Vittorio Camarchia

This work presents a GaN-based digital class-E Doherty power amplifier module for the 5G FR1 frequency band. The design integrates two identical class-E MMICs, driven by digital pulse-width-modulated signals, in a two-way wideband Doherty configuration. This configuration is co-optimized together with the class-E components of the amplifiers to achieve high efficiency over an extended output back-off range of up to 9 dB. At 7.5 dB back-off, the measured final-stage drain efficiency reaches 68% with a 3.4 GHz single-tone input. Across the (3.2–3.6) GHz band, the prototype maintains a minimum efficiency of 52% at 9 dB output power back-off, with a maximum saturated output power of 32.8 dBm. At 3.3 GHz, the module achieves efficiencies of 60% and 59% for 20 and 100 MHz digital OFDM signals with 9 dB peak-to-average power ratio, respectively.

Two fully integrated GaAs Doherty power amplifiers are presented for 5G FR1 applications requiring operation over the n77 (3.3-4.2 GHz) and n79 (4.4-5.0 GHz) bands. To assess the capabilities of different Doherty combiners in wideband scenarios, two versions are designed and compared: a first implementation using a direct quarter-wave-based combiner and a second version employing an inverted topology, theoretically expected to offer more broadband performance. Both designs employ stacked transistor cells and a balanced architecture to enhance output power and load mismatch resilience. Experimental results show that both implementations achieve comparable performance as the additional losses introduced by the inverted combiner limit its theoretical benefits in terms of efficiency and gain uniformity over the full band. Both achieve more than 34.5 dBm saturated output power, 20 dB small-signal gain, and power-added efficiency above 22% over a 6-dB output back-off.

The Doherty power amplifier (Doherty PA) is a popular back-off efficiency enhancement architecture proposed in 1936 for vacuum tubes and later adapted to solid-state transistor technology. Across the years the Doherty PA has evolved into various configurations which include asymmetric and multiway topologies to meet the stringent linearity and efficiency requirements of modern communication standards. Recent research efforts are focused on extending the Doherty concept toward millimeter-wave frequencies and monolithic integration. In this framework critical design trade-offs emerge: the gain penalty intrinsic to the Doherty architecture, often overlooked in favor of efficiency metrics, may become particularly severe and also impact the overall achievable efficiency. Bandwidth further constrains design choices, often forcing trade-offs between peak performance and wideband behavior. This also reduces the available design space to optimize linearity together with efficiency, which ultimately determines whether Doherty PAs provide sufficient advantages to justify deployment in place of non load-modulated class-AB PAs, generally more wideband, robust to process variations and simpler in terms of bias and architecture design.