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INDUSTRY MOBILES


ENVELOPE TRACKING and the future of the


CMOS PA


With envelope tracking enhancing the efficiency of CMOS PAs to the levels of their GaAs cousins, is the incumbent technology under threat?


BY JEREMY HENDY FROM NUJIRA


RECENTLY, much has been made of the prospect of increased competition for traditional GaAs PAs from CMOS alternatives. Interest in this has been piqued by the release of products such as the Qualcomm RF360, which unites a universal PA, switch and antenna in a single front-end solution.


However, although launches of products such as this have helped CMOS to make significant inroads into the cost-sensitive GSM GPRS and 3G markets, GaAs continues to dominate the mobile marketplace − more than 90 percent of new smartphones are built with GaAs PAs.


Whether GaAs will continue to dominate is a hot topic of discussion. One view that is being widely expressed is that the evolving market offers an opportunity for CMOS PAs to ‘kick GaAs’, but the fact remains that the CMOS PA must overcome some technical challenges before it will undergo widespread deployment within the handset space.


At the forefront of these is the fact that − despite success in the 2G market and some low-end 3G applications − significant performance drawbacks continue to limit deployment of CMOS PAs within high-end 3G and LTE applications.


Falling at the first hurdle Drawbacks associated with the CMOS PA include an inferior device gain, lower breakdown voltage, and challenges with RF simulation models. On top of this, CMOS designs have tended to be larger − roughly three times the size of a comparable GaAs die – due to their heavy use of inductors and transmission lines. However, these concerns, although important, are overshadowed by the biggest flaw of the CMOS PA: Its poor inherent linearity.


One of the merits of the GaAs amplifier is its flat gain characteristic until relatively close to its point of saturation; here the gain response rapidly falls off in the space of 2 dB (see Figure 1). By contrast, for CMOS, the PA curve is non-linear over a wide range of output powers, with a ‘soggy’ compression characteristic stretching over the top 10 dB of the power range.


This lack of linearity is particularly problematic when a PA is used to provide coverage of 4G networks, which employ high Peak-to-Average Power Ratios (PAPR). Both classes of PA are capable of producing high enough peak powers to satisfy the requirement for high power constant envelope signals, such as GSM or simple 3G; but, for linear signals, such as


40 www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor


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