Anritsu HFE0902 Pengelly
26 High Frequency Electronics
High Frequency Design
RF POWER AMPLIFIERS
Improving the Linearity
and Efficiency of
RF Power Amplifiers
Raymond S. Pengelly
Cree Microwave
A
growing number
of semiconductor
technologies are
being applied to RF
power transistor applications. These technologies
include Si LDMOS FET,
SiGe HBT, InGaP HBT,
GaAs MESFET, AlGaAs
pHEMT, SiC MESFET
and AlGaN/GaN HEMT. The dependencies of
linearity and efficiency of such technologies
are often common, such as transconductance
derivatives, capacitance variations, breakdown effects and parasitic resistances. This
article overviews the work that has been
achieved to date to maximize linearity and
efficiency in the most promising technologies,
as related specifically to infrastructure applications. The article also addresses the increasing number of device and circuit level techniques that are being used to enhance these
two important parameters as required for
IM3, ACPR and ACLR suppression in 3G systems such as W-CDMA/UMTS.
This article focuses on high power (that is
greater than 10 watt) RF transistor technologies where digital modulation techniques are
demanding higher and higher peak-to-average
ratios (PARs) and thus higher peak powers.
Peak and average DC-to-RF efficiencies have
become critical parameters, and much attention is being focused in decreasing multi-carrier intermodulation distortion, adjacent channel power ratios (ACPRs) and adjacent channel leakage ratios (ACLRs). Unfortunately,
improving transistor linearity often leads to
decreased efficiency which directly affects
overall system efficiency, heat removal, size
and cost.
Competing Technologies
The generation of solid state RF power has
been in existence since the late 1960s when
silicon bipolar transistors were introduced by
such companies as TRW and RCA (ref. 1).
Today there are a range of technologies available, including silicon bipolar, silicon LDMOS
FET, GaAs MESFET, GaAs pHEMT,
AlGaAs/InGaAs HFET, GaAs, InP, InGaP and
SiGe HBT as well as wide bandgap transistors
such as SiC MESFET and AlGaN/GaN
This article describes
improvements in device
technology and design
techniques that will enable
power amplifiers with
higher efficiency and better
linearity performance —
at higher frequencies
Technology Price/Watt Power Supply Linearity Frequency PAE
Density Voltage
Si BJT Low Cost Medium 26 V Poor <2 GHz Low
SiGe BJT Low Cost Medium <20 V Good >2 GHz High
Si LDMOS Low Cost Low 26 V Very Good <3 GHz Medium
GaAs MESFET Competitive Medium 12 V Good >2 GHz Medium
GaAs pHEMT Medium Medium 8 V to 12 V Very Good >2 GHz High
GaAs HBT Competitive High 8 V to 26 V Good >2 GHz High
SiC MESFET Competitive Very High 48 V Good >4 GHz Medium
GaN HEMT N/A Very High 48 V Promising >12 GHz High
Table 1 · Overview of competing solid-state RF power transistor technologies.
From September 2002 High Frequency Electronics
Copyright © 2002, Summit Technical Media, LLC
28 High Frequency Electronics
High Frequency Design
RF POWER AMPLIFIERS
HEMT. Table 1 presents a brief comparative overview of some of these
technologies. Figure 1 shows the
trends in discrete transistor output
powers and efficiencies as a function
of frequency for HEMTs and HBTs.
Single die peak powers for Si LDMOS
FETs have reached greater than 60
watts at 2 GHz.
Of particular interest today are
wide bandgap transistors such as silicon carbide (SiC) MESFETs and gallium nitride (GaN) HEMTs. Such
transistors exhibit very high RF
power densities (watts per mm of
gate width) compared to any other
technologies (by a factor of 10 over
GaAs MESFET for example) (ref. 3
and 4).
Wide band-gap transistors fabri-
cated from 4H-SiC and AlGaN/GaN
offer superior RF performance, particularly at elevated temperatures,
compared to comparable components
fabricated from GaAs or Si. RF output powers on the order of 4 to 7
W/mm and 10-12 W/mm are achievable from SiC MESFETs and
AlGaN/GaN HFETs respectively.
Achievement of higher power densities is a priority for RF power technologies as it reduces size, which is
important in both fixed and mobile
platforms. It also provides higher
working impedances, which are
important for wider bandwidth operation, simpler circuits and easier
manufacture.
Figure 2 shows a comparison of
the input and output impedances of a
20 mm GaN HEMT delivering
greater than 100 watts CW peak
power with a commercially available
Si LDMOS FET of similar power
capability. Clearly, the GaN HEMT
has much more convenient
impedance levels which can also
result in easier packaging whereby
no internal pre-matching is needed
(Figure 3). The higher gain of the
GaN device requires lower drive
drive. Initial linearity measurements
show similar performance for the two
device technologies.
Both SiC MESFETs and GaN
HEMTs show promising efficiencies
and linearities. For example, Figure
4 shows the peak efficiencies of a
GaN HEMT as a function of drain-tosource voltage over a range of 10 to
40 volts. Note that the drain efficiency remains almost constant at
greater than 60 percent over the
complete voltage range which
enables efficiencies to be optimized
at reasonable back-off powers (e.g.
up to 10 dB). Figure 5 shows an
example of the promising linearity
that can be obtained from such wide
bandgap transistors. The figure
shows a comparison between a 1.2
mm gate width GaAs pHEMT and a
1 mm gate width AlGaN HEMT.
Although these transistors have
comparable gate widths the AlGaN
HEMT provides >10 dBm more output power with improved third order
intermodulation distortion.
Figure 1 · Discrete device output powers and efficiencies versus frequency (after Nguyen and Micovic, ref. 2).
Modeled GaN Motorola
20 mm HEMT MRF18090B
P
out
100 Watts 100 Watts
Z
in
25 +j49 Ω 2 +j8 Ω
Z
out
3.8 +j0.8 Ω 1.3+j2.2 Ω
Gain >25 dB >13 dB
• Similar packages assumed—matching
capacitor included for GaN input
• No output matching for GaN device
•V
ds
= 35 V GaN, 26 V for LDMOS
Figure 2 · High power GaN HEMT in a cellular base station application.
Figure 3 · Example of a packaged 100 watt GaN HEMT (ref. 20). P
out
is >100 W and peak drain efficiency is 54 percent.
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