Impact of Nonlinear $C_{bc}$ on HBT Doherty Power Amplifiers

Theoretically, the load modulation of carrier amplifiers in Doherty power amplifiers (PAs) achieves a 3-dB higher gain and a higher power-added efficiency (PAE) at a 3-dB back-off power due to the two times larger load impedance (2R_opt), compared to those at the peak power. In this paper, we analyze the impact of nonlinear C_bc on the gain behavior of heterojunction bipolar transistors (HBT) that shows nearly identical gain and a lower PAE with 2R_opt. In the environment, the gain distribution of carrier and peaking amplifiers is explained for the linear operation of the Doherty PA with the imperfect load modulation. We also explore harmonic controls for the Doherty PA using class-F matching network to enhance efficiency at a given back-off power and propose a method using a second-harmonic short circuit at the input of the carrier amplifier to compensate for the signal distortion caused by nonlinear C_bc. To validate the efficiency improvement while achieving high linearity, a Doherty PA is fabricated and tested using a long-term evolution signal having a 10-MHz bandwidth and a 16-quadrature amplitude modulation (16-QAM) 7.5-dB peak-to-average power ratio. The proposed Doherty PA achieves a PAE of 45% and an adjacent channel leakage ratio of -34 dBc at an average output power of 29 dBm. The total consumed power of the proposed PA at this output power represents a reduction by 4.5% and 29%, respectively, to that of a conventional class-F-type Doherty and a class-AB PA.