TECHNICAL ARTICLE
Walt DeMore
Engineering Manager,
Analog Devices, Inc.
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GAUGING THE STATE
OF GaN POWER
AMPLIFICATION
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Abstract
Improvements in GaN power semiconductor technology and modular
design are making possible high power continuous wave (CW) and
pulsed amplifiers at microwave frequencies.
Gallium nitride (GaN) power semiconductor technology has contributed
a great deal to improved performance levels in RF/microwave power
amplification. By reducing devices' parasitic elements, using shorter gate
lengths, and using higher operating voltages, GaN transistors have
reached higher output power densities, wider bandwidths, and improved
dc-to-RF efficiencies. For example, by 2014, GaN-based X-band
amplifiers capable of 8 kW pulsed output power were demonstrated for
radar systems applications as replacements for traveling wave tube (TWT)
devices and TWT amplifiers. By 2016, 32 kW variants of these solid-state
GaN power amplifiers are expected. In anticipation of the availability of
those amplifiers, some of the key characteristics and features of these
high power GaN amplifiers will be reviewed.
In the recent past, GaN has been the technology of choice for counter
radio frequency electronic warfare (CREW) applications with tens of
thousands of amplifiers delivered for field use. The technology is now
being deployed in the airborne EW arena as well, with amplifiers under
development capable of delivering hundreds of watts of output power
across multiple octaves in the RF/microwave range. Several variants of
these wideband EW power amplifiers are due for release in the current
calendar year.
Areas for further research include improvements in linearity for high peak
to average power ratio (PAPR) waveforms employed in many military
communications systems, including common data link (CDL), wideband
networking waveform (WNW), soldier radio waveform (SRW), and wideband satellite communications (satcom) applications. The “Bits to RF”
initiative at Analog Devices will integrate the company’s strengths in
baseband signal processing and GaN power amplifier (PA) technologies.
This integration will enable advances in PA linearity and efficiency through
the use of such techniques as predistortion and envelope modulation.
In the last few years, GaN-based devices, both discrete field effect
transistors (FETs) and monolithic microwave integrated circuits (MMICs),
have been released and widely used in high power microwave amplifier
systems. These devices, available from several foundry sources and
device manufacturers, are typically fabricated on 100 mm silicon carbide
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(SiC) wafers. GaN on Si processes are also under consideration; however,
the relatively poor thermal and electrical conductivities of Si offset the
cost advantages in high performance, high reliability applications. These
devices feature gate lengths as small as 0.2 μm and support operation
into millimeter wave frequency bands. GaN-based devices have largely
replaced gallium arsenide (GaAs) and Si laterally diffused metal oxide
semiconductor (LDMOS) devices in many high frequency applications and
all but the most cost-sensitive, lower frequency applications.
GaN devices are of interest to the RF power amplifier designer because
they support very high operating voltages (three to five times higher than
those of GaAs), and roughly double the allowable current per unit FET gate
width compared to GaAs devices. These characteristics have an important
consequence to PA designers, specifically a higher load impedance for
a given output power level. Previous GaAs- or LDMOS-based designs
often had extremely low output impedances relative to typical system
impedances of 50 Ω or 75 Ω. Low device impedances place limits on
achievable bandwidth, that is, as the required impedance transformation
ratio between the amplifying device and its load increases, the number
of components and insertion loss increase as well. Due to these high
impedances, early users of these devices were in some cases able to
achieve partial results by merely installing one in an unmatched test
fixture, applying dc bias, and driving the device with an RF/microwave
test signal.
GaN devices are also finding their way into high reliability space
applications due to these operating characteristics and their exceptional
reliability. Life test data from several sources of these devices predict
mean time to failure (MTTF) durations (for single devices) in excess
of one million hours at junction temperatures of 225°C or higher. This
exceptional reliability is mainly due to the high band gap value for GaN
(3.4 for GaN vs. 1.4 for GaAs). This makes them more than suitable for
high reliability applications.
The chief impediment to wider use of GaN in high power applications has
been its relatively high fabrication costs—typically two to three times that
of GaAs and five to seven times that of Si LDMOS-based devices. This has
usually been a barrier to use in cost-sensitive applications such as wireless infrastructure and consumer handsets. GaN on Si substrate processes
are available, albeit with the performance issues noted earlier, and devices
from these processes are probably best suited for these cost-sensitive
applications. In the near future, cost reductions, perhaps on the order
of 50%, are expected as GaN device fabrication moves to larger wafers,
with diameters of 150 mm and larger, now underway at several leading
foundry sources for GaN devices.
