Power amplifier performance improvement using tunable matching

International Journal of Electronics and Communication Engineering & Technology (IJECET),
INTERNATIONAL JOURNAL OF ELECTRONICS AND
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME

COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Special Issue (November, 2013), pp. 230-238
© IAEME: www.iaeme.com/ijecet.asp
Journal Impact Factor (2013): 5.8896 (Calculated by GISI)
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IJECET
©IAEME

Power Amplifier Performance Improvement Using Tunable Matching
Network
Zakaria Settaf1, Claude Duvanaud2, Hervé Cam3, Jean Marie Paillot4, Francis Huin5, Smail
Bachir6
1,3,5ACCO,

36-38 rue de la Princesse 78430 Louveciennes, France
of Poitiers, XLIM Laboratory, SIC Department, CNRS 7252
86962 Futuroscope Chasseneuil Cedex, France
Bât. SP2MI, Téléport 2, Bd Marie et Pierre Curie

2,4,6University

1zakaria.settaf@acco-semi.com, 2claude.duvanaud@univ-poitiers.fr

ABSTRACT: In mobile handsets, the power performance, efficiency, and transmission quality of
wireless systems are degraded by the impedance mismatch between the transceiver and the
antenna. Such a mismatch can be due to the component and antenna variations over the
frequency and with environmental conditions. This paper presents a multiband tunable
matching network (TMN) able to control the antenna impedance mismatch using lumped
elements, and proposes a procedure to adapt the TMN. It has been tested in simulation on an
integrated amplifier designed by ACCO on its patented CMOS technology. The TMN maintains
the output power of the power amplifier around 31.4 dBm with a power added efficiency (PAE)
of 51.5% at 900MHz for VSWR 3:1, and maintains the power around 30.2dBm with 38% PAE at
1900MHz for VSWR 3:1. In worst VSWR phase case, improvements of 3dB for output power
and 10% for PAE are achieved.

KEYWORDS: Impedance matching, Mobile antenna, Reconfigurable RF front end, Tunable
matching network

I.

INTRODUCTION

When designing a front-end circuit, designers always optimize the operating conditions for a
given load impedance. In wireless systems, the power amplifier (PA) is loaded by the
transmitting antenna. Therefore, any change in the impedance of the antenna directly affects
the performances in terms of power and consequently degrades the transmission quality.
The mismatch between the power amplifier and the antenna is particularly spotted in wireless
applications based on multiple transmission formats, therefore multiple carrier frequencies
and wide frequency band. This is emphasized for mobile handset applications using compact
and narrow band antenna. For that reason, it is difficult to obtain a good matching for all
frequency bands. Besides, the mobile’s environment disturbs the antenna impedance. For
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India

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example, user interactions such as the user’s hand position, can cause a severe degradation of
the performances of a mobile handset. In addition, an excessive mismatch can damage the PA
or the devices linked to the PA. Using a tunable matching network solves all of these problems
and allows getting good power performances by matching the antenna impedance.
Various authors have shown the advantages of TMN. In [1], a digitally tunable capacitor,
manufactured with the CMOS-on-Sapphire process, has been evaluated for 2G, 3G and 4G for
various load VSWR conditions. Under VSWR 3:1, the defined Power Delivered Improvement is
lower than 1dB. Another work proposes a single tunable element matching [2] providing good
power results by optimizing the transducer gain, for GSM850 & GSM 900 bands. In the
previous articles, TMNs are evaluated in terms of power delivered to the antenna, using a 50
ohm source at the TMN input. However, the TMN will be used at the output of the power
amplifier, which does not operate like an ideal source. For this reason, in this paper,
performances are evaluated using a system approach including a multiband power amplifier.
Another advantage of the proposed evaluation procedure is that the improvement of the
amplifier power added efficiency (PAE) can also be investigated.
In the first part of this paper, we explain the choice of the TMN topology and the procedure
used to optimize the component values. The objective is to counteract the antenna impedance
variation by improving the TMN transducer gain for 700-915MHz & 1500-2100 MHz
bandwidth and at VSWR 3:1, which corresponds to a user’s interaction with the mobile
antenna [3]. Hence TMN performances are presented in terms of output power and power
added efficiency (PAE) improvements with and without the TMN, for a multiband power
amplifier designed by ACCO.

