Design and W-CDMA Characterization of a Wideband AlGaN/GaN

Design and W-CDMA Characterization of a Wideband AlGaN/GaN
Design and W-CDMA Characterization of a
Wideband AlGaN/GaN HEMT Power Amplifier for
Future 3G Multiband Base Station Applications
D. Wiegner1, U. Seyfried1, W. Templ1, T. Naß1, S. Weber1, S. Wörner1, I. Dettmann2, R. Quay3, F. van
Raay3, H. Walcher3, H. Massler3, M. Seelmann-Eggebert3, R. Reiner3, R. Moritz3, and R. Kiefer3
1
Alcatel SEL AG, Holderaeckerstrasse 35, 70499 Stuttgart, Germany (specification, board assembly and characterization),
fax: +49 711 821 32455, e-mail: Dirk.Wiegner@alcatel.de
2
Institute of Electrical and Optical Communication Engineering at University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart,
Germany (board design and simulation)
3
Fraunhofer Institute of Applied Solid State Physics, Tullastrasse 72, 79108 Freiburg, Germany
(GaN technology, modelling and packaging)
Abstract— A single stage wideband amplifier based on an
AlGaN/GaN HEMT power cell with a total gate-width of 16 mm
matched to 50 Ohm has been successfully developed and
characterized by use of a single-carrier W-CDMA signal. A very
wide bandwidth of more than 1.7 GHz covering several mobile
radio frequency bands within L- and S-band with peak output
power levels up to 44 dBm has been demonstrated with meeting
3GPP ACLR requirement in the complete measured frequency
range. The maximum measured output power level was 45.8 dBm
translating into a power density of 2.4 W/mm, which is superior
compared to that of established technologies like LDMOS. The
presented power amplifier shows the impressive potential of
AlGaN/GaN HEMT technology regarding output power and
bandwidth, supporting the development of future multiband/
multistandard capable base stations.
Keywords- multiband/multistandard, AlGaN/GaN HEMT
technology, wideband amplifier, W-CDMA test signal
I.
INTRODUCTION
Since the number of mobile radio standards (GSM, UMTS,
WiMAX, etc.) and frequency bands (900 MHz, 2100 MHz,
2500 MHz, etc.) increased in mobile communication in recent
years, the demand for multiband/multistandard capable base
stations arose [1], e.g. in order to reduce system manufacturers’
product diversity and to support the flexibility of mobile
operators. Additionally linearity and efficiency requirements of
future base stations increase, due to new modulation schemes
with peak-to-average ratios (PAR) of e.g. 10.5 dB for single
carrier W-CDMA signals, which are used in order to improve
data rate, necessary to allow new applications like music or
video download. These requirements of future mobile base
stations directly translate into the power amplifier module,
which is located in the transmit path of a base station and
which is one of the most challenging components when
realising a multiband/multistandard capable base station.
Established technologies like LDMOS e.g. provide high power
and good linearity but only a limited bandwidth especially for
high output power levels. New wide bandgap semiconductor
device technologies like GaN HEMT promise an improved
power and broadband capability, due to superior physical
properties like high breakdown voltage and high carrier
mobility. A high breakdown voltage allows to apply higher
supply voltages, resulting in an increased power density and
thus increased impedance level of the power transistor itself.
An increased impedance level facilitates realisation of
broadband matching circuit to the 50 Ohm in- and output of the
amplifier module. First reported promising results of assembled
wideband power amplifiers [2,3], covering several hundreds of
MHz and providing power levels up to 100 W, show a high
potential of GaN technology for future wideband/multiband
amplifier applications.
II.
DEVICE TECHNOLOGY
The AlGaN/GaN HEMT technology is based on multiwafer
MOCVD growth on 2-inch s.i. SiC substrates using an Aixtron
2000 multiwafer reactor. The T-gate technology is e-beam
defined with a gate length of lg = 300 nm. The basic sub cells
with a gate-width of Wg = 2 and 4 mm yield a maximum drain
current density of > 900 mA/mm and a transconductance of
> 200 mS/mm at VDS = 35 V. The current gain cut-off
frequency fT of the 2 mm device is beyond 30 GHz. Currently
the maximum operational voltage is VDS = 35 V with a
corresponding on-state breakdown voltage of BVDS > 70 V.
