Signal-Flow-Based Analysis and Design of Pseudo-Doherty Load-Modulated Balanced Amplifier Toward Unlimited RF Bandwidth

This article reports a first-ever decade-bandwidth pseudo-Doherty load-modulated balanced amplifier (PD-LMBA), designed for emerging 4G/5G communications and multiband operations. By revisiting the load-modulated balanced amplifier (LMBA) theory using an S-matrix-based signal-flow approach, a generalized theory for wideband LMBA operation is developed, taking into account the frequency-dependent nature of all components. In addition, by analyzing the signal-flow behavior of LMBA, a frequency-agnostic phase-alignment condition is identified as critical for ensuring intrinsic broadband load modulation. 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 ultrawideband 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%–72% for peak output power and 44%–62% for 10-dB output power back-off (OBO), respectively.