4.11: Summary

The realization of efficient power amplifiers is one of the more competitive aspects of RF and microwave design. Power amplifiers must operate at high efficiency and hence operation is strongly nonlinear. Linear amplifier design is only an approximate guide to power amplifier design. Power amplifiers significantly affect the cost and reliability of RF front ends. This is true of base station amplifiers producing hundreds of watts, and equally true of handset power amplifiers producing hundreds of milliwatts. The cost of a power transistor is significant, and so it must operate close to its power output capability. It is important that the power amplifier designer extract every bit of performance from the power transistors, as every tenth of a decibel is significant.

In a basestation the cost of electricity for the RF front end, largely determined by the power amplifier, and by the required air-conditioning can be a significant part of the operating expense of the basestation. With handsets, the efficiency of the RF power amplifier impacts battery life. While it is known how to build amplifiers with high efficiency by combining the concepts presented in this chapter, combining too many concepts can result in a lengthy and costly design effort. The design cost and the possible need to manually tune individual amplifiers can appreciably raise the unit cost of each amplifier. The cost of design must be managed and designers should consider the requirement for individual amplifier tuning. For example, it is not reasonable to tune individual handset power amplifiers but it is for basestation power amplifiers. The trade-off for basestation amplifiers will be different from the trade-off for handset amplifiers, which have unit volumes that can be many orders of magnitude larger. Handset amplifiers are powered directly from the battery supply without voltage regulation. Thus handset power amplifier design must contend with limited supply voltages that can drop as the battery discharges. The limited supply voltage also restricts the choice of transistor technologies.

As with most aspects of RF design, intuition and experience is important in guiding initial topology selection. This is typically followed by computer-aided design and then optimization at the bench. All power amplifiers require manual, at-the-bench, optimization during design, as there are many effects that cannot be fully accounted for in the models used in microwave circuit simulators. Power amplifiers must contend with signals whose average power can vary significantly from packet to packet, and which can have a very large ratio of peak envelope power to mean envelope power (i.e., large PMEPR). The standard design procedure is to arrive at a successful topology and initial layout using computer-aided design. Then load-pull is used in the laboratory to optimize the loading conditions for actual digitally modulated signals. It is possible that the topology may need to be changed to accommodate necessary changes identified during load-pull.

One of the significant costs and sources of reliability degradation in a handset is using multiple technologies for different parts of the RF front end. This requires heterogeneous integration rather than the more reliable monolithic integration. For example, in a handset it would be desirable to implement a power amplifier in silicon so that it can be integrated with the silicon driver and other circuitry. However, it is currently more feasible to obtain high efficiencies with compound semiconductor devices such as HBT and pHEMT transistors.

Ideally base station amplifiers would use silicon transistors, be able to efficiently amplifier many carriers (with PMEPRs exceeding \(20\text{ dB}\)) simultaneously, be able to operate from \(500\text{ MHz}\) to \(5\text{ GHz}\), and be reconfigurable for future unforeseen applications. Similarly, cellular handsets must support multiple bands and an important consideration is whether separate power amplifiers are used for each band. Ideally a handset would use a single silicon power amplifier as part of an RFIC that contains the rest of the RF front end, achieve efficiencies of \(60\%\) or greater, and operate from supply voltages that vary between \(2\) and \(3\) volts.

The RF spectrum is now being exploited beyond \(200\text{ GHz}\) and power amplifiers are required from \(100\)s of megahertz to more than \(200\) gigahertz. Currently the amplifiers with highest output power in this range merge semiconductor-based power amplifiers with vacuum-tube-based amplifier technologies.

Power amplifier design is a trade-off of available technologies that enable design with manageable design and operating costs while achieving requisite powers and efficiencies. There is a large trade-off space and there is a significant opportunity to provide competitive solutions.