A high-linearity SiGe RF power amplifier for Linköping University Post Print

A high-linearity SiGe RF power amplifier for Linköping University Post Print
A high-linearity SiGe RF power amplifier for
3G and 4G small basestations
Ted Johansson, Noora Solati and Jonas Fritzin
Linköping University Post Print
N.B.: When citing this work, cite the original article.
This is an electronic version of an article published in:
Ted Johansson, Noora Solati and Jonas Fritzin, A high-linearity SiGe RF power amplifier for
3G and 4G small basestations, 2012, International journal of electronics (Print), (99), 8, 11451153.
International journal of electronics (Print) is available online at informaworldTM:
http://dx.doi.org/10.1080/00207217.2011.651695
Copyright: Taylor & Francis
http://www.tandf.co.uk/journals/default.asp
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-79712
IJE 2011 rev2
A High-Linearity SiGe RF Power Amplifier for 3G and 4G Small Basestations
Ted Johansson (corresponding author)
Huawei Technologies Sweden AB, P.O. Box 54, SE-164 94 KISTA, Sweden.
Division of Electronic Devices, Department of Electrical Engineering, Linköping University,
SE-581 83 Linköping, Sweden.
email: ted.johansson@huawei.com, alt. ted@isy.liu.se
Postal address: Huawei Technologies Sweden AB, P.O. Box 54, 164 94 KISTA, Sweden.
Telephone: +46-70-6270237
Fax: +46-8-12060800
Noora Solati
Division of Electronic Devices, Department of Electrical Engineering, Linköping University,
SE-581 83 Linköping, Sweden.
Jonas Fritzin
Division of Electronic Devices, Department of Electrical Engineering, Linköping University,
SE-581 83 Linköping, Sweden.
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A High-Linearity SiGe RF Power Amplifier for 3G and 4G Small Basestations
Abstract
This paper presents the design and evaluation of a linear 3.3 V SiGe power amplifier (PA) for 3G and 4G
femtocells with 18 dBm modulated output power at 2140 MHz. Different biasing schemes to achieve high
linearity with low standby current were studied. The ACPR linearity performance with WCDMA (3G) and LTE
(4G) downlink signals were compared, and differences analysed and explained.
Keywords
Power amplifier, WCDMA, LTE, biasing, linearity, ACPR
1. Introduction
Long Term Evolution (LTE) is becoming a widely used standard for 4G to meet demands for
increased data rates of cellular transmissions (
, Dahlman, Furuskär, Jading, Lindström
and Parkvall 2009). While 3G WCDMA uses fixed 5 MHz channels with transmission
bandwidth of 3.84 MHz (bandwidth utilisation of 77 %) (3GPP TS25.104 2010), LTE uses
bandwidths between 1.4 MHz and 20 MHz (all but the lowest band with utilisation of 90 %)
(3GPP TS36.104 2010), which allows for efficient use of spectrum and high data rates up to
100 Mbit/s in the downlink (DL) and 50 Mbit/s in the uplink (UL).
With the introduction of advanced modulation schemes, such as used in EDGE (2.5G),
WCDMA (3G) and OFDMA (4G), the linearity of the power amplifier (PA) has become an
important research topic. The air interface of LTE is based on orthogonal frequency division
multiple access (OFDMA) in the DL and single carrier frequency division multiple access
(SC-FDMA) in the UL direction. The use of SC-FDMA reduces the peak to average power
ratio (PAPR) by 2-4 dB, which mitigates the linearity requirement and also helps the design
of high efficiency terminal power amplifiers, in contrast to WiMax which uses OFDMA for
both DL and UL. However for the DL from the basestation, 3G WCDMA and 4G LTE use
high Peak-to-Average Power Ratio (PAPR) of 10-12 dB. This requires a large power backoff from peak power, reducing the efficiency. If using a linear class-A/AB amplifier design
for this application, DC-to-RF conversion efficiency plays a minor role, instead heat
dissipation from the DC biasing current is a major concern. A linear PA with low DC biasing
current therefore becomes essential.
