Very often sensitivity of a power amplifier's power output to the impedance of a non-linear load (such as a plasma load) connected to the output of the amplifier gives rise to system instabilities. A common, but not exclusive, method to control the source impedance of a power amplifier to reduce system instabilities is to use a balanced amplifier configuration, as described by K. Kurokawa, “Design theory of balanced transistor amplifiers”, Bell System Technical Journal, October 1965, to apply power to a load.
If the source impedance of the individual amplifiers used in the balanced amplifier configuration is close to the desired output impedance, no substantial penalty is paid for using such a balanced amplifier, however in this case there is no need to use a balanced amplifier configuration to restore the generator source impedance. Many common amplifier technologies, however, such as class D and E amplifiers, have a source impedance that is very different from the desired output impedance. In such a case, a large fraction of the reflected power from a load connected to the output of the balanced amplifier is dissipated in a terminating resistor of an output hybrid of the balanced amplifier. In addition, the individual amplifiers experience larger excursions in the impedance into which they deliver power than the excursion of the load impedance itself. This means that when two amplifiers are combined in a balanced amplifier configuration, the power delivery capability into load impedances different from the nominal load impedance (typically 50 ohm) is typically not much better than that of a single amplifier of the same type.
In industrial applications, it is common to accept reflected power equal to 20% of the output capability of an amplifier, which puts strain on the on the design of the hybrid combiner terminating resistor and individual amplifiers and often limits the voltage-current capability of the balanced amplifier.
It is thus apparent that there is a need in the art for an improved method and apparatus for modifying the output impedance of a generator to a desired value without having to dissipate power in a dump resistor and without needing almost double the power capability compared to a single amplifier configuration to ensure good power delivery into non-standard load impedances