Decade-Bandwidth RF-Input Pseudo-Doherty Load Modulated Balanced Amplifier using Signal-Flow-Based Phase Alignment Design

TL;DR

The paper tackles the challenge of sustaining load modulation over a broad in-band frequency range in load-modulated power amplifiers. It introduces a signal-flow-graph framework that yields a frequency-agnostic phase-alignment condition, enabling independent optimization of the BA and CA in a pseudo-Doherty LMBA configuration. A decade-bandwidth PD-LMBA prototype is designed using GaN devices, with a CA at 10 W and two BA devices at 15 W, operating from 0.2 to 2 GHz; measurements show peak efficiencies reaching 51–72% and 10-dB OBO efficiencies of 44–62%, validating the approach. The work provides a rigorous design methodology, explains the wideband behavior of LMBA, and demonstrates an unprecedented bandwidth improvement over existing DPAs/LMBAs, with potential applicability to other LMBA topologies.

Abstract

This paper reports a first-ever decade-bandwidth pseudo-Doherty load-modulated balanced amplifier (PD-LMBA), designed for emerging 4G/5G communications and multi-band operations. By revisiting the LMBA theory using the signal-flow graph, a frequency-agnostic phase-alignment condition is found that is critical for ensuring intrinsically broadband load modulation behavior. This unique design methodology enables, for the first time, the independent optimization of broadband balanced amplifier (BA, as the peaking) and control amplifier (CA, as the carrier), thus fundamentally addressing the longstanding limits imposed on the design of wideband load-modulated power amplifiers (PAs). To prove the proposed concept, an ultra-wideband RF-input PD-LMBA is designed and developed using GaN technology covering the frequency range from 0.2 to 2 GHz. Experimental results demonstrate an efficiency of 51% to 72% for peak output power and 44% to 62% for 10-dB OBO, respectively.

Figures (7)

• Figure 1: (a) General circuit schematic of PD-LMBA; (b) Comparison with state-of-the-art DPAs/LMBAs. ($f_{high}$/$f_{low}$ refers to the ratio between the upper and lower boundaries of frequency range. Average cross-band efficiency is defined as the average of maximum and minimum efficiency at a specific output power level.)
• Figure 2: Proposed signal-flow graph for the wideband PD-LMBA with circuit implementation using wideband coupled line couplers and GaN transistors
• Figure 3: (a) BA and CA signal path phase offset based on (\ref{['eq:phase']}) at different frequencies. (b) BA Intrinsic load impedance trajectories
• Figure 4: Fabricated PD-LMBA prototype.
• Figure 5: Measured peak output power, gain and efficiency at various OBO levels from $0.2$ to $2$ GHz.