Performance assessment of nonlinear active devices to design broadband microwave power amplifiers via virtual gain optimization

Abstract

In this thesis, we proposed a structured set of sequential procedures to design broadband microwave power amplifiers. A power amplifier is a major building block in transceivers for wireless communications. The output stage of a transmitter, amplifies the modulated electrical signal over a power amplifier connected to an antenna. To design solid-state microwave power amplifiers, active devices such as radio frequency (RF) power transistors are used. Nowadays, it is a common practice to employ Gallium Nitrate (GaN) transistors in RF power amplifier designs due to their high electron mobility and high-power delivering capacity. In practice, power amplifier design process starts with careful selection of the power transistor considering the design parameters such as the required output signal power to be delivered, drain/power-added efficiency of the amplifier, transducer power gain over the specified bandwidth, etc. Once the power transistor is selected, its nonlinear behavior is characterized by determining the optimum source-pull (SP) and load-pull (LP) impedances to design an RF power amplifier for optimum gain and efficiency. As they are obtained, these impedances may not be realizable network functions over the desired frequency band to yield the input and the output matching networks for the amplifier. Therefore, in this thesis, first, we introduce a new method to test if a given impedance is realizable. Then, a novel "Real Frequency Line Segment Technique" based numerical procedure is introduced to assess the gain-bandwidth limitations of the given source and load impedances, which in turn results in the ultimate RF power-intake and power-delivery capacity of the amplifier. During the numerical performance assessment process, a robust tool called "Virtual Gain Optimization" is presented. In the course of performance assessment process, a new definition called "Power-Performance-Product" is introduced to measure the quality of an active device. Examples are presented to test the realizability of the given source/load-pull data and to assess the gain-bandwidth limitations of the given source/load-pull impedances for a 45W-GaN power transistor, over 0.8-3.8 GHz bandwidth. In the second part of the thesis, we present the actual design and implementation of the novel methods in a sequential manner. As the result of the proposed methods, firstly, the power intake and power-delivery capacity of the active device are assessed for a 10W-GaN power transistor over 800 MHz-3.0 GHz bandwidth. We determined the optimum realizable source and load impedance data via the virtual gain optimization. Then, the optimum source and load impedance data is modelled as realizable network functions. Generated realizable network functions are synthesized using the Darlington synthesis which in turn yields optimum input and output matching network topologies with component values. Eventually, the designed power amplifier is manufactured. It is shown that the computed and the measured performance of the amplifier agrees within acceptable limits. Hence, we obtained an avaragre of 10 Watts output power with 11.4±0.6 dB gain and 49% to 76 % power added efficiency. In the third part, we introduce a new matching concept, so-called Virtual or interchangebly Fictitious Matching (FM), which may be defined between the artificially generated non-Foster passive immittances, over a lossless equalizer [E]. These immittances may not necessarily belong to physical devices, rather, they are fabricated like a source-pull or load-pull impedances to maximize the gain, the output power, the efficiency, and to minimize the output harmonics of the nonlinear-active device under consideration. In Fictitious Matching problems, equilezer [E] is constructed between the virtually produced generator immittance data K_GF and the load immittance data K_LF to optimize the power transfer in the passband. In this regard, [E] is described by means of its back-end driving point input immittance in the Darlington sense, and it is determined as the outcome of the optimization process. The input and the output impedances are synthesized as commensurate transmission lines as they are cascaded. It is demonstrated that the new concept of virtual matching can be utilized to build broadband power amplifiers. In this part, solving the virtual matching problem successively, the input and the output matching networks of a power amplifier are designed over 500 MHz-3 GHz with an average gain of 11.5dB, an output power of 40.5 dBm, and an average drain efficiency of 61.7%.