double shunt stub impedance matching network based concurrent dual-wlan-band amplifier
DESCRIPTION
TRANSCRIPT
Double Shunt Stub Impedance Matching Network based
Concurrent Dual-WLAN-Band Amplifier
Chandranshu Garg #1
, Vivek Sharma #2
, Nagendra P. Pathak #3
#Radio Frequency Integrated Circuits (RFIC) Group
Department of Electronics and Computer Engineering (E&CE)
Indian Institute of Technology (IIT) – Roorkee
Uttarakhand, India [email protected] [email protected]
Abstract — The paper presents design of a concurrent dual-band microwave amplifier that operates simultaneously at the two WLAN frequencies around 2.44-GHz and 5.25-GHz. The amplifier employs concurrent dual-band input and output impedance matching networks, each of which utilizes the conventional double open-circuited shunt-stubs structure. Such tuning network is designed to allow simultaneous matching of arbitrarily different complex impedances of transistor to the standard 50- impedance at any two desired frequencies, while rejecting a wide-band in between them.
Index Terms — Amplifier, concurrent, dual-band, double stub impedance tuner, hybrid microstrip integrated circuit, wireless LAN.
I. INTRODUCTION
Rapid global development of wireless communication
systems has expanded their applications into various
diversified fields, such as, GSM, WCDMA, GPS, Bluetooth,
WLAN, etc. Several different communication standards have
been introduced for such distinct areas so as to not only
ensure their feasible performance, but also, minimize
interference among their signals. Further advancements in
wireless domain target implementation of multi-standard
mobile devices that allow simultaneous support for multiple
distinct applications. This requires realization of multi-mode
or multi-channel, individual, radio-frequency (RF)
components that will bring about reduction in both the
components’ count and the system complexity.
One of the challenging issues in effective realization of a
multi-band transceiver involves realization of multi-band RF
transmit amplifiers. Various techniques have been devised to
develop multi-band amplifiers that include wide-band
operation [1], parallel architectures [2], or multi-band
switching [3]. A broad-band amplifier supports multiple
frequencies by covering them within its pass-band. The next
approach employs multiple, separate, single-band amplifiers
in parallel, wherein, each amplifier is optimized to one
particular band of interest. The third scheme suggests
switching among distinct matching networks using CMOS or
MEMS based switches, while utilizing a single active device.
Yet, the afore-mentioned design configurations exhibit
certain issues. In particular, a wide-band amplifier
additionally augments undesired frequency bands,
interference signals, inter-modulation products and noise,
which, in turn, degrades amplifier’s linear performance.
Besides it, the simplified parallel approach leads to increase
in power dissipation, chip area and overall module cost.
Furthermore, switching based design results in non-
concurrent operation. Hence, current need is to design
concurrent multi-band microwave amplifiers [4]–[5], which
can simultaneously support the required frequencies.
One of the ways to achieve simultaneous multi-band
amplification is through utilization of concurrent multi-band
impedance transforming networks at both the input and the
output ends of active device. Impedance matching at both the
input and the output ends allows maximum power transfer,
which, in turn, results in power gain at all selected
frequencies. In this regard, efforts are being put in to design
multi-band matching networks that match frequency-
dependent complex impedances to the standard 50-
impedance at multiple uncorrelated transmission frequencies.
This paper explores dual-frequency operation of the
conventional double open-circuited shunt-stubs based
impedance transformer. The structure is shown to match
arbitrary frequency-dependent complex load impedances to
the standard resistance, simultaneously at any two, arbitrarily
selected, uncorrelated frequencies of interest. Thereafter, as
mentioned earlier, the structure’s utility is highlighted by
realizing both the input and the output impedance matching
networks for implementation of a concurrent dual-band
amplifier. The fabricated prototype amplifier is designed for
operation around dual-WLAN-bands.
II. CONCURRENT DUAL-BAND MATCHING NETWORK
The basic concept for matching a load at any frequency is
to achieve zero reflection co-efficient for the power coming
from the source towards the load. Design of the proposed
dual-band matching network is based on this concept and aim
is to achieve zero reflections for two different complex load
impedances at any two arbitrary frequencies. In order to
simultaneously match unequal complex load impedances at
any two arbitrary frequencies, Colantonio, et. al., [6] first
transformed complex impedances to equal real impedances at
two desired frequencies, using dual-band shunt-stub
transmission-line (TL) sections. Thereafter, dual-band two-
section stepped impedance transformer [7] was employed to
match the resulting real impedances. However, such structure
doesn’t allow matching at harmonically related frequencies. A
simplified topology of three-section stepped impedance
transmission-line transformer was presented by Liu, et. al.,
[8] for dual-band matching of unequal complex impedances.
However, both the above mentioned approaches share a
common constraint that the required characteristic
impedances of TL sections do not always lie in the feasible
range for fabrication. This limitation was mitigated by
Chuang [9] through the observation that concurrent dual-band
matching of frequency dependant arbitrary complex
impedances requires only four design parameters. In
distributed elements based matching network design, each TL
section provides two adjustable parameters, viz.,
characteristic impedance and physical length. Using physical
lengths of four TL sections as design parameters, the scheme
allowed their characteristic impedances to be chosen
arbitrarily for feasible fabrication. The approach utilized a
two-section stepped impedance transmission-line transformer
(TLT) to first equalize the real parts of resulting admittances
to 1/50–1
. Their imaginary parts were subsequently
cancelled out using two-section shunt-stubs, thereby,
achieving dual-band matching.
