In this paper, an lateral diffused metal-oxide-semiconductor-based very high-frequency class-E power amplifier has been investigated theoretically and experimentally. Simulations were verified by amplifier measurements and a record-high class-E output power was obtained at 144 MHz, which is in excellent agreement with simulations. The key of the results is the use of efficient device models, simulation tools, and the invention of a novel high-Q inductor for the output series resonance network. The latter allows for low losses in the output network and, simultaneously, a wide tuning range for maximum output power or maximum efficiency optimization

Rutledge, David B.

Zirath, Herbert

English

Abstract — In this paper, an lateral diffused metal–oxide– semiconductor-based very high-frequency class-E power amplifier has been investigated theoretically and experimentally. Simulations were verified by amplifier measurements and a record-high class-E output power was obtained at 144 MHz, whic is in excellent agreement with simulations. The key of the results is the use of efficient device models, simulation tools, and the invention of a novel high-Q inductor for the output series resonance network. The latter allows for low losses in the output network and, simultaneously, a wide tuning range for maximum output power or maximum efficiency optimization. Index Terms—High efficiency, power amplifier. I

Herbert Zirath

David B. Rutledge

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An LDMOS VHF Class E power amplifier using a High Q novel variable inductor

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2534 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
An LDMOS VHF Class-E Power Amplifier Using
a High- Novel Variable Inductor
Herbert Zirath, Member, IEEE, and David B. Rutledge, Fellow, IEEE
Abstract— In this paper, an lateral diffused metal–oxide–
semiconductor-based very high-frequency class-E power
amplifier has been investigated theoretically and experimentally.
Simulations were verified by amplifier measurements and a
record-high class-E output power was obtained at 144 MHz,
whic is in excellent agreement with simulations. The key of the
results is the use of efficient device models, simulation tools, and
the invention of a novel high-Q inductor for the output series
resonance network. The latter allows for low losses in the output
network and, simultaneously, a wide tuning range for maximum
output power or maximum efficiency optimization.
Index Terms—High efficiency, power amplifier.
I. INTRODUCTION
THE class-E amplifier is an attractive solution to obtain ahigh output power with a high efficiency; the principle
of the circuit was first described by Ewing [1] in 1964. Ewing
demonstrated an amplifier with 20-W output power with 94%
efficiency at 500 kHz.
Eleven years later, Sokal and Sokal [2] showed an ampli-
fier working in the low megahertz range with 26-W output
power and a drain efficiency of 96%. Closed-form equa-
tions have been developed by many authors to aid the de-
sign of the class E-amplifier [3]. Due to the evolution of
advanced MOS transistor technologies like lateral diffused
metal–oxide–semiconductor (LDMOS), and the increasing de-
mand of high-efficiency amplifiers for mobile communication,
the class-E concept is again receiving attention, and output
powers of up to 400 W have been obtained in the high-
frequency band [4]. In the VHF range and higher, some
experimental results have been obtained, but the output power
has thus far been limited [5], [6]. In this paper, a novel
concept of the output network and the usage of the newly
developed Silicon LDMOS device made it possible to achieve
a record-high output power of 54 W with a comparatively high
efficiency of 70% at a frequency of 144 MHz. We believe that
the results can be scaled up both in power and frequency.
II. THE CLASS-E AMPLIFIER
The switched-mode class-E amplifier is ideally based on the
characteristic topology, according to Fig. 1, where a switch
Manuscript received March 26, 1999; revised July 16, 1999. This work
was supported by the Army Research Office and by D. Antsos, Jet Propulsion
Laboratory, Pasadena, CA.
H. Zirath is with the Department of Microelectronics, Chalmers University
of Technology, 412 96 Go¨teborg, Sweden, and is also with Ericsson Mi-
crowave Systems, Mo¨lndal, Sweden.
D. B. Rutledge is with the Department of Electrical Engineering, California
Institute of Technology, Pasadena, CA 91125 USA.
Publisher Item Identifier S 0018-9480(99)08452-5.
Fig. 1. Ideal class-E amplifier circuit.
is charging a choke with magnetic energy during the “on”
time of the switch, while during the “off” time, the energy
is released partly to the parallel capacitor and partly to
the output network, which consists of a load in series with a
series resonance circuit. In our work, the switch is realized
by an LDMOS transistor driven by a voltage applied at the
gate. At frequencies in the VHF band and higher, neither the
switch nor the passive components can be regarded as ideal,
and the analytical equations described by different authors
cannot be used anymore for successful optimization of the
circuit. However, the recent improvements in the modeling
of active devices and simulation tools have made it possible
to design such amplifiers with sufficiently good accuracy by
computer simulation. The LDMOS transistor used in this study
