Compact Load Network Having a Controlled Electrical Length for Doherty Power Amplifier

Abstract

The load network of the carrier amplifier for the conventional Doherty power amplifier (DPA) consists of an impedance matching circuit, an offset line, and a λ/4 transmission line (TL), so that the overall electrical length of the network can easily exceed the minimum value of 90°. Then for appropriate impedance modulation, it should be 270° with an additional 180°. This excessive electrical length of the load matching network limits the bandwidth at either the low-power or peak-power level. In this paper, a compact quasi-lumped λ/4 impedance transformer (ITF) having simultaneous multiple functions of impedance matching and load impedance modulation with a controlled electrical length of 90° is presented. The proposed load network includes the internal components of the transistor, the simplest high-pass network using a shunt inductor, and a low-pass L-C network. Using the optimized value of the shunt inductor, the electrical length of the load network can be adjusted to 90°, while other components are accordingly changed to match the optimum load impedance. To verify the proposed load network, a DPA was designed and implemented using 10 W GaN-HEMTs for both carrier and peaking amplifiers. Using a 5G New Radio (NR) signal with signal bandwidth of 100 MHz and peak-to-average power ratio (PAPR) of 7.8 dB, a drain efficiency (DE) of 47 - 54.2%, and adjacent channel leakage power ratio (ACLR) of –27.9 - –23 dBc were achieved at an average output power level of 35.8 - 36.3 dBm for the frequency band of 3.4 - 3.8 GHz.