The proposed matching network employs series TL
sections and stubs to transfer the complex impedance seen at
the transistor terminals to 50- at the port. The two stubs are
connected in parallel to the main line and are open-circuited.
Fig. 1 shows the impedance transformer structure that has
been used for dual-band matching. Besides, the afore-
mentioned drawback of infeasible characteristic impedances
is mitigated by considering the four physical lengths of both
TL sections and stubs as design parameters. Hence, designer
can arbitrarily set characteristic impedances of all TL
sections. Such consideration not only allows dual-band
matching of unequal complex impedances, but also, feasible
fabrication of microstrip transmission-line sections.
Fig. 1. Concurrent dual-band impedance matching network.
Consider the network as shown in Fig. 1. Let YL be the load
admittance, which is converted to YB by the first series TL
section of length l2 and an open-circuited shunt stub of length
d2. This admittance is further transformed into standard
admittance Y0 by another series TL section of length l1 and an
open-circuited shunt stub of length d1. For ease of analysis
and feasible fabrication, characteristic impedances of all TL
sections and stubs are set to standard 50-.
Considering, the normalized admittances with respect to Y0
as yL, yB and yA, the transmission line theory leads to
following design relations:
(1)
(2)
. (3)
Further consider the two frequencies of interest as f1 and f2.
Consequently, two different load admittances, YL1 and YL2,
need to be matched at the two frequencies f1 and f2,
respectively. Moreover, the propagation constant, β, also
varies with operating frequency. Accordingly, six equations
are achieved for simultaneous impedance matching at the two
design frequencies. Given the values for yL1 and yL2, the
lengths l1, l2, d1 and d2 are adjusted such that all equations are
satisfied simultaneously. This will mean that load impedances
at f1 and f2 are simultaneously matched to 50-. Based on
derived equations, a MATLAB code was developed to
provide all possible solutions for feasible length parameters
for dual-band impedance matching.
IV. PROTOTYPE CONCURRENT DUAL-BAND AMPLIFIER
This section details out design and implementation of
concurrent dual-band microwave amplifier, supporting the
two commercially popular IEEE 802.11 WLAN (Wireless
Local Area Network) frequencies around 2.44-GHz and
5.25GHz. Design employs the conventional double open-
circuited shunt stubs impedance transformer for simultaneous
dual-frequency impedance matching at both the input and the
output ends of the active device. The circuit schematic of the
proposed amplifier is depicted in Fig. 2.
Fig. 2. Circuit schematic for the concurrent dual-band amplifier. The amplifier is realized using hybrid microstrip integrated
technology (HMIC). Hence, all surface mount devices
(SMDs) are mounted on the commercial NH9320 substrate,
which is a Poly-Tetra-Fluoro-Ethylene (PTFE)/glass/ceramic
dielectric. The substrate is characterized by the dielectric
constant (r) of 3.2 and the substrate height (h) of 60-mil.
Matching networks consist of only distributed elements of
microstrip TL sections.
Furthermore, the active device, used in the design, is
ATF54143 from AVAGO Technologies. The device is a low-
noise enhancement mode pseudomorphic high electron
mobility transistor (E-pHEMT). Both the drain and the gate
DC bias circuits of the amplifier employ a common topology,
which consists of a short-circuited, quarter-wavelength, high-
impedance, TL stub. The high-impedance lines provide DC
paths for their respective supply voltages, while, acting as
open circuits for ac signals in the circuit, thereby, avoiding
unwanted ac coupling. Apart from that, coupling capacitors
are employed at both the input and the output ends of the
transistor to couple only RF power, while, blocking DC
signals. Non-linear model of the selected active device,
including package parasitic effects, is used in all circuit
simulations using Advanced Design System (ADS). The DC
bias point selected for the amplifier design is VDS = 3.0V and
VGS = 0.59V. The chosen bias point operates the transistor in
class-A operation.
Source-pull and load-pull characterizations of the biased
transistor identify the required matching source and load
impedances for the maximum power transfer at 2.44-GHz and
5.25-GHz. These impedances are listed in Table I. The
scattering (S-) parameters for the transistor confirm that the
impedances do not lie in the unstable region of operation and,
therefore, stabilization of the device is validated.
TABLE I
MATCHING SOURCE AND LOAD IMPEDANCES
Frequency
(GHz)
Required Matching Impedances ()
Source Load
2.44 19.95 – j51.25 43.50 – j36.65
5.25 127.90 + j163.25 82.30 – j10.30
Using the complex matching impedances as target loads,
design parameters for the input and the output matching
networks are obtained through the MATLAB code. The code
is written to solve the design equations (1) to (3) for
concurrent dual-frequency complex impedance matching
through the conventional double open-circuited shunt stubs
structure. These design relations are derived in the previous
section. The inputs to the program are the two design
frequencies along with corresponding source and load
matching impedances. Moreover, substrate parameters, such
as, the dielectric constant, etc., are also provided in order to
take their effects into account while performing computations
at the two frequencies of interest. The program provides all
possible solutions in terms of the physical lengths of the
series TL sections and the open-circuited shunt-stubs. The
circuit physical design parameters are further tuned through
ADS harmonic and EMDS simulations to increase power
transfer at both the desired frequencies. Table II shows
resulting design parameters for the input and the output
matching networks.
