Efficienc Enhancement - Linear CMOS RF Power Amplifiers for Wireless Applications - page 5

Chapter 2
Efficienc Enhancement
Abstract This chapter presents some basic principles of RF power amplifiers,
mainly dealing with the linearity-efficiency trade-off, to prepare the reader for the
design of a dynamic supply CMOS RF power amplifier, subject of the next chapter.
It also surveys the main efficiency-enhancement techniques found in the literature
to establish a basis for comparison with the results presented in Chap. 4
2.1 Introduction
According to Kundert in [ 16 ], the two primary goals in a transmitter design are:
First, they must transmit a specified amount of power while consuming as little power as
possible. Second, they must not interfere with transceivers operating on adjacent channels.
Figure 2.1 depicts a simplified block diagram of a direct conversion transmitter.
The RF power amplifier is the last component in the transmitter chain. Hence, the
objectives stressed by Kundert are also valid for a PA design. These goals are better
expressed through two performance metrics: efficiency and linearity. Therefore, the
PA should be as efficient as possible and as linear as required by the system in
which it operates. The major problem is that efficiency and linearity sit at opposite
sides of a seesaw: when one goes up, the other goes down. This is the well-known
linearity-efficiency trade-off inherent to RF power amplifier design.
2.2 Basic Principles
2.2.1 Linearity
The linearity of a PA is not of fundamental importance 1 in communication sys-
tem transceivers with constant envelope modulation schemes. Since the RF signal
1 But care should be taken to respect spurious emission limits.