A linearity improvement technique for two-stage amplifiers with second harmonic driving

This paper presents a linearity improvement technique for two-stage nonlinear amplifiers. Third-order intermodulation terms (IM3) at the output of an amplifier can be cancelled out by second harmonic injection at the input. The first stage is biased at highly nonlinear bias point to generate second harmonic. A dual-band interstage network between the first stage and the second stage is implemented to cover the fundamental and the second harmonic frequencies. Then, large second harmonic terms are injected to the input of the second stage. With appropriate biasing of two amplifiers and second harmonic phase shifting at the interstage network, the IM3s are cancelled out at the output of the second stage. A two-stage amplifier has been designed and implemented with discrete transistors on a PCB. The measurement results show 10-dB suppression on the IM3 at 920 MHz.