioned above, a triode amplifier with a power gain of the order of 20 is used. Thus, the intermediate stage before the amplifier output stage must output, for example 1 kW, for a generator with an output power of 20 kW. An intermediate amplifier capable of producing a power of the order of one kW is the maximum that is achievable with routinely used transistors such that this type of amplifier can be produced at a reasonable price.
For higher power generators, for example as described with reference to FIG. 1, the difficulties in making an intermediate stage and consequently the price increase quickly. The gain of triode amplifiers remains constant in class B and C; therefore the intermediate stage must output a power proportional to the required output power. An intermediate power amplifier outputting a high power may be obtained by replacing the transistors by a tube. This replacement is expensive firstly due to the price of the tube, and secondly due to the need for an additional high voltage power supply to operate the said tube. Under these conditions, rather than using such an intermediate stage with a tube, it is preferred to use a tetrode to build high power high frequency generators, because a tetrode can have a power gain between 50 and 100. Thus, although the tube itself is more expensive and it also requires an additional electronic power supply to polarize the additional screen grid, the price of the generator remains lower due to the resulting simplification of the intermediate amplifier. The main difficulty with generators using a tetrode is due to the need to neutrodine the tube by a suitable counter-reaction circuit. This circuit is difficult to make. It must be placed very close to the tube and it is difficult to adjust, particularly because it is sensitive particularly to thermal variations induced by the tube. Therefore, there is a need for an easy-to-make generator output amplifier outputting a high frequency high power current that does not require an intermediate amplifier outputting a relatively high power.