is an MRF 183 from Motorola, which is able to deliver an
output power of 45 W with 33% drain efficiency at 900 MHz.
A nonlinear model based on the Root model is available
on Motorola’s website and can be easily downloded to the
simulator.
In order to obtain a practical class-E amplifier topology
some modifications to Fig. 1 had to be made. The goal was to
obtain the same power from the transistor compared to when
it is working in class AB and to investigate the possibility
of achieving a high efficiency simultaneously. The capacitor
is partly absorbed by the transistor through its output
capacitance, which for the particular device is 38 pF. The
input impedance according to the datasheet is quite low,
(at 930 MHz) so a matching network consisting
of a ferrite transformer and a series inductor was used at the
input. A turn ratio of four was used in the simulations. In
order to increase the output power to the load, the 50- load
impedance was transformed to 1.5 by a parallel capacitance
and a series inductance. This series inductance is absorbed
0018–9480/99$10.00  1999 IEEE
ZIRATH AND RUTLEDGE: LDMOS VHF CLASS-E POWER AMPLIFIER 2535
Fig. 2. Class-E amplifier circuit.
TABLE I
COMPONENT VALUES OF THE OPTIMIZED DESIGN
in the series resonance circuit. The -value of the resonance
circuit was chosen to be low, of the order of five, in order to
have reasonable bandwidth and low sensitivity of the circuit
performance to the series resonator values.
III. SIMULATIONS
The device model was inserted in the simulator and the
circuit topology in Fig. 2 was used for the simulation and
optimization of the circuit. The impedance transforming net-
work at the output was determined by a linear simulation,
while the choke and the capacitor were optimized
for highest efficiency at maximum output power. was
chosen to have the smallest possible value without sacrificing
the efficiency. Component values of the optimized design are
listed in Table I.
The operation of a class-E amplifier is determined by
the series resonance network, i.e., the output power and the
efficiency is critically dependent on the tuning of this network.
The output power, efficiency, and different waveforms were,
therefore, investigated as a function of the inductance of the
series resonance. Fig. 3 shows the output power and Fig. 4 the
drain and power-added efficiency when this inductor is swept
from 10 to 30 nH. Point refers to high output power while
points and are points tuned out from the resonance. It is
clear from the figures that the highest efficiency is obtained at a
higher value of than the value obtained at maximum power,
i.e., the resonance circuit should be tuned somewhat lower in
frequency compared to the input frequency in order to obtain
high efficiency. The predicted maximum drain efficiency is
92% at an output power of 20 W. At the maximum power
point, i.e., 60 W, the drain efficiency is 70%. From this
Fig. 3. Output power (in watts) versus series inductance.
Fig. 4. Drain efficiency and power-added efficiency versus series inductance.
simulation, it is clear that the output power can be readily
tuned by the series resonance inductor. The drain voltage,
the sum of the currents through the device and , and the
gate voltage at points – is shown in Fig. 5. The power
is approximately the same for the and cases, but the
efficiency is optimum for . Note the high choke-current in
case compared to case . Also note the absence of ringing
2536 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
(a)
(b)
(c)
Fig. 5. (a) Waveforms: choke current (ichoket), drain current (idt), drain
voltage (vdst), and load voltage (vloadt) for point a) in Fig. 3. (b) Waveforms
for point b) in Fig. 3. (c) Waveforms for point c) in Fig. 3.
in the drain voltage waveform when the device is in its on state,
when is tuned optimally [case ]. At maximum power,
the peak current through the transistor and is 11 A with
a mean value of about 4 A.
IV. DESIGN OF THE AMPLIFIER
When all component values were optimized, passive com-
ponents were selected from the basis of -parameter char-
acterization. was fabricated by using a tinned wire with
a diameter of 0.5 mm, and the input transformer was made
by using a binocular-type nickel–zinc ferrite of type Fair-rite
28 43 002 402. For all capacitors, except the decoupling
capacitors, ATC-100 capacitors were used. All components
were tested on an HP 8510 vector network analyzer (VNA)
Fig. 6. Inductance of 50-mm-long ribbon with a 4 mm 1 mm cross section
versus distance from ground in millimeters.
Fig. 7. Photo of the amplifier.
for verification. Since simulations predicted that the circuits
are critically dependent of the output series resonance circuit,
special care was taken in the design of this part. A novel
inductor was invented, using a copper ribbon 5 mm above the
ground plane. By inserting a wedge-shaped aluminum tuning
plate between the ribbon and ground plane, the inductance
can be tuned by at least a factor of two. The value of
the inductor, as measured by the VNA, is more than 500 at
144 MHz, somewhat dependent on the distance to the ground
plane. Fig. 6 shows the simulation of the inductance versus
distance from the ground plane for a 5-cm-long ribbon with
a cross section of 4 mm 1 mm. This result was also
experimentally verified in a special fixture. The transistor was
mounted on a heatsink and all components were connected
by soldering (see Fig. 7). After assembling the circuit, the
