Design and optimization of a high-efficiency current-biased reverse load modulated power amplifier with impedance and performance constraints

Abstract

We propose an optimization method based on evolutionary computation for the design of broadband high-efficiency current-biased reverse load-modulation power amplifiers (CB-RLM PAs). First, given the reverse load-modulation characteristics of CB-RLM PAs, a comprehensive objective function is proposed that combines multi-state impedance trajectory constraints with in-band performance deviations. For the saturation and 6 dB power back-off (PBO) states, approximately optimal impedance regions on the Smith chart are derived using impedance constraint circles based on load-pull simulations. These regions are used together with in-band performance deviations (e.g., saturated efficiency, 6 dB PBO efficiency, and saturated output power) for matching network optimization and design. Second, a multi-objective evolutionary algorithm based on decomposition with adaptive weights, neighborhood, and global replacement is integrated with harmonic balance simulations to optimize design parameters and evaluate performance. Finally, to validate the proposed method, a broadband CB-RLM PA operating from 0.6 to 1.8 GHz is designed and fabricated. Measurement results show that the efficiencies at saturation, 6 dB PBO, and 8 dB PBO all exceed 43.6%, with saturated output power being maintained at 40.9–41.5 dBm, which confirms the feasibility and effectiveness of the proposed broadband high-efficiency CB-RLM PA optimization and design approach.

1. Introduction

With the modern communication systems advancing toward higher data rates and broader coverage, the peak-to-average power ratio (PAPR) of modulated signals increases significantly (Iqbal et al., 2025). This trend imposes more stringent requirements on the design of power amplifiers (PAs) with high efficiency at a larger power back-off (PBO) (Zhou et al., 2022; Giofrè et al., 2024; Zhang Y et al., 2025b). So, the Doherty power amplifier (DPA) has attracted wide attention because its circuit is relatively simple and it maintains high efficiency and linearity at PBO. However, conventional DPAs typically rely on a quarter-wavelength (λ/4) impedance transformer to realize the required load modulation; the dispersive characteristic of this element severely limits the effective operating bandwidth in broadband operation (Doherty, 1936; Akbarpour et al., 2012).

To extend the bandwidth, various improvements have been proposed, such as reactance compensation (Xia et al., 2016; Yang et al., 2019; Xiao F et al., 2021), modified load-modulation structures (Pang et al., 2015; Zhou et al., 2017), and reciprocal bias configurations (Gao et al., 2022; Zhang Y et al., 2025a). Although these methods expand the operating band to some extent, they remain fundamentally constrained by the λ/4 transformer architecture, which makes it difficult to achieve a large fractional bandwidth while ensuring high efficiency. To address the aforementioned issues, Akbarpour et al. (2017) proposed a novel power amplifier architecture using a constant current source at the drain for direct current (DC) biasing. By exploiting the reverse load-modulation behavior of current-biased transistors, the operating band at PBO can be effectively expanded without a conventional λ/4 impedance transformer. Therefore, this architecture is a promising approach to broadband high-efficiency PA design.

However, owing to the intrinsic characteristics of transistors and the reverse load-modulation mechanism, it remains difficult to match impedances that meet high-efficiency requirements across different operating states over a wide band. Given its excellent global search capabilities, evolutionary computation has been widely used in radio frequency (RF) design and automated optimization (Zhang H et al., 2024; Fan et al., 2025; Hong et al., 2025; Jin et al., 2025; Ni et al., 2025b, 2025c). Chen et al. (2020) used the multi-objective evolutionary algorithm based on decomposition (MOEA/D) to optimize output power, efficiency, and gain for a high-efficiency DPA at 3.5 GHz. In Kong et al. (2024), MOEA/D was applied to optimize the output matching network (OMN) impedances for different operating states. Although these studies have made progress, the constant current-biased reverse load-modulation power amplifier (CB-RLM PA) has a unique load-modulation mechanism, and how to apply this methodology effectively to CB-RLM PA design remains to be further explored.