Radio Frequency Integrated Circuit Design - Webs

10
Power Amplifiers
10.1 Introduction
Power amplifiers, also known as PAs, are used in the transmit side of RF circuits,
typically to drive antennas. Power amplifiers typically trade off efficiency and
linearity, and this tradeoff is very important in a fully monolithic implementation.
Higher efficiency leads to extended battery life, and this is especially
important in the realization of small, portable products. There are some additional
challenges specifically related to being fully integrated. Integrated circuits
typically have a limited power supply voltage to avoid breakdown, as well as a
metal migration limit for current. Thus, simply achieving the desired output
power can be a challenge. Power amplifiers dissipate power and generate heat,
which has to be removed. Due to the small size of integrated circuits, this is a
challenging exercise in design and packaging. Several recent overview presentations
have highlighted the special problems with achieving high efficiency and
linearity in fully integrated power amplifiers [1–3].
Power amplifiers are among the last circuits to be fully integrated. In
many instances, there is no choice but to design them with discrete power
transistors, or at least separately from the rest of the radio frequency front-end
circuits. There is a lot of interest in discrete and semi-integrated power amplifier
design, and for years the main reference was the classic book by Krauss, Bostian,
and Raab [4]. Only recently has this book been complemented by a number
of fine new books on the topic [5–7], showing that this subject matter is still
of interest and may in fact be growing in importance.
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