The basic cell of 2 mm yields a saturated output power density
of 6 W/mm and a linear gain of 23 dB at VDS = 35 V and
2 GHz single carrier operation. Single carrier power added
efficiency (PAE) can be as high as 58 % with proper second
harmonic tuning. The devices are diced and soldered by Au/Sn
process. They are packed in standard LDMOS packages
optimised for broadband operation without internal
prematching. Thus, special care is taken for the stabilisation of
the devices with respect to low- and high frequency oscillations
without compromising the broadband operation. The
optimisation of the bond wires within the package is found
useful for that purpose. Further, odd mode suppression is
achieved by various means, such as the introduction of
additional losses and additional bond wires.
III.
VDrain
VGate
RFC
RFC
RL
Zopt
/in
Pavs
Input
Matching
Output
Matching
/out
Pout
Figure 1. Power amplifier module
To obtain the maximum output power, the reference impedance
Z0 (typically 50 9) must be transformed to the optimum output
impedance Zout_opt of the transistor. Zout_opt can be derived from
load-pull measurements, simulation or by the load line method
[4]. With the load line method the optimum resistance Ropt
inside the transistor can be approximately calculated to
1 V - VK Ropt = × DS
8
Pout
Gate
LS
C in
POWER AMPLIFIER DESIGN
Fig.1 shows the typical block diagram of a power amplifier
module. The circuit consists of an input and output matching
block and a bias circuit for the gate and drain of the transistor.
While the output matching network is designed for highest
output power, the input matching network is designed for flat
gain and high bandwidth. The bias circuits also guarantee the
stability of the amplifier. The bias point was found by power
sweep simulation over different bias points. At the optimum
bias point the gain is nearly constant over the power sweep.
RS
Z0
2
(1)
where VK is the knee voltage, VDS the drain voltage and Pout the
output power of the transistor. For high voltage power
transistors as GaN transistors, Ropt is closer to the reference
impedance Z0 compared to e.g. a LDMOS transistor with the
same output power. However, due to the parasitic capacitances
of the transistor and the housing Ropt is transformed to Zout_opt
with a value well below 50 9. The parasitic output capacitance
limits the bandwidth of the transistor. Since the output
capacitance of a GaN transistor is smaller than e.g. that of an
LDMOS device with comparable output power, GaN
transistors are more attractive for broadband applications. At
the input of the transistor a matching network is required to
obtain a flat gain characteristic over the required frequency
range. The input circuit of the transistor can be modelled as a
series LC resonance circuit in series with the reference
impedance Z0 as shown in Fig.2.
Figure 2. Simplified input circuit of the transistor
To get a high bandwidth, Z0 must be transformed to low values.
The required matching network should have a low quality
factor Q in order not to limit the bandwidth performance of the
transistor and to get a flat gain characteristic. In addition,
matching networks should also guarantee low reflection
coefficients at the input and output of the amplifier. However,
both high bandwidth and low reflection can not be fulfilled at
the same time for a single transistor amplifier at the input, as
bandwidth and reflection coefficient are coupled by Fanos limit
[5]. At the output the reflection coefficient is determined due to
the necessity of matching the reference impedance Z0 to Zout_opt.
Therefore input and output reflection coefficients are secondary
design goals for a broadband single transistor power amplifier.
Since a large signal model has been available from FhGIAF Freiburg, Zout_opt has been found by load pull simulation.
At 2000 MHz, Zout_opt of the GaN power cell has a value of (3.7
+ j1.2) 9. The change of Zout_opt over the required frequency
range is sufficiently small. The output matching network has
been designed using the Smith chart. Capacitors in the output
matching network for matching purposes have been avoided
since they introduce losses in this network, which degrades the
overall efficiency of the amplifier. The matching network
consists of five line sections. The quality factor Q of the
network is kept low enough in order not to affect the large
signal bandwidth of the transistor over the required frequency
range.
To obtain a large small signal bandwidth and a flat gain
characteristic, the input matching network transforms the 50 9
reference impedance to 1.5 9. A multisection transformer at
the input has been designed using the Smith chart. The
multisection transformer consists of line sections with varying
line impedances. Each line section is terminated by a shunt
capacitor. Using shunt capacitors instead of e.g. quarter-wave
transmission lines reduces the overall size of the matching
network and allows tuning the network after production. Three
sections have been used to get a sufficiently low Q factor of the
network.