II.

TUNABLE MATCHING NETWORK

The first step is to choose a TMN architecture allowing for wide impedance coverage in the
smith chart with low loss, for both 700-915MHz Low band (LB) and 1500-2100 MHz High band
(HB). Among the various architectures studied, the π architecture (Fig. 1) was determined the
most suitable because of a good tradeoff between impedance coverage and low losses.

Fig. 1: π-architecture
Fig. 2 presents the impedance coverage in the smith chart for the TMN at VSWR 3:1. A single π
network is appropriate to cover the requirements in HB, but in LB, a 2-π network is preferred
to achieve our goals. Furthermore, when the TMN is not required, a bypass mode can be useful.
For those reason, switches are introduced in order to bypass either the 1st/2nd network, or

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both. The final TMN architecture is illustrated in Fig. 3. The ranges of L and C values are shown
in Table 1.

Fig. 2: Impedance coverage of the TMN for the π architecture for VSWR 3:1 at: (a) 700MHz (b) 915 MHz
(c) 1500 MHz (d) 2100 MHz, with circle of VSWR 3:1.

Table 1: Capacitors and inductor value for each band

Fig. 3: Final TMN architecture

III.

ADAPTATIVE IMPEDANCE TUNER

Having the TMN network, we will first evaluate the various metrics, to choose the one allowing
the best optimization for the TMN performance; then, we will insert this metric into the

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optimization procedure. Different metrics can be used to optimize the performances of the
TMN:
 S11 parameter: to check the impedance matching, without getting any
information about losses.
 Transducer gain GT: allows to take into account both mismatches and component
losses [4].
The transducer gain (GT) is the ratio of the power delivered to the load (PL) to the power
available from the source (Pf).
(1)
Using the transfer matrix of the TMN, we can obtain the following expression of GT as a
function of the TMN component.

(2)

Fig. 4: TMN structure
In the case of the TMN (2π-architecture) described in Fig. 4, the transfer matrix is expressed as:

(3)

Having the transfer matrix, we can calculate GT of the TMN.

(4)

When we maximize this value, the power delivered to the antenna is maximized at the same
time; the losses due to both the matching network and the components are minimized.
The previous expression allows to calculate GT for given values of components L1, L2, C1, C2
and C3. At first, the procedure is based on the research of all the capacitor combinations
allowing a S11 target level of –20dB. If no combination satisfies the requirement, the S11

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criterion is relaxed for a lower S11 exigency. In a second step, the capacitor combination giving
the lowest GT is retained.

IV.

RESULTS

A TMN has been simulated in ADS Agilent with a multimode power amplifier (MMPA) designed
by ACCO on its patented CMOS technology. The simulations have been made for both 900MHz
and 1900MHz. The snapshot of the MMPA with TMN is shown in Fig. 6 and the MMPA
performances are given in Table 2.

Fig. 6: ADS scheme of TMN with MMPA

Table 2: MMPA performance at VSWR 1:1
The TMN is based on ideal lumped elements, where the losses are modeled through their
quality factor. Because of an insufficient impedance configuration at HB, the topology of the
TMN has been changed for each band as can be seen in Table 3. The new TMN structure is
shown in Fig. 7.

Table 3: New capacitor and inductor values for each frequency band


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Fig. 7: New TMN structure.
The simulations are made for:


The MMPA only at VSWR 3:1 – all phases.



The MMPA with TMN using only S11 as optimization metrics at VSWR 3:1– all
phases.



The MMPA with TMN using both S11 andGT as optimization metrics at VSWR 3:1 –
all phases.

The S11 criterion is set to -20 dB for all simulations.
Simulation results are shown in Fig. 8 for LB and Fig. 9 for HB.


