Anritsu HFE0503 Leong

42 High Frequency Electronics

High Frequency Design

BROADBAND DESIGN

Broadband VHF/UHF
Amplifier Design Using
Coaxial Transformers

By C. G. Gentzler and S.K. Leong
Polyfet RF Devices

T

he desire of the
armed forces to

maintain instant
communications with all
forces requires the design
of miniature broadband
power amplifiers with
greater than decade
bandwidth (30 to 512
MHz). This bandwidth is

required for all-band transceivers that cover
tactical ground and air frequencies in addition
to civil telecommunication frequencies and the
frequencies of our allies. All-band radios are
commonplace in virtually every deployment of
new platforms, as well as in the retrofitting of
existing communications systems.

This paper will discuss the design of minia­ture coaxial structures and examine the
implementation of improved design tech­niques to enable the designer to obtain insight
into matching the load line of power MOSFET
transistors over decade bandwidths.

This article presents the development of
large signal parameters for a typical power
MOSFET device and the development of a
suitable load line using coaxial transmission
line transformers in conjunction with embed­ded lumped structures, enabling an efficient
load line match across a decade of bandwidth.

Simulation Methodology.

Linear simulation assumes that a circuit
with active devices is operated at such a low
power that the simulated measurements are
no longer power dependent. This simulation
can be achieved by two methods. First the cir­cuit uses a nonlinear model and nonlinear
simulator. The quiescent current is set at a

nominal condition and the power level used in
the simulation is set to a low level so as not to
make the output data power dependent.
Another linear simulation method is to use
tabular data to describe an active device and
simulate with a linear simulator. Usually the
data file is in S-parameter format although
other formats have been used in the past at
lower frequencies (e.g. impedance magnitude
and angle). If the nonlinear model and the
data file agree, both simulations will yield the
same measurement data. In the case of using
a non-linear model with a nonlinear simula­tor, the simulation results are generally very
close to actual amplifier performance.

Nonlinear simulators provide gain com­pression, power output, efficiency and har­monic power data. With somewhat less accu­racy, intermodulation distortion can be simu­lated, but not with the same accuracy as the
single tone measurements. To obtain accurate
results, the device model would have to track
an actual device transfer curve closer than 5
percent. Five percent accuracy is generally
acceptable for gain compression and efficiency
measurements, but not for the slight nonlin­earity that causes low to intermediate levels
of intermodulation distortion. Modeling tech­nology is slowing improving and it is expected
that intermodulation performance may be
accurately modeled in the future. Nonlinear
simulators generally are more costly, but are
really the only choice if large signal perfor­mance simulation is desired, as in the case of
this article.

Amplifier Design

First one must determine the optimum
load line impedance required by the device.

This article describes the

methods used to design

broadband coaxial trans-

former matching networks

for an LDMOS power ampli-

fier that delivers consistent

performance over more

than a decade bandwidth

From May 2003 High Frequency Electronics

Copyright © 2003 Summit Technical Media, LLC

44 High Frequency Electronics

High Frequency Design

BROADBAND DESIGN

Computer load pull or optimization is
required since any actual load pull
techniques are only generally avail­able for much higher frequencies. The
physical structures for generating
load impedances at frequencies below
500 MHz are too large to be practical
to implement. Additionally, since the
band width is multi-octave, broad­band matching structures must be
used to determine the load line
rather than multiple narrow band
measurements. A computer with suit­able software and good device models
is the most practical approach. In
this article we will use popular soft­ware packages such as Applied Wave
Research’s (AWR) Microwave Office
and Agilent Technologies’ ADS, used
together with Polyfet RF Spice
Models to demonstrate broadband
matching techniques.

Impedance Behavior of
Transistors

At low frequencies, the device’s
output impedance is relatively high
compared with the calculated load
line required to produce the desired
power. As the operating frequency is
increased, the output capacitance
(C

oss

), reverse capacitance (C

rss

) and
an increased saturation voltage low­ers the optimum load line to achieve
satisfactory performance.

Over a decade of bandwidth, the

optimum impedance can drop by a
factor of two. That is to say that if the
low frequency load line is 6 ohms, the
upper operating frequency could
require an impedance of 3 ohms with
some amount of inductive or capaci­tive reactance. Figure 1 shows real
value of Z

out

dropping from 11 ohms
at low frequencies to 2 ohms at high
frequency for the transistor LR401.

There has been considerable
experimental and developmental
work published on the attributes of
coaxial transformers to achieve
extremely wide bandwidths. This
paper will explore how to combine
the coaxial transformer with lumped
components to achieve optimal power
matching in a MOSFET power ampli­fier over more than a decade of band­width.

Computer simulated load pulling
will be utilized to extract the first
order magnitude of load line match­ing. This impedance information is
only the starting point, since it will
be extracted by a narrow band tech­nique. Broadband extraction is an
area that will be explored in the
future as the results will take into
account more realistic harmonic load­ing and allow more accurate broad­band design implementation. In the
case of Polyfet transistors, Z

in/Zout

data can be found for each transistor
in its respective data sheet.

Once the approximate load line
has been determined, let us review
the coaxial transformer matching
techniques and explore the use of
physical length, cable impedance,
and lumped components in addition
to ferrite loading to achieve optimum
performance.

Of all the coaxial transformer
designs, one of the most practical for
wideband impedance matching is the
4:1 design with a balun transformer
to achieve optimum balance. The
standard accepted equation for trans­formation is that the Z

0

of the cable
should be the square root of the prod­uct of the input and output
impedances. For example, if the input
impedance is 12.5 ohms and the out­put impedance is 50 ohms, then the
square root of 12.5 × 50 = 25. A 25­ohm impedance cable would give the
optimum results across a wide oper­ating bandwidth.

Figure 3 shows a uniform
impedance transformation ratio of
four across the frequency band. It
should be noted for the purpose of
load line design, impedance is mea­sured drain to drain. This allows sin­gle ended impedance measurements.
Simply divide the impedance data by
two to obtain individual device load
impedance. At 30 MHz the ratio falls
off due to reactive shunt losses,
which could be compensated with

Figure 1 · Zinand Z

out

vs. frequency. Figure 2 · Conventional 4:1 transformer with balun.