TABLE II
OPTIMIZED MATCHING NETWORK PHYSICAL LENGTHS
Matching
Networks l1 (mm) l2 (mm) d1 (mm) d2 (mm)
Input 11.5 4.5 15.3 11
Output 8.7 7.8 13.85 11.9
Fig. 3 shows a fabricated prototype of the proposed
amplifier, using concurrent dual-WLAN-band impedance
matching networks at both the input and the output ends of
the E-pHEMT active device.
Fig. 3. Fabricated concurrent dual-WLAN-band amplifier.
V. EXPERIMENTAL RESULTS
Laboratory setup for measurement of amplifier’s concurrent
dual-WLAN-frequency response on Network Analyzer is
illustrated in Fig. 4.
Fig. 4. Complete setup for measurement of dual-band amplifier’s
S-parameter response on the Network Analyzer.
Measured small-signal scattering (S11, S22, and S21)
parameter characteristics of the fabricated dual-band
amplifier are displayed in Fig. 5(a), (b) and (c).
(a)
(b)
(c)
Fig. 5. Measured small-signal |S11|, |S22| and |S21| parameters for
the designed dual-band amplifier (plots depict 1-GHz to 7-GHz
frequency range with major steps of 500-MHz).
Plots for the input (|S11|) and the output (|S22|) reflection
coefficients establish dual-band performance of the two
impedance matching networks. Moreover, measured transfer
characteristic of |S21| parameters are 5.6 dB and -5.3 dB at
2.44-GHz and 5.25-GHz, respectively. Hence, as required,
the amplifier passes the desired WLAN frequencies while
rejecting the undesired frequencies.
V. CONCLUSION
The paper presents concurrent dual-band impedance matching
characteristics of conventional double open-circuited shunt-
stubs structure. The impedance transformer is shown to match
frequency-dependent complex load impedances to standard
50- line at any two distinct frequencies. This is established
through design of a concurrent dual-WLAN-band amplifier
that utilizes the proposed structure for both the input and the
output impedance matching. Measured performance of the
fabricated amplifier exhibits the required dual-band response
with a wideband rejection in between the two operating
WLAN frequencies of 2.44-GHz and 5.25-GHz.
ACKNOWLEDGEMENT
Authors wish to acknowledge assistance and support for
their work through research fund grant from SERC, DST.
REFERENCES
[1] P. S. Wu, T. W. Huang, and H. Wang, “An 18-71 GHz multi-band and high gain GaAs MMIC medium power amplifier for millimeter-wave applications,” 2003 IEEE MTT-S Int. Microwave Symp. Dig., vol. 2, pp. 863-866, June 2003.
[2] S. Zhang, J. Madic, P. Bretchko, J. Mokoro, R. Shumovich, and R. McMorrow, “A novel power-amplifier module for quad-band wireless handset applications,” IEEE Trans. Microwave Theory & Tech., vol.51, no.11, pp. 2203- 2210, June 2003.
[3] A. Fukuda, H. Okazaki, and S. Narahashi, “A novel compact reconfigurable quad-band power amplifier employing RF-MEMS switches,” 36th European Microwave Conf., pp. 344-347, September 2006.
[4] A. Cidronali, N. Giovannelli, I. Magrini, and G. Manes, “Compact concurrent dual-band power amplifier for 1.9GHz WCDMA and 3.5GHz OFDM wireless systems,” 38th European Microwave Conf., pp. 1545-1548, October 2008.
[5] D. T. Bespalko, and S. Boumaiza, “Concurrent dual-band GaN power amplifier with compact microstrip matching network,” Microwave & Optical Technology Letters, vol. 51, no. 6, pp. 1604-1607, March 2009.
[6] P. Colantonio, F. Giannini, and L. Scucchia, “A new approach to design matching networks with distributed elements,” 15th Int. Conf. on Microwaves, Radar & Wireless Communications (MIKON), vol. 3, pp. 811-814, May 2004.
[7] C. Monzon, “A small dual-frequency transformer in two sections,” IEEE Trans. Microwave Theory & Tech., vol. 51, no. 4, pp. 1157-1161, April 2003.
[8] X. Liu, Y. Liu, S. Li, F. Wu, and Y. Wu, “A three-section dual-band transformer for frequency-dependent complex load impedance,” IEEE Microwave & Wireless Components Letters, vol. 19, no. 10, pp. 611-613, October 2009.
[9] M. L. Chuang, “Dual-band impedance transformer using two-section shunt stubs,” IEEE Trans. Microwave Theory & Tech., vol. 58, no. 5, pp. 1257-1263, May 2010.