ACPR (Adjacent Channel Power Ratio, also ACLR), which is affected by multi-carrier
intermodulation, is a good indication of "wideband linearity" for devices operating with
multicarrier signals compared to other linearity measures such as P-1dB compression point,
third-order intercept (TOI), or IMD products. The 3GPP standards define limits for ACPR
and EVM (Error Vector Magnitude), which both relate to linearity, but as a rule of thumb the
ACPR requirements are harder to fulfil.
Commonly, GaAs HBT PAs have been used for linear PA applications at low-to-medium
power levels for both handsets (usually in multichip modules (MCM)), and basestations
(Franco 2009; Li, Zhu, Prikhodko and Tkachenko 2010). Advances in SiGe BiCMOS
technologies, using the SiGe bipolar transistors for the amplifier and with possibility of
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higher integration using CMOS parts, make it interesting to evaluate the potential of this
technology. As GaAs-based devices generally are regarded to be more linear than Si-based
devices, it i e pecia in ere ing o inve iga e he 3G and 4G DL modu a ion cheme ’
impact on performance and linearity of Si-based PA designs.
Recent published work has explored SiGe-based devices for terminal UL applications. Li,
Lopez, Lie, Chen, Wu and Yang (2010) designed a SiGe cascode differential PA chip and
used it in an Envelope-Tracking amplifier measurement setup to achieve high efficiency and
high linearity for LTE applications. Krishnamurthy et al. (2010) describe a commercial multistage design in SiGe technologies, however focusing on WiMax performance with PAPR of
about 6 dB. Good linearity is achieved by dynamically balancing the biasing and generation
of harmonics between three amplifier stages. (The idea has similarities with the work by
Aoki, Kunihiro, Miyazaki, Hirayama and Hida (2004) where two amplifiers with different
gain expansion/compression characteristics are connected in series to cancel harmonics.)
This paper describes a linear SiGe PA design capable of delivering 18 dBm of modulated
linear output power in the 2110-2170 MHz DL band using a 3.3 V supply voltage for
WCDMA and LTE signals. Target is an output stage for a home basestation (femtocell) for
3G and 4G operation, but could also be a basestation linear predriver amplifier stage.
2. Amplifier Design
2.1 Amplifier core design
The goal of the PA design in this paper was an amplifier for 3GPP basestation (femtocell)
downlink Band 1 (2110-2170 MHz) with 18-20 dBm modulated output power, aimed for a
home base station. Carrier frequency of 2140 MHz and supply voltage of 3.3 V were used in
all simulations and evaluations. As modulated signal power plus PAPR for the signal roughly
translates to peak power with linear performance (Cripps 2002), the goal for the P-1dB output
power was set to 27 dBm.
IBM 5PAe 0.35 μm SiGe BiCMOS foundry technology was used. The process includes an
RF-PA optimized SiGe-bipolar device with high breakdown voltages, BVCEO of 8.3 V, and
BVCBO of >20 V, which allows linear PA class-A/AB operation up to 5 V supply voltage with
margins for load mismatch and long-term reliability without the need for cascode amplifier
topology. The technology is aimed for integrated PA design, with features such as two thick
top metal layers, TWV (through-wafer vias), 50 Ohm-cm substrate resistivity, and 0.35 um
CMOS. The TWV is especially useful for PA applications since it provides a low-reactance
path to ground, and increases the RF gain.
72 transistor elements of 3 x 0.8 x 20 um2 emitter area were needed to achieve the target
output P-1dB output power according to estimations and initial load-pull simulations in
Agilent ADS. A single-stage single-ended amplifier topology was used. Seven TWV close to
the transistor elements array assured good emitter ground connections to the amplifier.
