Orthogonal Load-Modulated Balanced Amplifier for Back-Off Efficiency Enhancement and Agile Frequency Configuration

This article presents the theory of operation and the design guidelines for an orthogonal load-modulated balanced amplifier (OLMBA) with back-off efficiency enhancement. The single-input balanced amplifier is operated by asymmetrically biasing the devices, similar to a Doherty configuration, where the main amplifier operates in class AB and the auxiliary in class C. The article shows how the reflective termination at the output isolated port, a key element of the OLMBA, can compensate for the nonnull phase delay of the output matching network. This means that the operational frequency can be optimized electronically by controlling the reflective termination at the output isolated port. At the same time, the matching network design can focus on the maximum power case without phase delay constraints. The article also quantifies the impact of losses in the electronically configurable elements and the power-handling requirements. The theory is validated with the design, simulation, and experimental characterization of a prototype based on GaN active devices. The tunable reflective termination is achieved through an RF single-pole, four-throw switch that switches between fixed reflective loads. The measurements on the prototype compare well with the state-of-the-art. When tested on the 0.8–1.6-GHz octave bandwidth, the single-tone maximum output power is between 45 and 47.3 dBm, and the 6-dB back-off efficiency is better than 48%. The system-level measurements using a 5-MHz channel signal with 6-dB crest factor show, at different carrier frequencies across the band, an average output power between 40 and 42 dBm, an efficiency better than 41%, and the linearizability of the prototype to meet spectral distortion requirements.