RF-Input Sequential Circulator Load Modulated Amplifier with Back-Off Efficiency Enhancement

Paper in proceeding, 2025

This paper presents the analysis and design of a single RF-input sequential circulator load modulated amplifier (SCLMA), which operates similarly to a sequential power amplifier (PA). The sequential architecture extends the back-off efficiency range without requiring load modulation. We provide a theoretical analysis of the proposed design, examining its performance under varying isolation levels and losses. Additionally, we outline a detailed design methodology for implementing the single RF-input SCLMA prototype. To validate the concept, we constructed a prototype RF-input SCLMA using GaN HEMT transistors and a surface-mount (SMT) circulator. Measured results demonstrate a peak output power drain efficiency of 62% and a 9.5-dB back-off efficiency of 52%, achieved at a peak delivered output power of 44.9 dBm at 3.35 GHz. Furthermore, by adjusting the main bias voltage, the amplifier attains a 12-dB back-off efficiency of 50% at the same operating frequency.