By extracting a portion of RF signals from an input side of a multistage RF amplifier
with a detector, and converting extracted signals into envelope signals, low-frequency
second-harmonic distortion components are efficiently extracted. Then, the extracted
low-frequency second-harmonic distortion components are amplified with a low-frequency
amplifier, and phase adjusted with a phase shifter, after which they are injected
into a gate or base bias of the final stage of the multistage RF amplifier. As
a result, the low-frequency second-harmonic distortion components are converted
into third-harmonic distortion due to the non-linearity of transistors, and the
third-harmonic distortion thus obtained cancels out the third-harmonic distortion
originally present in the multistage RF amplifier.
