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High efficiency and high linearity Doherty amplifiers for portable wireless communications

Abstract

The design of power amplifiers for wireless systems has become a challenging task due to increasingly stringent requirements on transmitters. The Doherty amplifier is an important candidate for this type of application due to its feature of high efficiency over an extended power range. However, Doherty amplifiers in practice have often failed to exhibit the superb efficiency predicted by theory and their linearity is not always adequate to meet strict specifications of systems with high Peak-to-Average Ratio (PAR) signals such as CDMA systems. The objective of this dissertation is to provide a path for Doherty amplifiers that leads to a simultaneous high efficiency and high linearity performance. First, based on detailed understanding of Doherty amplifier theory and careful investigation of behavior of Doherty amplifiers, a series of guidelines for designing a Doherty amplifier was created. A Doherty amplifier based on GaAs FET devices was accordingly designed in simulation and then fabricated. Simulation and measurement results were compared to provide a description of performance issues of the Doherty amplifier. Following the studies on characteristics of the Doherty amplifier, a new digital signal processing (DSP) control strategy was developed, which involves control of the gate bias voltage of the auxiliary amplifier. DSP was utilized to mitigate gain variations through this bias control and to eliminate phase distortion through baseband phase predistortion. The strategy was shown to significantly improve linearity of the Doherty amplifier, enabling it to meet ACPR specifications with IS-95 signals. The Doherty amplifier also maintained its high efficiency performance. Advantages of analog component based predistortion were also studied and demonstrated by experiments. Finally, in an effort to find a circuit solution for high linearity Doherty amplifiers, nonlinear behavior in Doherty amplifiers was investigated. A simple analytical model was developed based on full transistor models. The model enables nonlinearity analysis associated with each nonlinear component in the transistor. This results in accurate distortion prediction for a Doherty amplifier implemented with HBTs. The model provides diagnosis of nonlinearity in the Doherty amplifier. Accordingly, simple circuit modifications were made to significantly enhance linearity. Experimental results demonstrate the efficacy of this model

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