The bias networks at the input and the output of the
transistor have been designed by using quarter-wave
transformers. The length of the lines are determined by the
upper frequency limit. This guarantees an inductive behaviour
of the bias network over the whole frequency range. The
impedance of the bias network is several times higher than the
impedance of the matching network over the whole frequency
range and therefore the bias network has no impact on the
matching network. A proper termination of the bias network
for both high and low frequencies is important. The impedance
of the bias network should be constant from DC to the
modulation bandwidth. This reduces nonlinearities when the
amplifier is driven near the compression point [6].
IV.
PROTOTYPE ASSEMBLY
V.
CHARACTERIZATION RESULTS
First characterization measurements at a supply voltage of
VDS = 35 V have been performed by using a single-carrier WCDMA test-signal with a peak-to-average ratio (PAR) of
approximately 10.5 dB which means no reduction of signal
dynamic. Furthermore, no linearisation was used for the
measurements. The amplifier has been characterized with
respect to 3GPP Adjacent Channel Leakage Ratio (ACLR)
specification, which requires a signal suppression of - 45 dBc
at 5 MHz frequency offset and - 50 dBc at 10 MHz frequency
offset. Fig.5 illustrates the frequency characteristic of the
assembled amplifier within a wide measured frequency range
from 1000 MHz up to 2800 MHz at a peak input power level of
approximately 32.5 dBm.
Single Carrier W-CDMA Performance vs. Frequency
@ approx. 32.5 dBm peak input power
Power [dBm], Gain [dB],
The designed wideband GaN amplifier has been assembled
as a first amplifier demo-board by Alcatel SEL model
workshop. In- and output matching has been realised by
microstrip lines and lumped elements. The realised amplifier
prototype is illustrated in Fig.3.
Figure 4. Packaged GaN power cell (TGW = 16 mm)
Pout (peak)
Pout (average)
Gain
ACLR -5 MHz
-15+5 MHz
ACLR
55
50
45
40
-25
35
30
-35
25
20
-45
15
10
-55
5
0
-65
900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900
ACLR [dBc]
Stability is an important issue for power amplifiers. Since
power transistors show a very high transconductance gm, they
tend to become instable due to parasitic feedback capacitances.
Inband stability is guaranteed by the design of the matching
networks. However, to achieve outband stability additional loss
must be included. This can be realised either in the output or in
the input matching network. However, it is not recommendable
to add loss in the output matching network because this will
degrade the efficiency of the amplifier. In many cases stability
problems occur at frequencies near the lower frequency limit of
the amplifier pass band. At these frequencies the matching
network is blocked by the decoupling capacitors and the
transistor sees mainly the impedance of the bias network.
Inductive behaviour of the gate and drain bias network at these
frequencies will potentially cause oscillation. Adding a small
series resistor in the gate bias network improves the stability of
the amplifier. Placing a series resistor in the drain bias network
is not possible, since a high current is flowing through the drain
bias line. Therefore a small shunt resistor decoupled by a
capacitor is placed in the bias network. The design and
simulation of the wideband amplifier has been done at the
Institute of Electrical and Optical Engineering at University of
Stuttgart.
Frequency [MHz]
Figure 5. Frequency characteristic at nearly constant input power
Figure 3. 16 mm GaN wideband amplifier prototype
The GaN power bar with a total gate-width (TGW) of 16 mm
housed in a single standard LDMOS package is depicted in
Fig.4. The power bar itself consists of eight basic GaN power
cells with a TGW of 2 mm, each. Packaging and bonding was
done at FhG-IAF Freiburg. The GaN HEMT power cells
themselves are based on a FhG-IAF Freiburg proprietary
AlGaN/GaN HEMT technology. In order to reduce the source
inductance and to support stabilisation of the power cells,
bonding was done over an additional substrate. Due to the
current stress on drain side, a plurality of bond wires has been
used.
The effective ripple of the presented final amplifier stage can
be seen by means of the gain curve. The gain is about 10 dB
over a wide frequency range. At 32.5 dBm peak input power,
the amplifier achieves a maximum measured peak output
power of approximately 43 dBm at 1200 MHz and is above
40 dBm within the complete frequency range. The related
average output power is above 30 dBm for the used W-CDMA
signal. Since measured ACLR is clearly better than the
required - 45 dBc for wide frequency ranges, an improved
output power characteristic can be achieved by adjusting the
input power level considering 3GPP ACLR conformance, as
shown by Fig.6. Since the amplifier is intended to cover
frequency bands within the frequency range from 1800 MHz
up to 2700 MHz, comprising GSM, UMTS and WiMAX
bands, the power adaptation has been performed within this
limited frequency range.
