This section encompasses low noise amplifiers (LNAs), and power amplifiers (PAs).

The LNA is the first gain stage in the receiver path. From Friis' equation, we know that its noise figure adds directly to the receiver's overall noise figure. In addition, the noise contributed by the stage immediately succeeding the LNA decreases as the LNA gain increases. Hence, high gain and very low noise are key targets of an LNA designer. In addition, we want good impedance match at its input and its output, good linearity, and, as with any other active circuit, we also desire rock-solid stability and the lowest power consumption we can get away with. Simultaneously meeting all these criteria makes for an iterative and complicated design process that typically requires the designer to utilize multiple software tools beyond the circuit simulator. These tools may include MATLAB for post-processing simulated/measured data, ASITIC for characterizing integrated spiral inductors, and an EM simul...

This section encompasses low noise amplifiers (LNAs), and power amplifiers (PAs).

The LNA is the first gain stage in the receiver path. From Friis' equation, we know that its noise figure adds directly to the receiver's overall noise figure. In addition, the noise contributed by the stage immediately succeeding the LNA decreases as the LNA gain increases. Hence, high gain and very low noise are key targets of an LNA designer. In addition, we want good impedance match at its input and its output, good linearity, and, as with any other active circuit, we also desire rock-solid stability and the lowest power consumption we can get away with. Simultaneously meeting all these criteria makes for an iterative and complicated design process that typically requires the designer to utilize multiple software tools beyond the circuit simulator. These tools may include MATLAB for post-processing simulated/measured data, ASITIC for characterizing integrated spiral inductors, and an EM simulator to characterize package parasitics, bondwire inductances, and mutual couplings. Given the paramount role of LNA's in the receiver chain and the complexity of their design process, they have been and will continue to be the focus of intensive investigation for years to come.

Key performance targets of PA designers include efficiently delivering the required output power, stability (specially when the antenna presents a mismatched impedance), gain, and ruggedness, the ability not to self-destruct when the antenna presents a *very* mismatched impedance to the PA. On certain wireless standards, PA linearity is supremely important; in others, not at all. Although low- and medium- power PA's (0.5 Watts or less) have been fabricated and produced successfully in CMOS or SiGe fabs, PA's for cell phones (1-Watt or more) are fabricated almost exclusively in compound semiconductor materials such as gallium arsenide (GaAs). One big reason is that the fabrication process for these wafers is typically much shorter than that of CMOS or SiGe wafers. Why go through an iterative and complicated design process and run a few thousand long and probably inaccurate large-signal circuit simulations and do a detailed package characterization to verify that the part you will get in 6-8 weeks will meet everyone's expectations when you can submit 20 design iterations to a GaAs fab, have real parts in the lab in 10 days, and quickly decide on the design that meets the required performance? Also, compound semiconductors are miles ahead in terms of ruggedness, i.e., the PA's resilience to withstand a 10:1 mismatch at the antenna without blowing up. For these reasons and the fact that power amplifiers have stubbornly resisted any elegant, reliable design approach thrown at them, PA's that deliver more than 0.5 Watts of power are typically standalone parts designed in dedicated fabs rather than being integrated with the rest of the SoC. In any case, new wireless standards such as 3G and WiMax have opened the door to a tremendous amount of opportunity for PA designers and companies (that can deliver) everywhere. For example, whereas Apple's first iPhone, released in 2007, utilized a single Skyworks multi-band PA module, the iPhone 3G released in the summer of 2008 utilizes the same Skyworks part plus 3 Triquint PA's. That's right: 4 PA chips in a single cell phone. This means that the PA dollar content in newer cell phones will be double or triple that of the previous generation of cell phones, which bodes very well for PA companies that can execute, and also probably siginifies that there will be no shortage of demand for good PA designers in the foreseeable future, given the enormous amounts of dedication and effort that each PA requires.

[1] B. Razavi, RF Microelectronics, Prentice Hall, Upper Saddle River, NJ, 1998, pp 43-45, pp 166-167, pp 298-300.

[2] J. Cheng, C. Ecker, K. Fisher, iPhone in Depth: The Ars Review, http://arstechnica.com/reviews/hardware/iphone-review.ars/14

[3] http://www.ifixit.com/Guide/First-Look/iPhone3G/images/XiuvbfUecK3GsDUm-large.jpg