2
Gauging the State of GaN Power Amplification
Currently deployed radar systems for weather prediction and target
acquisition/identification rely on TWT-based power amplifiers operating
at C-band and X-band frequencies. The amplifiers run at high supply voltages (10 kV to 100 kV) and temperatures and are susceptible to damage
from excessive shock and vibration. The reliability in the field for these
tube-based amplifiers is typically 1200 h to 1500 h, which leads to high
costs for maintenance and for spare parts.
As an alternative to these high power TWT-based amplifiers, Analog Devices
developed an 8 kW, solid-state, X-band power amplifier based on GaN
technology. The design uses an innovative, layered combiner approach
to sum the contributed RF/microwave output power of 256 MMICs,
each developing approximately 35 W output power. The combining
methodology results in graceful performance degradation in the event
of individual MMIC failures. This is in contrast to TWT amplifiers, which
tend to suffer failures that are catastrophic in nature due to low levels
of redundancy. For these solid-state GaN power amplifiers, the RF/
microwave combining architecture must present a reasonable balance
between the isolation required between MMICs and the RF/microwave
insertion loss of the overall network.
The 8 kW amplifier topology is modular, comprised of four 2 kW
amplifier assemblies with their output power combined using wavguide
structures (Figure 1). The amplifier can be mounted in a standard
19" rack enclosure. The current design of the amplifier (Figure 2) is
configured for use with water cooling, although variants of the amplifier
using air cooling are under development. Table 1 provides a summary of
the performance levels for the water cooled 8 kW GaN PA.
Figure 1. GaN-based, solid-state power amplifier capable of 8 kW output power at
X-band frequencies.
Table 1. Typical 8 kW PA Performance
Rated Output Power
Frequency range
Rise/fall time (max)
8 kW
8 GHz to 11 GHz
200 ns
0.05 μs to 100 μs
20%
1.50:1
–70.0 dBc
–40.0 dBc
SMA
Waveguide
Pulse width
Duty cycle
Input/output VSWR
Out of band spurious noise (max)
Second-order harmonics (max)
RF input connector
RF output connector
The 8 kW SSPAs as designed in such a way that these modular SSPAs can
be combined to produce even higher power levels. Work is under way to
develop an amplifier that will combine three of these 8 kW SSPA modules
to achieve a unit with a peak output power level of 24 kW across the same
frequency range. Other configurations achieving power levels to 32 kW are
feasible and are under consideration for further evaluation.
Analog Devices is currently developing an advanced power module, also
based on GaN technology, that will double the RF/microwave output
power of current modules. The module is designed to be hermetically
sealed to support operation in extreme environments. This, along with
next-generation combining structures with reduced insertion loss
(compared to current approaches), will extend pulsed output power to
levels approaching 75 kW to 100 kW at RF/microwave frequencies.
These advanced, high power SSPAs will include control and processor
functionality to enable fault monitoring, built in test (BIT) functionality,
remote diagnostic testing, and control of fast, real-time bias control
circuits for the MMIC devices that power the amplifiers.
These GaN-based solid-state power amplifiers address the industry’s
need for amplifiers with wide instantaneous bandwidths and high output
power levels. Some systems attempt to meet these requirements using
channelized or multiple amplifiers, each covering a portion of the required
spectrum and feeding a multiplexer. This leads to increased cost and
complexity and results in coverage gaps at the frequency crossover
points of the multiplexer. A more effective alternative solution is
continuous coverage of wide frequency ranges at elevated power levels,
as has been accomplished with two different GaN-based amplifiers
covering VHF through L-band frequencies, as well as 2 GHz to 18 GHz.
W.G.
Combiner
2 kW Assemblies
4:1
1:8
8:1
1:8
8:1
1:8
8:1
1:8
8:1
Splitter
1:4
Figure 2. Block diagram representing the structure and components of the GaN, X-band, solid-state power amplifier.
Visit analog.com For use from VHF through S-band frequencies, Analog Devices has
developed a very small, feature rich, multiple octave amplifier capable
of 50 W output power from 115 MHz to 2000 MHz. The amplifier achieves
output power levels of 46 dBm (typically 40 W) across that full frequency
range when fed with a nominal input signal of 0 dBm.
Packed in a compact housing with dimensions of 7.3" × 3.6" × 1.4,"
the amplifier includes BIT functionality for thermal and current overload
protection, telemetry reporting, and an integrated dc-to-dc converter for
uncompromised RF performance with input supplies ranging from 26 VDC
to 30 VDC. Figure 3 provides a photograph of the amplifier, with typical
measured performance data for output power vs. frequency presented
in Figure 4.
