Development of High-Efficiency Switch-Mode Concurrent Dual-Band RF Power Amplifiers

For the past ten years, we’ve seen the rapid development of wireless communication along with the number of frequency bands to be supported in a wireless device. RF power amplifiers (PAs), as the last and most important stage in a transmitter, then have to support the increasing number of frequency bands and operation modes. The solution that has been used in industry is simply to increase the number of PAs with each of them covers several adjacent frequency bands. It might be feasible from 2G to 4G since the frequency range is confined within low GHz (<3 GHz), however, as 5G comes, not only the number of frequency band will keep increasing, but the frequency range will expand to much higher ranges (~6 GHz, 30 GHz, 60 GHz, etc.). Higher frequency range will need much more PAs and thus make the traditional solution impractical due to constrained cost and area. In addition, carrier aggregation technique used in 4G and future 5G requires additional filters (diplexer/triplexer) to combines different single-band PAs which will introduce extra power loss. Multi-band PAs that are able to operate at several frequency bands (not adjacent to each other) simultaneous could potentially reduce the number of PAs and filters thus making the increasingly complicate RF front end feasible in terms of area and cost with reduced power loss. Such PAs are defined as concurrent multi-band PAs. Unfortunately, traditional multiband PAs were designed for operate one band at a time and thus experienced significant efficiency and output power drop when operate concurrently. A few concurrent dual-band PAs were designed in recent years targeting concurrent operation. However, the drop in both efficiency and output power were still too much to make those PAs useful in actual applications. x The performance of existing concurrent dual-band PAs are mainly limited by their linear-type topology. In this thesis, a switch-mode concurrent dual-band PAs was developed for the first time which, as expected, could achieve higher efficiency with minimum drop in both efficiency and output power. A concurrent dual-band current-switching class-D PA was proposed in this thesis, and developed from fundamental theories, design methodology, to actual implementation and finally measurement results. The theoretical analysis showed that, the proposed PA could provide a concurrent-mode (two carriers simultaneously) drain efficiency of 87% at 6 dB over drive which was only 5% lower than single-mode operation (one carrier at a time). A concurrent dual-band class-B PA (one of linear-type PAs) on the other hand only have a maximum concurrent-mode drain efficiency 62.5%, 16% lower than single-mode case. The output power drop was also reduced from 3 dB in linear-type PAs to 1.2 dB in the proposed PA. The design of the proposed PA however was complicated due to a large number of harmonics and intermodulation components (IMs) to be properly terminated at the output. To reduce the design complexity, the tradeoff between number of harmonics/IMs to be properly terminated and efficiency was discussed based on ADS simulation. It was found out that, the 2 harmonics and IM2 were critical to maintain high efficiency, 3 harmonics and IM3 however had smaller effect on efficiency thus can be neglected (partially) to greatly reduce design complexity with tolerable efficiency degradation. The bias effect was also explored and was suggested that the PA should be bias into triode (defined in Chapter 4) or in other words, bias above class A, in order to achieve high efficiency. The proposed PA was implemented in a push-pull structure which need a balun at both output and input. The design equations of balun were derived in this thesis together with some xi parameter optimization for minimum imbalance and largest bandwidth. The output balun provides not only differential to single-ended conversion but also a 1:4 impedance transformation. The final PA was fabricated and measured in lab. A drain efficiency of 60% was achieve when operating concurrently at 880 MHz and 1.49 GHz with less than 0.5 dB output power drop compared single-mode operation. The performance was among the best concurrent dual-band PAs. Measurement results together with simulation results show that the proposed PA has the ability to achieve much higher efficiency than linear-type concurrent dualband PAs with minimum efficiency and output power drop, and thus is capable to make increasingly complicated RF FEM feasible.