2.2 Bias circuit design
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Besides supporting the PA with a well-defined DC-bias point, the biasing circuit can have a
large influence on the linearity of the PA. Fig. 1 shows the schematic of a one-stage PA with
a conventional resistive bias. The circuit consists of LC-based input and output matching
networks, series capacitors for signal DC blocking (not shown), an RF choke (RFC) to the
power supply, a load, RL (50 Ohm), and a bias resistor, Rbias. The biasing may also consist of
two resistors arranged as a voltage divider between supply and ground.
The PA core and biasing were simulated using Agilent GoldenGate simulator in the Cadence
Virtuoso design environment. Ideal LC input and output matching networks were used to
tune the PA to achieve maximum output power while keeping ACPR <-45 dBc for the
adjacent channel according to the 3GPP standards for WCMDA (3GPP TS25.104 2010) and
LTE (3GPP TS36.104 2010) signals. Fig. 2 shows the bias current dependence on the
simulated maximum WCDMA modulated linear output power while maintaining ACPR<-45
dBc for the PA in Fig. 1. The ACPR performance was optimised in each point by tuning
matching and input power. The highest modulated power for the resistor-biased amplifier,
20.2 dBm, was achieved using a collector bias current in the 400-450 mA range.
As discussed in Section 1, the efficiency of the studied amplifier is of less importance;
instead low DC standby current while maintaining linear operation becomes essential. If we
could reduce the biasing current without degrading the linearity, this would be a significant
advantage.
The drop in linearity at high output power levels is related to early gain compression caused
by Vbe (base-emitter voltage) DC bias drop at the RF transistor (Yoshimasu, Akagi, Tanba
and Hara 1998). Fig. 3a shows the Vbe characteristics for the PA in Fig. 1 for two bias
currents representing weak Class-A (Ic=500 mA) and Class-AB (Ic=300 mA) operation, with
similar behaviour for both bias currents.
To avoid the bias drop, what we want is a constant voltage source (and an RF block). In
in Fig. 3b the Vbe characteristics have been added for the circuits in Fig. 4a (biasing using a
constant voltage source and an ideal RF block (Vsource)), and Fig. 4b (implementation of the
constant voltage source biasing using a current mirror (Current mirror)). A modified Wilson
current mirror (Wilson 1968; Hart and Barker 1976) was used, as it is well known to provide
robust and distortion-free constant current or voltage. For simplicity, same 3 x 0.8 x 20 um2
emitter area transistor elements as used for the amplifier were used for the current mirror
design, one element for each transistor in the schematic. A 3 nH on-chip inductor was used as
RF block between the bias circuitry and the RF path.
In Fig. 5, the linear Pout characteristics is compared for the three different biasing circuits.
The improvement by using a voltage source is clearly visible at lower bias currents where an
additional 0.5 dBm of modulated power could be obtained using bias currents in the 275-300
mA range (Class-AB) without loss of ACPR. The reduced DC current during maintained
linear operation is the real advantage of the improved biasing circuitry.
3. Measurements
A.
3.1 Basic RF evaluation
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Several PA design variants were fabricated. Fig. 6 shows a micrograph of a fabricated PA
with an additional on-chip inpu prema ch. The chip ize of a varian wa 715 x 830 μm2.
The chips were directly bonded on the testboard, which was a two-layer 0.2 mm thick PCB
wi h εr of 4.7 and an δ of 0.019. Matching to 50 Ω was achieved by passive SMD
components. The testboard was designed in Agilent ADS and from the simulations, starting
values for the component values and positions on the PCB were obtained. An external SMD
resistor (Rmirror in Fig. 4b) was used during the evaluations to select transistor bias current.
RF measurements were done using Rohde & Schwarz SMBV100A Vector Signal Generator
and Anritsu 2692 Vector Signal Analyzer.
The PA was matched for highest saturated output power and high gain using a bias current of
around 300 mA (class-AB). If higher output power is needed, the PA can be operated 5 V,
which will further increase the output power an estimated 2-3 dB. However, with the used
PCB with no additional cooling or heatsink, the temperature at 5 V supply became too much
elevated to perform any reliable measurements.
