Radio frequency amplifiers use linear amplifiers that are not always “clean,” and in operation, often produce intermodulation distortion. This distortion creates interference at the operating frequencies used by radio frequency, cellular, and other similar communications circuits. As cellular and other communication systems become more important and prevalent, it is necessary to reduce the intermodulation distortion produced in amplifier systems, and especially linear power amplifiers, which play an important part in these systems.
Some prior art techniques suppress intermodulation distortion by generating an inverse distortion signal and applying it to the input end of the amplifier. Another prior art technique uses a negative feedback system for negatively feeding back the distortion signals with an output signal at its input end. Other prior art techniques use a feed forward system that extracts the intermodulation distortion signals, and inverts the phase of those signals to cancel the problematic signals. This type of system is widely used in base stations, satellites, and mobile communication systems. This system has high efficiency in suppressing the signals as compared to other types of linear power amplifiers, but is typically complex in structure and large. Sometimes the circuits cause increased power losses. Decreasing intermodulation distortion becomes even more important in multiple carrier linear amplifiers that are operative with multiple carriers in communications systems. Even small amounts of intermodulation distortion can create extreme performance problems.