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Fig. 8: Output power and PAE for LB


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Fig. 9: Output power and PAE for HB.
As expected, at some phases for TMN with S11 optimization only, compared with optimization
using S11 and GT, the output power and PAE are affected because of TMN losses. Hence, by
optimizing both S11 and GT, the following maximum improvement of output power and PAE are
reached: 1.8dB and 9% for 900MHz, and 0.3dB and 3% for 1900MHz. In comparison with the
amplifier without TMN, an improvement up to 3dB has been reached for the output power, and
10% for the PAE in the worst case of VSWR condition for the 2 bandwidths. Furthermore, less
variation of the output power and PAE in VSWR phase conditions are obtained by using both
S11 and GT optimizations. This point is particularly important for 3/4 G operation where the
transmitted power is constantly monitored by the system.
The output power is maintained around 31.4dBm with PAE of 51.5% at LB for VSWR 3:1 and
maintained around 30.2 dBm with PAE of 38% at HB for VSWR 3:1.

V.

CONCLUSION

In this paper, a tunable matching network is presented, which reduces effectively the impact of
the antenna mismatch. A 2 π-architecture, with switching elements, has been selected to cover
the requirements for 900 MHz and 1900 MHz applications. The TMN has been evaluated at the
output of a multimode power amplifier. By optimizing S11 and GT, for VSWR 3:1 and in the
worst phase conditions, the output power and PAE are improved respectively by more than
3dB and 10%. Hence the TMN allows to maintain the power performances of the amplifier
whatever the VSWR phase.

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VI.

ACKNOWLEDGMENTS

The authors would like to thank Guillaume Blamon for his helpful assistance. The authors also
would like to thank ACCO for their support in conducting this project.

REFERENCES
[1] R. Whatley, T. Ranta and D. Kelly, CMOS Based Tunable Matching Networks for Cellular
Handset Applications, Peregrine Semiconductor, San Diego, CA, 92121, USA.
[2] Wesley N. Allen and D. Peroulis, Bandwidth-optimal Single-tunable-element Matching
Network for Antenna Tuning in Mobile Handsets, School of Electrical and Computer
Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA,
2011
[3] Maurice A. de Jongh, A. van Bezooijen, K. R. Boyle, T. Bakker, Mobile Phone Performance
Improvements using an Adaptively Controlled Antenna Tuner, TDK-EPC, SAW Components
Division, Bijsterhuizen 11-22, 6545 AS Nijmegen, The Netherlands, 2011
[4] Morris A.S, QizhengGu, Ozkar M., Natarajan S.P., High performance tuners for handsets,
Microwave Symposium Digest (MTT), 2011 IEEE MTT-S International

BIOGRAPHY
Zakaria SETTAF received the Master degree in electronics and optical microwave from the
University of Limoges (France). He is currently a second year PhD student of Poitiers
University and junior engineer with ACCO semiconductor society, Louveciennes, FRANCE.
Claude Duvanaud received the Ph.D. Degree in electronics and communication engineering
from the University of Limoges, France, in 1993 and the “Habilitation à Diriger des Recherches”
from the University of Poitiers. He is currently assistant professor at the Poitiers University
and member of the XLIM Laboratory. His research interests include modeling, simulation and
design of nonlinear power amplifiers and communication systems.
Jean-Marie PAILLOT (M’95) received a PhD degree in Electronics form the University of
Limoges, France, in 1990. After graduation, he joined the Electronics Laboratory of PHILIPS
Microwave, as R&D engineer. Since October 1992, J.M. Paillot is with the University of Poitiers,
where he currently is Full Professor of Electronics Engineering and member of the XLIM
Laboratory (Limoges-France). In charge of several contracts with industry, and author of
papers published in scientific journals, J.M. Paillot is presently interested in phase noise
reduction techniques for microwave oscillators, as well as in the research and development of
circuits to command the antenna arrays.
SmailBachir received his degree in Signal Theory from Polytechnic school of Algeria in 1997.
He joined the scientific department of Leroy Somer Society and the University of Poitiers in
france where he received his Ph.D. degree in Automatic and Electrical Engineering in 2002. He
is presently an Associate Professor at the University of Poitiers and a researcher in XLIM
laboratory with the department of SIC. His research interests include signal processing,
parameter identification, telecommunication and radio communication circuits.
Hervé Camand Francis Huin: Biography not available at the time of publication.


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Power amplifier performance improvement using tunable matching

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Power amplifier performance improvement using tunable matching