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CMOS PAs and Pervasive RF

by Asif Anwar | 5月 24, 2010

Dr David Allstot from the University of Washington kick-started the 2010 IEEE Radio Frequency Integrated Circuits Symposium with a review of the challenges and ongoing research into CMOS power amplifiers for use in the cellular radio front-end. Highlighting the energy consumption and CO2 emissions from cellular handset usage, Dr Allstot emphasized the need for more efficient PAs as continued motivation for development of CMOS PA technologies.  Research into CMOS PAs has been ongoing for around 20 years, with current designs achieveing around 1W power outputs and up to 60% PAE. The performance of typical power amplifier PAs including Class A, B and E were highlighted, and then put into context with the demands of current networks which place greater importance on PAs operating backed off from peak power with schemes such as 64QAM resulting in probability distributions that translate into actual efficiencies of only 5%.  Dr Allstot provided examples of digital PAs comprising multiple PA cells and power combining techniques that have been put forward to address these issues, and highlighted the potential for Class G power PAs that feature a hump-shaped curve with the first peak aligning with probability distribution curves of a 64QAM spectral signal and offering the potential for efficiencies as high as 24%.  While these results are promising, Strategy Analytics still sees a gap in performance capabilities between GaAs and CMOS PAs which is compounded by cellular handset requirements continuing to evolve. This translates into a moving target that places greater emphasis on linearity, efficiencies and peak-to-average ratios (PAR) and in our opinion will continue to favour the capabilities offered by compound semiconductor technologies.  Nevertheless, the pervasive nature of RF technology will open up doors for CMOS and SiGe PAs, while also continuing to provide growth the compound semiconductor technologies, and this was highlighted by the second plenary talk, given by Gregory Waters of Skyworks Solutions Inc.  Mr Waters provided an overview of how the growth in the cellular handset market has transformed the RF industry into a mainstream technology with future trends pointing to greater pervasive use of RF in multiple applications. Smartphones are leading the initial trends towards more complexity in the radio chain which translates into greater content and provides continued growth for the industry.  Video is the primary driver for the RF market moving forwards with social media driving demands for always-on connectivity. This in turn will translate into more RF content going into terminals to meet these needs. Skyworks estimated that average RF content has increased by around 2.5 to 4x the content required when delivering voice-only capabilities.  The different approaches in the cellular front-end to handle these growing needs were highlighted, from multiple PAs through broadband PAs able to tackle high-band and low-band RF chains, to solutions that will cover all bands through a converged solution. This will be coupled with techniques such as envelope tracking and digital predistortion to provide more linear, more efficient RF solutions.  The other key message from Skyworks was that RF technology will become even more pervasive in its application moving forwards, with embedded RF translating to multiple applications, examples including smart metering for utilities, vending machines and passive RF technology for tracking applications.  To serve market needs moving forwards, RF companies will need to continue developing a broad base of technologies that incorporate not only the RF, but also interconnects and packaging   Strategy Analytics sees this pervasiveness as a catalyst that will open doors for Si technologies while the growing complexity needed to serve the always-on connectivity will present continued opportunities for compound semiconductor technologies.

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