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Doherty amplifier structures for modern microwave communication systems

Lees, Jonathan. 2006. Doherty amplifier structures for modern microwave communication systems. PhD Thesis, Cardiff University.

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Abstract

The Power Amplifier (PA) is a critical component in any mobile communications system with performance that is very sensitive to RF envelope dynamics. Achieving the required linearity demanded by evolving communications systems invariably involves increasing PA complexity at the cost of reducing PA efficiency the consequences of which are severe and include for example reduced operational time for portable communications devices and perhaps less obviously the significant running, capital and thermal management costs associated with mobile communication system base-stations. The Doherty PA is one of a number of elegant architectures that have been developed to address this problem, and although conceived and patented in the 1930's, has only recently become established as a means of enhancing efficiency in microwave PA applications. The Doherty is renowned for its elegant simplicity however, the realisation of functional Doherty PAs using modern microwave devices is problematical and hindered by many hidden complexities, which are in general brought about by the complex, 'load-pulling' action of two active devices that conspire to cause a variety of performance related problems. Although harmonic behaviour is important, understanding device interaction at a fundamental level has been found to be the critical factor in achieving good overall Doherty performance. With this in mind, this thesis concentrates initially on developing an extensive understanding of fundamental device interaction through the use of a novel Doherty measurement approach which involves replacing the classical Doherty's symmetrical input power division arrangement with independent, phase-coherent excitations. The resulting insight has meant that it has been possible to introduce more focused measurement techniques including harmonic analysis and waveform engineering in order to further explore individual device behaviour. The extensive use of harmonic load-pull measurement systems and the direct synthesis of the impedance environments that exist within the Doherty have allowed a number of device technologies to be considered within the application environment resulting in the realisation of GaAs and GaN Doherty prototypes. As a direct result of this analysis, a number of optimisation approaches have been identified that involve the dynamic adjustment of relative input magnitude, relative input phase and relative device bias, which has in turn exposed the various design trade-offs that exist between linearity and efficiency within the Doherty. Other work includes the development of modulated measurement systems and specialised excitations that allow the meaningful comparison between measured single-tone and modulated performance, as well as allowing more comprehensive investigations into Doherty linearity under varying IF impedance conditions.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
ISBN: 9781303205101
Date of First Compliant Deposit: 30 March 2016
Last Modified: 04 Jun 2017 06:03
URI: http://orca.cf.ac.uk/id/eprint/56074

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