Table 2. Typical Wideband SSPA Performance
Output Power
Frequency range
Duty cycle
Input/output VSWR
Out-of-band spurious noise (max)
Gain stability
RF input connector
RF output connector
50 W
2 to 18 GHz
100%
1.50:1
–70.0 dBc
2.5 dB
SMA
Type N
Figure 5. Amplifier capable of 50 W, CW output power from 2 GHz to 18 GHz.
Figure 3. Continuous wave (CW), 50 W, solid-state power amplifier operating from
115 MHz to 2000 MHz.
80.0
49.0
75.0
70.0
47.0
65.0
60.0
46.0
55.0
45.0
Power Gain (dB)
Output Power (dBm)
48.0
50.0
44.0
2000
1900
1800
1700
1650
1350
15000
1200
900
1050
750
600
450
300
150
115
43.0
45.0
40.0
Frequency (MHz)
Figure 4. Plot displaying the output power vs. frequency for the 50 W, 115 MHz to
2000 MHz power amplifier.
To address wideband applications above 2 GHz, Analog Devices has also
developed a GaN amplifier that produces 50 W continuous wave (CW)
output power across the entire 2 GHz to 18 GHz band. This amplifier uses
commercially available 10 W GaN MMICs with output power contributions
summed by means of a wideband, low loss combiner circuit. Multiple
amplifiers may in turn be combined to develop output powers as high as
200 W across this same 2 GHz to 18 GHz bandwidth. The driver amplifier
chain is also based on GaN active devices. The amplifier operates from
48 VDC and features an internal voltage regulator and high speed switching
circuits to enable pulsed operation with good pulse fidelity and fast rise and
fall times. Table 2 lists specifications for this amplifier. Figure 5 offers a
photograph of the amplifier, while Figure 6 presents the amplifier’s output
power as a function of frequency from 2 GHz to 18 GHz.
Figure 6. Plot displaying the output power vs. frequency for the 50 W, 2 GHz to 18 GHz
power amplifier.
This 50 W amplifier is one of a family of amplifiers covering the 2 GHz
to 18 GHz band. ADI has also developed a compact, benchtop amplifier
capable of 12 W output power (Figure 7) and a rack-mount unit developing
100 W output power (Figure 8). Other amplifiers, with frequency coverage
from 2 GHz to 6 GHz and 6 GHz to 18 GHz, are under development. ADI is
also working to increase the output power of these broadband amplifiers
from their present levels to power levels of 200 W and higher. To achieve
these higher output power levels, the company is developing modules
with increased output power, as well as broadband RF power combiners
with greatly improved combining efficiency and less loss than current
power combiners.
Figure 7. Wideband 2 GHz to 18 GHz power amplifier yielding 12 W CW output power
across its full frequency range.
3
About the Author
Walt DeMore received his B.S.E.E. from UCLA in 1983 and his
M.S.E.E. under the Hughes Aircraft Company fellowship program
in 1987. While at Hughes, he worked on several national security
programs including the Milstar satellite constellation and the
Wideband Global satellite system. His contributions included
microwave module and MMIC development, phased array subsystem
architecture and design, and leadership of engineering design teams.
He is also the holder of five U.S. patents.
Figure 8. 2 GHz to 18 GHz, solid-state power amplifier producing 100 W CW output
power across its full frequency range.
These are a few examples of the performance levels possible with
GaN-based, solid-state amplifiers. The unit costs of these amplifiers is
expected to decrease in the future as more GaN semiconductor vendors
move to larger wafer sizes and continue to improve their yields of devices
per wafer. Systems operating at millimeter wave frequencies will see
more use of GaN devices as gate lengths are reduced, enabling higher
frequency operation for GaN-based SSPAs. It is clear that current GaN
trends of improving performance and reducing costs should continue for
some time.
In 2002 Walt joined TRW, later acquired by Northrop Grumman,
where he led the development of high efficiency wideband RF
power amplifiers operating from 30 MHz to 45 GHz, and C, X, and
Ku-band SATCOM and terrestrial communication terminals. Walt
was elected as a Northrop Grumman Technical Fellow in 2010 in
recognition of this work.
Walt is currently the engineering manager leading the High Power
Subsystems Group at Analog Devices in San Diego, CA. The group
is focused on the design and development of very high power,
wideband solid state power amplifiers for radar and electronic
warfare applications.
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