Efficienc Enhancement - Linear CMOS RF Power Amplifiers for Wireless Applications - page 7

means longer battery lifetime which translates directly into user comfort. For non-
portable applications, efficiency is also important as heat dissipation is often an
issue.

2.2.2.1 Metrics

In the context of RF power amplifiers, two main efficiency metrics are commonly
used: drain efficiency ( η d ) and Power-Added Efficiency (PAE). The drain efficiency
is calculated as the output power ( P out ) divided by the DC power consumption ( P DC )
as expressed below:

P out
P DC

η d = 100 × . (2.1)

The PAE is defined by Giannini and Leuzzi in [ 10 , p. 164] as

the ratio between the RF power added by the amplifier and the DC power required for this
addition,

as shows the following equation.

P out − P in

PA E = 100 × . (2.2)

P DC

The PAE is a better indicator than the drain efficiency because it includes the
input power ( P in ) and, hence, the gain of the power amplifier. When the power gain
( G ) is higher than 10 dB, the PAE can be approximated by the drain efficiency with
an error smaller than 10% as suggested by the equations below:

P out
P in ,

=

G (2.3)

η d 1

.

1
G

PA E = − (2.4)

2.3 Overview of the Main Efficiency-Enhancemen Techniques

The linearity-efficiency trade-off is a problem that the RF PA designer has been try-
ing to solve for a long time. Linearization as well as efficiency-enhancement tech-
niques are possible solutions and the choice between them depends on the applica-
tion and on the expertise of the designer. For application in WLAN, the requirement
on the PA linearity is very stringent and, hence, it makes sense to choose to use a
linear PA (class A, AB, or B) together with an efficiency-enhancement technique.
This chapter provides a review of the most important works recently published
in the field of efficiency enhancement of RF power amplifiers. For more details re-
garding linearization techniques, the interested reader can refer to a dedicated text-
book [ 14 ].