Power Combining Solutions for High-Power GaN MMICs at mm-Wave Frequencies
The work at hand is concerned with the design of a broadband solid-state power amplifier system (SSPA) with very large output power in the millimeter-wave frequency range. Itis divided into two major parts. The first part consists of a comprehensive investigation of the parameters that limit the bandwidth of a power amplifier. To that end, the Bode-Fano limit is illustrated, which represents the fundamental boundary of the bandwidth a transistor can be reactively matched over. In addition, the effects of this limit on an interstage matching network is analyzed. It turns out that in many cases, the interstage network determines the bandwidth of the overall amplifier system. Furthermore, the conditions are examined under which the Bode-Fano limit bounds the bandwidth of an amplifier and which impact this results in for the design of the amplifier. In amplifier designs that are aimed at high output powers, it is generally necessary to parallelize a large number of transistors. Consequently, the boundary conditions that arise from this fact are examined in case of the most complex amplifier constituent, the interstage network. From this, a minimum prototype network is developed, which satisfies these boundary conditions. As extensive coupling effects occur in miniaturized millimeter-wave layouts, it is often a considerable obstacle to transfer prototype networks to electrically equivalent layouts. Therefore, for the first time, space-mapping algorithms are evaluated for the design of high-power amplifiers. To that end, the formulation and concrete application of several algorithm realizations shows their advantages and disadvantages. Additionally, a fast and quickly applicable reference implementation is introduced. In the following, this implementation enables the design of several demonstrator MMICs which surpass the current state of the art in important aspects. The second part of this work is concerned with a study of several approaches to large-scale power combining. Particularly, the insertion loss, the achievable bandwidth hand manufacturability properties are examined with a focus on the scaling of the designs towards millimeter-wave frequencies. Apart from topologies based on binary-tree and traveling-wave structures, radially symmetric approaches are analyzed in depth. As a result, the radial-line combiner turns out to be the topology with the best compromise of the aforementioned properties. Subsequently, the radial-line combiner is examined more closely. The analysis shows that three major components can be considered separately: the central feeder, the radial line and the peripheral transition. For the realization of these constituents, several new components are developed in the course of this work. As an example, a new peripheral transition to rectangular waveguide is proposed, which integrates compensation structures in the transitional area. Consequently, it reaches a considerably higher operational bandwidth than the previous art. The results of these analyses and developments are used to design a 16-way radial-line combiner demonstrator in the Ka band. Its characterization shows very good matching properties and record values with respect to the insertion loss in the entire Ka band. Using the MMIC modules designed in the first part and the combiner demonstrator from the second part of this work, a two-stage SSPA is manufactured in the following. The in-depth small and large-signal characterizations show a transducer gain of over20 dB in the entire Ka band and an output power of over 100W in a large part of the Ka band. These results impressively underline the value of the analyses and new approaches that have been developed here.
Freiburg/Brsg., Univ., Diss., 2021