Fig. 7 shows a plot of the measured and simulated output power and PAE vs. input power at
3.3 V supply voltage and a frequency of 2140 MHz. The PA delivered 26.4 dBm P-1
saturated power with a maximum collector efficiency of more than 50 %. The small-signal
gain was 14 dB. There is a good correlation between the measured and simulated
characteristics as can be seen from the plots.
A.
3.2 Linearity performance
In this section, we will evaluate linearity performance using ACPR as defined in the 3GPP
standards (3GPP TS25.104 2010; 3GPP TS36.104 2010). We will compare WCDMA signals
with LTE signals at the same frequency and try to understand differences in the obtained
values.
The 3GPP standards define a number of Test Models for the DL, i.e. predefined signal sets
aimed for measuring certain parameters or performance. For WCDMA, Test Model 1 (TM1)
should be used for ACPR tests (3GPP TS25.141, 2009). LTE should use E-UTRA test Model
1.2 (E-TM1.2) (3GPP TS36.141, 2010). The ACPR was measured using the same setup as in
the previous section and was also simulated with additional parasitics from external
components and PCB using Agilent's GoldenGate simulator in Cadence Virtuoso design
environment with Test Model signals generated in Agilent Signal Studio as baseband inputs
in the simulations.
Fig. 8 shows the measured ACPR performance for a WCDMA TM1-64QAM signal and an
LTE E-TM1.2-64QAM signal, with different LTE bandwidths. The WCDMA signal reached
the -45 dBc limit at a measured modulated output power of 17.9 dBm. The LTE signals,
except for 1.4 MHz bandwidth, reached the limit at slightly lower output power, around 17.017.4 dBm, while the 1.4 MHz signal reached the limit at 18.4 dBm. Simulated values for the
measurement case (not shown) were similar (18.2, 16.7-17.4, and 18.8 dBm), but lower than
expected from section 2 where all parameters (bias, matching component values) were
optimized for each point and just a few parasitics included in the simulations.
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Interestingly, Fig. 8 shows a clear visual grouping of the measured ACPR curves (simulated
curves show same visual grouping), which appears to be characteristic for the different
signals. This behaviour can be understood by considering the different bandwidth utilisation
of the signals. Higher utilisation results in more signal power closer to the band limit, with a
larger part of the "signal tail" going into the next band, which will increase ACPR (Sesia,
Toufik, and Baker 2009). All LTE signals, except the 1.4 MHz, have the same utilisation (90
%) and fall into one group. The 1.4 MHz has lower utilisation (77 %) and thus ACPR is
higher. Although the 1.4 MHz LTE signal and the WCDMA signal have the same utilisation
(77 %), the occupied bandwidth (99 % of the power) is somewhat larger for the WCDMA
signal. It was measured to 4.2 MHz or 84 % of the band, and for the LTE signal it was 1.09
MHz or 78 % of the band. This will contribute to an increase in ACPR, which may well
explain the observed difference.
To increase the ACPR even further (or more correctly, the output power before ACPR
limitation), different approaches are needed. Using an analogue Dynamic Predistorter,
Yamanouchi, Aoki, Kunihiro, Hirayama, Miyazaki, and Hida (2007), were able to improve
the output power by over 15 dB for WCDMA terminal (UL) signals. But the most commonly
used approach today is Digital Pre-Distortion (DPD) (Kim, Stapleton, Kim and Edelman
2005) where algorithms in the baseband are used to correct for the non-linearities of the PA.
4. Conclusions
A 3.3 V linear 18 dBm modulated output power SiGe-based PA have been designed and
evaluated for WCDMA and LTE downlink signals. Different biasing schemes for high
linearity with low standby current were studied. By using a current mirror instead of a
conventional resistor biasing, increased ACPR at lower DC biasing was achieved.
