Anritsu HFE0503 Raab

22 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

R

F and microwave
power amplifiers

and transmitters
are used in a wide variety
of applications including
wireless communication,
jamming, imaging, radar,
and RF heating. This
article provides an intro­duction and historical

background for the subject, and begins the
technical discussion with material on signals,
linearity, efficiency, and RF-power devices. At
the end, there is a convenient summary of the
acronyms used—this will be provided with all
four installments. Author affiliations and con­tact information are also provided at the end
of each part.

1. INTRODUCTION

The generation of significant power at RF
and microwave frequencies is required not
only in wireless communications, but also in
applications such as jamming, imaging, RF
heating, and miniature DC/DC converters.
Each application has its own unique require­ments for frequency, bandwidth, load, power,
efficiency, linearity, and cost. RF power can be
generated by a wide variety of techniques
using a wide variety of devices. The basic
techniques for RF power amplification via
classes A, B, C, D, E, and F are reviewed and
illustrated by examples from HF through Ka
band. Power amplifiers can be combined into
transmitters in a similarly wide variety of
architectures, including linear, Kahn, enve-

lope tracking, outphasing, and Doherty.
Linearity can be improved through techniques
such as feedback, feedforward, and predistor­tion. Also discussed are some recent develop­ments that may find use in the near future.

A power amplifier (PA) is a circuit for con­verting DC input power into a significant
amount of RF/microwave output power. In
most cases, a PA is not just a small-signal
amplifier driven into saturation. There exists
a great variety of different power amplifiers,
and most employ techniques beyond simple
linear amplification.

A transmitter contains one or more power
amplifiers, as well as ancillary circuits such as
signal generators, frequency converters, mod­ulators, signal processors, linearizers, and
power supplies. The classic architecture
employs progressively larger PAs to boost a
low-level signal to the desired output power.
However, a wide variety of different architec­tures in essence disassemble and then
reassemble the signal to permit amplification
with higher efficiency and linearity.

Modern applications are highly varied.
Frequencies from VLF through millimeter
wave are used for communication, navigation,
and broadcasting. Output powers vary from 10
mW in short-range unlicensed wireless sys­tems to 1 MW in long-range broadcast trans­mitters. Almost every conceivable type of mod­ulation is being used in one system or anoth­er. PAs and transmitters also find use in sys­tems such as radar, RF heating, plasmas, laser
drivers, magnetic-resonance imaging, and
miniature DC/DC converters.

With this issue, we begin a

four-part series of articles

that offer a comprehensive

overview of power amplifier

technologies. Part 1 covers

the key topics of amplifier

linearity, efficiency and

available RF power devices

RF and Microwave Power
Amplifier and Transmitter
Technologies —

Part 1

By Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington,
Zoya B. Popovic, Nick Pothecary, John F. Sevic and Nathan O. Sokal

This series of articles is an expanded version of the paper, “Power Amplifiers and Transmitters for RF and
Microwave” by the same authors, which appeared in the the 50th anniversary issue of the IEEE Transactions on
Microwave Theory and Techniques, March 2002. © 2002 IEEE. Reprinted with permission.

From May 2003 High Frequency Electronics

Copyright © 2003 Summit Technical Media, LLC

24 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

No single technique for power
amplification nor any single trans­mitter architecture is best for all
applications. Many of the basic tech­niques that are now coming into use
were devised decades ago, but have
only recently been made practical
because of advances in RF-power
devices and supporting circuitry such
as digital signal processing (DSP).

2. HISTORICAL DEVELOPMENT

The development of RF power
amplifiers and transmitters can be
divided into four eras:

Spark, Arc, and Alternator

In the early days of wireless com­munication (from 1895 to the mid
1920s), RF power was generated by
spark, arc, and alternator techniques.
The original RF-power device, the
spark gap, charges a capacitor to a
high voltage, usually from the AC
mains. A discharge (spark) through
the gap then rings the capacitor, tun­ing inductor, and antenna, causing
radiation of a damped sinusoid.
Spark-gap transmitters were rela­tively inexpensive and capable of
generating 500 W to 5 kW from LF to
MF [1].

The arc transmitter, largely
attributed to Poulsen, was a contem­porary of the spark transmitter. The
arc exhibits a negative-resistance
characteristic which allows it to oper­ate as a CW oscillator (with some
fuzziness). The arc is actually extin­guished and reignited once per RF
cycle, aided by a magnetic field and
hydrogen ions from alcohol dripped
into the arc chamber. Arc transmit­ters were capable of generating as
much as 1 MW at LF [2].

The alternator is basically an AC
generator with a large number of
poles. Early RF alternators by Tesla
and Fessenden were capable of oper­ation at LF, and a technique devel­oped by Alexanderson extended the
operation to LF [3]. The frequency
was controlled by adjusting the rota­tion speed and up to 200 kW could be

generated by a single alternator. One
such transmitter (SAQ) remains
operable at Grimeton, Sweden.

Vacuum Tubes

With the advent of the DeForest
audion in 1907, the thermoionic vac­uum tube offered a means of elec­tronically generating and controlling
RF signals. Tubes such as the RCA
UV-204 (1920) allowed the transmis­sion of pure CW signals and facilitat­ed the transition to higher frequen­cies of operation.

Younger readers may find it con­venient to think of a vacuum tube as
a glass-encapsulated high-voltage
FET with heater. Many of the con­cepts for modern electronics, includ­ing class-A, -B, and -C power ampli­fiers, originated early in the vacuum­tube era. PAs of this era were charac­terized by operation from high volt­ages into high-impedance loads and
by tuned output networks. The basic
circuits remained relatively un­changed throughout most of the era.