Pout (peak)
Pout (average)
Gain
Pin (peak)
-15-5 MHz
ACLR
ACLR +5 MHz
Single Carrier W-CDMA Performance vs. Frequency
55
50
45
40
35
30
25
20
15
10
5
0
1700
-25
-35
-45
ACLR [dBc]
Power [dBm], Gain [dB]
@ Adjusted Peak Input Power Considering 3GPP ACLR Conformance
-55
1900
2100
2300
2500
2700
Figure 8. Single-carrier W-CDMA spectrum at 2.0 GHz
and 43.4 dBm peak output power
-65
2900
Frequency [MHz]
Figure 6. Improved frequency characteristic by adjusted input power level
The achieved peak output power level is now above 43 dBm,
with a related average output power level of approximately
34 dBm over a very wide frequency range. Maximum achieved
peak output power is 45 dBm at 1800 MHz. 3GPP ACLR
specification is met within the complete measured frequency
range.
Fig.7 shows the related power sweep measurement at
2000 MHz by use of a single-carrier W-CDMA test-signal.
1 dB compression point can be found approximately at a peak
output power level of 44 dBm. 3GPP ACLR at 5 MHz offset
will be met up to a peak output power of nearly 45 dBm,
ACLR at 10 MHz is well below the required - 50 dBc and thus
completely uncritical.
CONCLUSION
This work presents promising results of a GaN HEMT
based wideband power amplifier matched to 50 Ohm for base
station applications in 3G mobile communication. The used
design approach in order to achieve a wideband characteristic
has been discussed and the assembled amplifier has been
characterized with respect to 3GPP ACLR specification. By
use of a single-carrier W-CDMA signal, the presented
wideband amplifier covers a very wide frequency range from
1000 MHz up to 2800 MHz, meeting 3GPP ACLR. By input
power adaptation, a peak output power level of approximately
43 dBm can be achieved over a wide frequency range.
Maximum measured output power at 2000 MHz is 45.8 dBm
translating into a power density of 2.4 W/mm, which exceeds
power densities of established technologies like e.g. LDMOS
or GaAs.
mean power
Gain
ACLR@5MHz
-5
ACLR@10MHz
P1dB
-15
-25
-35
-45
ACLR [dBc]
Output Power [dBm]
1-Carrier W-CDMA Signal, PAR approx. 10.5dB
50
45
40
35
30
25
20
15
10
5
0
ACKNOWLEDGMENT
peak power
Output Power vs. Input Power @ 2000 MHz
The authors wish to acknowledge the support of the
German Ministry of Education and Research (BMBF) under
contract 01BU382 and 01BU386 in the context of the
‘Leitinnovation “Mobile Internet”’. The continuous support of
the epitaxial and technology department at Fraunhofer IAF as
well as the support of the radio department and model
workshop at Alcatel R&I Stuttgart is gratefully acknowledged.
-55
REFERENCES
-65
22
24
26
28
30
32
34
36
38
[1]
Peak Input Power [dBm]
Figure 7. Power sweep measurement
The measured gain is approximately 10.8 dB for low input
power levels, when the applied test-signal is not driven into
compression. Fig.8 shows the spectrum of a single-carrier WCDMA signal directly at the output of the 16 mm wideband
GaN amplifier, at 2000 MHz. The shape of the spectrum at
2000 MHz suggests that 3GPP ACLR requirements at 5 MHz
offset (required: - 45 dBc, measured: - 47.48 dBc, - 46.66 dBc)
as well as at 10 MHz offset (req.: - 50 dBc, meas.: - 59.08 dBc,
- 59.55 dBc) are clearly met at a peak output power of approx.
21.8 W (43.4 dBm). The corresponding average output power
is approx. 2.2 W (33.4 dBm).
[2]
[3]
[4]
[5]
[6]
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Karlsruhe.
A. Maekawa, M. Nagahara, T. Yamamoto and S. Sano, “A 100W high
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D. Wiegner, T. Merk, U. Seyfried, W. Templ, S. Merk, R. Quay, F. van
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Steve C. Cripps, “RF power amplifiers for wireless communications”,
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