Differences in ACPR linearity between WCDMA and LTE signals were observed and
explained.
Acknowledgments
The authors thanks Rohde & Schwarz Sweden for support during the evaluations, Per Landin,
Gävle University, Sweden, and Jan Sjögren, Agilent Sweden, for valuable discussions on the
ACPR measurements interpretations.
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References
3GPP TS25.104 (2010), ‘Ba e S a ion (BS) radio ran mi ion and recep ion (FDD)’, V9.5.0,
2010, subclause 6.6.2.2.1.
3GPP TS25.141 (2009), ‘Ba e S a ion (BS) conformance e ing (FDD)’, V9.5.0, 2010,
subclause 6.1.1.1.
3GPP TS36.104 (2010), ‘Evo ved Univer a Terre rial Radio Access (E-UTRA); Base
S a ion (BS) radio ran mi ion and recep ion’, V9.5.0, 2010, ubc au e 6.6.2.1.
3GPP TS36.141 (2010), ‘Evo ved Univer a Terre ria Radio cce
S a ion (BS) conformance e ing’, V9.5.0, 2010, ubc au e 6.1.1.1.
(E-UTRA); Base
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'LTE: The Evolution of Mobile Broadband', IEEE Communications Magazine, vol.47, no.4,
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Cripps, S. C. (2002), Advanced Techniques in RF Power Amplifier Design, Norwood, MA:
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Aoki, ., Kunihiro, K., i aza i, T., Hira ama, T., and Hida, H. (2004), ‘ 20-mA
quiescent current two-stage W-CDMA power amplifier using anti-phase intermodulation
di or ion’, IEEE Radio Frequency Integrated Circuits (RFIC) Symposium, 357-360.
Hart, B. L., and Bar er, R. W. . (1976), ‘D.C. ma ching error in he Wi on curren ource’,
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Kim, W.-J., Stapleton, S.P., Kim, Kim, J. H., Edelman, C., 'Digital predistortion linearizes
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Nomadic Applications’, IEEE Radio Frequency Integrated Circuits Symposium (RFIC), 569572.
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SiGe Differential Power Amplifier Using An Envelope-Tracking Technique for 3GPP LTE
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Li, ., Zhu, R., Pri hod o, D., and T achen o, . (2010), ‘LTE Power mp ifier odu e
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Sesia, S., Toufik, I., Baker, M. (2009), LTE, the UMTS long term evolution: from theory to
practice, Chicester: Wiley, 512-517.
Yamanouchi, S., Aoki, Y., Kunihiro, K., Hirayama, T., Miyazaki, T., and Hida, H. (2007),
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Figure Captions
Figure 1. PA design schematics with conventional resistor biasing.
Figure 2. Simulated maximum WCDMA modulated output power while ACPR<-45 dBc for
the PA in Fig.1 with the conventional resistor biasing.
Figure 3a. Vbe vs. Pout for the PA in Fig.1 with the conventional resistor biasing for two
biasing currents.
Figure 3b. Vbe vs. Pout for different biasing schemes and biasing currents.
Figure 4a. PA design schematics an ideal voltage source for the biasing.
Figure 4b. PA design schematics an current mirror circuit for the biasing.
Figure 5. Simulated maximum WCDMA modulated output power while ACPR<-45 dBc for
PA design schematics with conventional resistor biasing in Fig. 1 (Resbias), ideal voltage
source in Fig. 4a (Vsource), and current mirror circuit in Fig. 4b (Current mirror).
Figure 6. Chip photo. Chip size was 715 x 830 μm2 for all chip variants.
Figure 7. Measured and simulated output power and efficiency vs. input power at 2140 MHz
and 3.3 V supply voltage.
Figure 8. Measured ACPR performance for WCDMA and LTE signals.
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Figure 1
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Figure 2
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Figure 3a
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Figure 3b
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Figure 4a
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Figure 4b
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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