Vacuum tube transmitters were
dominant from the late 1920s
through the mid 1970s. They remain
in use today in some high power
applications, where they offer a rela­tively inexpensive and rugged means
of generating 10 kW or more of RF
power.

Discrete Transistors

Discrete solid state RF-power
devices began to appear at the end of
the 1960s with the introduction of sil­icon bipolar transistors such as the
2N6093 (75 W HF SSB) by RCA.
Power MOSFETs for HF and VHF
appeared in 1974 with the VMP-4 by
Siliconix. GaAs MESFETs introduced
in the late 1970s offered solid state
power at the lower microwave fre­quencies.

The introduction of solid-state
RF-power devices brought the use of
lower voltages, higher currents, and
relatively low load resistances.
Ferrite-loaded transmission line
transformers enabled HF and VHF

PAs to operate over two decades of
bandwidth without tuning. Because
solid-state devices are temperature­sensitive, bias stabilization circuits
were developed for linear PAs. It also
became possible to implement a vari­ety of feedback and control tech­niques through the variety of op­amps and ICs.

Solid-state RF-power devices
were offered in packaged or chip
form. A single package might include
a number of small devices. Power out­puts as high as 600 W were available
from a single packaged push-pull
device (MRF157). The designer basi­cally selected the packaged device
that best fit the requirements. How
the transistors were made was
regarded as a bit of sorcery that
occurred in the semiconductor houses
and was not a great concern to the
ordinary circuit designer.

Custom/Integrated Transistors

The late 1980s and 1990s saw a
proliferation variety of new solid­state devices including HEMT,
pHEMT, HFET, and HBT, using a
variety of new materials such as InP,
SiC, and GaN, and offering amplifica­tion at frequencies to 100 GHz or
more. Many such devices can be oper­ated only from relatively low voltages.
However, many current applications
need only relatively low power. The
combination of digital signal process­ing and microprocessor control allows
widespread use of complicated feed­back and predistortion techniques to
improve efficiency and linearity.

Many of the newer RF-power
devices are available only on a made­to-order basis. Basically, the designer
selects a semiconductor process and
then specifies the size (e.g., gate
periphery). This facilitates tailoring
the device to a specific power level, as
well as incorporating it into an RFIC
or MMIC.

3. LINEARITY

The need for linearity is one of the
principal drivers in the design of

26 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

modern power amplifiers. Linear
amplification is required when the
signal contains both amplitude and
phase modulation. It can be accom­plished either by a chain of linear
PAs or a combination of nonlinear
PAs. Nonlinearities distort the signal
being amplified, resulting in splatter
into adjacent channels and errors in
detection.

Signals such as CW, FM, classical
FSK, and GMSK (used in GSM) have
constant envelopes (amplitudes) and
therefore do not require linear ampli­fication. Full-carrier amplitude mod­ulation is best produced by high level
amplitude modulation of the final RF
PA. Classic signals that require lin­ear amplification include single side­band (SSB) and vestigal-sideband
(NTSC) television. Modern signals
that require linear amplification
include shaped-pulse data modula­tion and multiple carriers.

Shaped Data Pulses

Classic FSK and PSK use abrupt
frequency or phase transitions, or
equivalently rectangular data pulses.
The resultant RF signals have con­stant amplitude and can therefore be
amplified by nonlinear PAs with good
efficiency. However, the resultant
sinc-function spectrum spreads sig­nal energy over a fairly wide band­width. This was satisfactory for rela-

tively low data rates and a relatively
uncrowded spectrum.

Modern digital signals such as
QPSK or QAM are typically generat­ed by modulating both I and Q sub­carriers. The requirements for both
high data rates and efficient utiliza­tion of the increasingly crowded spec­trum necessitates the use of shaped
data pulses. The most widely used
method is based upon a raised-cosine
channel spectrum, which has zero
intersymbol interference during
detection and can be made arbitrari­ly close to rectangular [4]. A raised­cosine channel spectrum is achieved
by using a square-root raised-cosine
(SRRC) filter in both the transmitter
and receiver. The resultant SRRC
data pulses (Figure 1) are shaped
somewhat like sinc functions which
are truncated after several cycles. At
any given time, several different data
pulses contribute to the I and Q mod­ulation waveforms. The resultant
modulated carrier (Figure 2) has
simultaneous amplitude and phase
modulation with a peak-to-average
ratio of 3 to 6 dB.

Multiple Carriers and OFDM

Applications such as cellular base
stations, satellite repeaters, and
multi-beam “active-phased-array”
transmitters require the simultane­ous amplification of multiple signals.

Depending on the application, the
signals can have different ampli­tudes, different modulations, and
irregular frequency spacing.

In a number of applications
including HF modems, digital audio
broadcasting, and high-definition
television, it is more convenient to
use a large number of carriers with
low data rates than a single carrier
with a high data rate. The motiva­tions include simplification of the
modulation/demodulation hardware,
equalization, and dealing with multi­path propagation. Such Orthogonal
Frequency Division Multiplex
(OFDM) techniques [5] employ carri­ers with the same amplitude and
modulation, separated in frequency
so that modulation products from one
carrier are zero at the frequencies of
the other carriers.

Regardless of the characteristics
of the individual carriers, the resul­tant composite signal (Figure 2) has
simultaneous amplitude and phase
modulation. The peak-to-average
ratio is typically in the range of 8 to
13 dB.

Nonlinearity

Nonlinearities cause imperfect
reproduction of the amplified signal,
resulting in distortion and splatter.
Amplitude nonlinearity causes the
instantaneous output amplitude or

Figure 1 · SRRC data pulses. Figure 2 · RF waveforms for SRRC and multicarrier signals.