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 1cgarg@cgarg.com 2vivs@ieee.org

3nagppfec@iitr.ernet.in

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.

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