amplifier was connected to the bias supply and measurement
equipment and the amplifier worked immediately.
V. EXPERIMENTAL RESULTS
The frequency of the generator was set to 144 MHz and the
input and output power was measured with a Bird 4421 power
meter, which has an accuracy of 3%. The output versus
input power was first measured and the output inductance was
tuned for maximum power. The data from this measurement
is plotted in Fig. 8. The drain supply voltage was 20 V. From
ZIRATH AND RUTLEDGE: LDMOS VHF CLASS-E POWER AMPLIFIER 2537
Fig. 8. Output power, gain, drain efficiency, and power-added efficiency as
a function of input power at 144 MHz.
Fig. 9. Efficiency and drain current versus output power. This plot is made
by varying the series inductance.
the figure, we can conclude that an input power of 5 W was
needed in order to achieve saturated operation, i.e., a state
where the output power is not too dependent on the input
power. The drain efficiency and power-added efficiency are
relatively constant from 3 to 5 W.
In the next measurement, the input power was kept constant
to 5 W, while the output inductance was tuned manually by
changing the position of the tuning plate. The result of this
measurement is plotted in Fig. 9, which shows the output
power, drain supply current, drain efficiency, and power-added
efficiency versus the tuning of the series inductance, i.e., the
amplifier is tuned between the high-efficiency point toward the
high power point, as was described in the theory (point to
point in Fig. 4). As can be seen from the figure, the power
and efficiency behavior is similar to the simulations with a
maximum power of 54 W and an associated drain efficiency
of 70%. The experimental efficiency is increasing when
is increased from the maximum power point in accordance
with the theory, at 14-W output power, the drain efficiency is
88%. The power-added efficiency is decreasing at low output
power levels since the output power is decreased, while the
Fig. 10. Output power and drain efficiency as a function of the drain supply
voltage.
input power is constant. Decreasing the input power could
possibly increase the power-added efficiency, but this was not
investigated in this study.
One interesting feature with the class-E amplifier is the
variation of the output power versus the drain supply voltage.
The peak drain voltage is of the order three times the drain
supply voltage, as can be seen from the waveform simulations,
up to a point when the drain-breakdown voltage is reached.
In Fig. 10, the output power and drain efficiency is plotted as
a function of the drain supply voltage. The drain efficiency
is constantly between 5–18 V. Supported by the simulations,
we believe the decrease of the efficiency above V
is due to breakdown in the transistor. The power versus drain
supply voltage is clearly quadratic up to a point when the
drain efficiency start to drop. The output spectrum was also
monitored by an HP 8563 Spectrum analyzer. The second
harmonic dominated and was typically 38 dB below the carrier.
VI. SUMMARY
A class-E amplifier based on LDMOS transistors working
at VHF was demonstrated by simulations and experiments. A
record-high power was obtained in class E at this frequency,
i.e., 62 W, from a single device with 121-mm gatewidth. At
a lower power, i.e., 14 W, the drain efficiency was as high
as 88%. The key of this result is proper tuning of the series
resonator network, computer simulations with good models,
and the use of a novel inductor design. The drain efficiency
was experimentally found to be independent on the supply
voltage up to the breakdown voltage of the transistor.
REFERENCES
[1] G. G. Ewing, “High-efficiency radio-frequency power amplifiers,” Ph.D.
dissertation, Oregon State Univ., Corvallis, OR, 1964.
[2] N. O. Sokal and A. D. Sokal, “Class-E a new class of high-efficiency
tuned single-ended switching power amplifiers,” IEEE J. Solid-State
Circuits, vol. SC-10, pp. 168–176, June 1975.
[3] F. Raab, “Idealized operation of the class-E tuned amplifier,” IEEE
Trans. Circuits Syst., vol. CAS-24, pp. 239–247, Apr. 1978.
[4] J. F. Davis and D. B. Rutledge, “A low-cost class-E power amplifier
with sine-wave drive,” in Proc. IEEE MTT-S Microwave Symp. Dig.,
Baltimore, MD, 1998, pp. 1113–1116.
2538 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
[5] S. Li, “UHF and X-band class-E amplifiers,” Ph.D. dissertation, Dept.
Elect. Eng., California Inst. Technol., Pasadena, CA, 1998.
[6] T. B. Mader and Z. B. Popovic´, “The transmission-line high efficiency
class-E amplifier,” IEEE Microwave Guided Wave Lett., vol. 5, pp.
290–292, Sept. 1995.
Herbert Zirath (S’84–M’86), for photograph and biography, see this issue,
p. 2357.
David B. Rutledge (S’77–M’77–SM’89–F’93), for photograph and biogra-
phy, see this issue, p. 2176.


An LDMOS VHF class-E power amplifier using a high-Q novel variable inductor

A low-cost class-E power amplifier with sine-wave drive,” in

Class-E a new class of high-efficiency tuned single-ended switching power amplifiers,”

High-efficiency radio-frequency power amplifiers,”

Idealized operation of the class-E tuned amplifier,”

The transmission-line high efficiency class-E amplifier,”

UHF and X-band class-E amplifiers,”

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.397.9263

An LDMOS VHF class-E power amplifier using a high-Q novel variable inductor

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