Investigation of Second Harmonic Injection on the Linearity of Doherty Power Amplifier

In this paper, we investigate the impact of the second harmonic injection on the linearity of a Doherty power amplifier (DPA). The second harmonic signal is injected into different points of the circuit, i.e., the input and output of the main and auxiliary amplifiers, and the results are compared. The comparison shows that injection of the second harmonic signal into the input of the main amplifier is the most effective way to linearize the PA without degrading the efficiency. A DPA is designed in a 3.55 to 3.7 GHz frequency band employing two 25-watt GaN transistors. The designed DPA has an output power of 56-64 watts with a drain efficiency higher than 73% in its saturation. It also provides a drain efficiency of better than 57% at 6dB-output back-off. By applying a second harmonic signal to the input of the main amplifier in single-tone analysis, the gain of the DPA structure becomes flatter. In two-tone simulations, the second harmonic injection significantly improves the third-order intermodulation (IMD3) in a wide range of output power.