Theory and Design of Wideband Doherty Power Amplifiers

Licentiate thesis, 2012

The Doherty power amplifier (DPA) is one of the most popular power amplifier architectures for obtaining high average efficiency for modern communication signals with high peak-to-average power ratios (PAPR). However, the DPA suffers from often having narrowband performance which limits its capabilities in wideband and/or multi-standard microwave and radio frequency applications. In this thesis, the theoretical and practical bandwidth limitations of the DPA are examined and it is demonstrated, by theory and measurements, how to design high performance and wideband DPAs. A Tee-line network of transmission lines is presented that enables simple design of Doherty combining networks in many situations where traditional approaches would fail. The utility of the Tee-line network is demonstrated by implementation in a 7-8 GHz gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) DPA. Continuous wave measurements showed a power added efficiency (PAE) larger than 30 % at 9 dB output power back-off across a 6.7-7.8 GHz frequency range. The maximum output power was maintained within 35 ± 0.5 dBm from 6.6 to 8.5 GHz. Linearized modulated measurements, employing a signal with 7.8 dB (PAPR), reported average PAE larger than 35 %, with an average output power of 27.5 ± 0.2 dBm and an adjacent channel power ratio (ACPR) less than -45 dBc, across a 6.8-8.5 GHz frequency range. The results demonstrates state-of-the art performance both in terms of PAE and bandwidth. To extend the inherent bandwidth of the DPAs a new type of power amplifier, based on the DPA topology, is presented. It is theoretically shown that the proposed amplifier can simultaneously provide high efficiency at full output power and at power back-off, as well as reconfiguration of the efficiency in power back-off without the need of tunable elements. The theoretical findings were confirmed by the fabrication and measurements of a demonstrator circuit. Measurements reported higher than 48 % drain efficiency at both full output power and at power back-off from 1.5-2.4 GHz, thereby demonstrating a unique combination of high fractional bandwidth and high back-off efficiency. The reported fractional bandwidth that was simultaneously achieved at full output power and at 6 dB output power back-off is to the author’s knowledge larger than what has been reported for any power amplifier architecture