Computational Engineering in Systems Applications (Volume II)
Design and Measurement of 2.5GHz Driver Amplifier for IEEE
802.16e Mobile WiMAX using a Small-Signal Method
S. KASSIM, F. MALEK
School of Computer and Communication Engineering, University Malaysia Perlis (UniMAP),
No 12 & 14, Jalan Satu, Taman Seberang Jaya, Fasa 3, 02000 Kuala Perlis,
Perlis, Malaysia.
mfareq@unimap.edu.my
Abstract – In this paper, the crucial and the needs for a highly linear driver amplifier for IEEE 802.16e Mobile
WiMAX is being addressed. This was done by demonstrating a high linearity and low power driver amplifier which is
adequate to accommodate a high Peak-to-Average Power Ratio (PAPR) and high efficiency requirements in Mobile
WiMAX systems. A discrete E-pHEMT transistor is used to design a 5V, 300mA, 2.5GHz amplifier with a SmallSignal design method. The amplifier exhibits an Input and Output Return Loss (IRL & ORL) that are better than
10dB, Small Signal Gain (SSGain) of 12dB, Third-Order Intercept Point (OIP3) of 44dBm and Output 1-dB Gain
Compression (OP1B) of 27dBm. The results demonstrate that it can provide a reasonable efficiency, linearity and
good output power.
Keywords – IEEE 802.16e, High linearity, OIP3, PAPR, Driver Amplifier.
1
based on a 1% packet error rate. EVM and RCE are
interchangeable terms since RCE describes EVM
measurement that is calculated over an entire Fixed
WiMAX frame. So as to achieve a robust link, Mobile
WiMAX driver amplifier or PA must not exceed an
EVM of 2.5% for 64QAM and 4% for 16QAM
modulation [9].
INTRODUCTION
1.1
The Needs for High Linearity in WiMAX
Transmitter
Similar to Power Amplifier (PA), driver amplifier is
one of the key components in IEEE 802.16e Mobile
WiMAX system. It needs some characteristics, such as
high output power, high linearity and high power added
efficiency (PAE) in order to support high Peak-toAverage Power Ratio (PAPR) results from complex
digital modulation i.e. 64-Quadrature Amplitude
Modulation (64QAM) used in Mobile WiMAX. This
paper intended to highlights a 2.5GHz highly linear
driver amplifier that is suitable for adaptation into
Mobile WiMAX applications [2 & 5].
As for comparison, Mobile WiMAX needs to have a
better linearity than IEEE 802.11 (WLAN) in order to
supply robust link at high data rates. Mobile WiMAX
supports multiple modulation formats, including
64QAM, 16QAM and Quadrature Phase-Shift Keying
(QPSK), with 64QAM systems demand the highest
linearity performance. Since Mobile WiMAX standards
have been enhanced with additional features, such as
quality of service (QOS), the amplifier in transmitter
chain require a very low levels of distortion for a given
modulation, as defined by system Error Vector
Magnitude (EVM). EVM is a measure of the distortion
in a QAM constellation diagram and the resulting
uncertainty (or error) for each point therein. The IEEE
802.16e Mobile WIMAX standard specifies that
Relative Constellation Error (RCE) be held to -31dB,
ISBN: 978-1-61804-014-5
2
METHODOLOGY
2.1
E-pHEMT biasing
The enhancement mode technology provides
superior performance while allowing a DC grounded
source amplifier with a single polarity power supply to
be easily designed and built. As opposed to a typical
depletion mode pHEMT where the Gate must be made
negative with respect to the Source for a proper
operation, an enhancement mode pHEMT requires that
the Gate be made more positive than the source for
normal operation. Biasing an enhancement mode
pHEMT is as simple as biasing a Bipolar Junction
Transistor (BJT). Instead of a 0.7V Base to Emitter
Voltage (Vbe), the enhancement mode pHEMT requires
about a 0.6V potential between the Gate and Source,
Vgs, for the target drain current, Ids [1].
2.2
Circuit Description
DC biasing is accomplished by the use of a
Resistive Voltage Divider network consisting of R2 to
R9. The voltage for the divider is derived from the Drain
Voltage (Vd) which provides a form of voltage feedback
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Computational Engineering in Systems Applications (Volume II)
board. Multiple via-holes around ATF-50189 help to
increase the thermal dissipation effectiveness of the
to help keep Drain Current (Id) constant. At the input
side, the combination of R1 and C7 enhance the ATF50189’s stability by terminating the gate resistively at
low frequency. L1 and C5 form the bias-decoupling
network. In order to reduce circuit loss, L1 should have
high unloaded Q, (QUL) and operated below its Self
Resonant Frequency (SRF). Capacitor C5 is
dimensioned for low reactance at the operating
frequency (fopr). Capacitors C1 and C2 form a capacitive
tap matching for ATF-50189’s input.
evaluation board.
2.4
Linear Simulation
An RF simulator from Agilent Technologies,
Advanced Design System (ADS) allows the input and
output matching networks to be designed accurately
given the evaluation board was properly modeled i.e.
dielectric materials and RF connector used were taken
into considerations. In addition, critical parameters such
as Rollet Stability Factor (K-Factor) and Small Signal
Gain (SSGain) can be predicted during the preliminary
design stage i.e. if simulation forecasts a strong
tendency to self-oscillation, the designer can anticipate
the problem by incorporating additional stabilization
network into the preliminary circuit.
Figure 1: 2.5GHz Driver Amplifier Schematic
At the output, the ferrite bead, L3, works in
combination with C8 to provide a resistive termination
down to the tens of MHz range. Although a resistor can
provide the same function, the power dissipation will be
high. Inductor, L2 and capacitor, C6 form the biasdecoupling network. L2 and C6 are chosen with the same
criteria as L1 and C5. Capacitors C3 & C4 form a
capacitive tap matching for ATF-50189’s output.
2.3
Figure 2: ADS Simulation of Input Matching Network
Evaluation Board Design
In order to demonstrate the performance, an
evaluation board was designed with the material cost
and actual space constraints were taken into
considerations. The evaluation board was designed
using 0.031inch (31mils) of FR-4 dielectric and 0603
small surface mount components. RF connections to the
evaluation board were made via PCB edge-mounted
microstrip to SMA coax transitions, J1 and J2.
Similar to bipolar transistors, which exhibit a
wide variation in hFE within the same part number, the
ATF-50189’s forward transconductance, gm, can also
vary from unit to unit. For that reason, the resistor
network, R3 to R7, on the evaluation board allows a finetuning the Gate Bias Voltage, Vg, to be made in order to
cover the range of gm variation. The individual PCB
traces connecting to R3 to R7 is cut one at a time until
the evaluation board draws the target current range of
315 ± 15mA. This would results 4.5V of Vds and 280mA
of Ids at the device-under-test, ATF-50189. Two 3mm
holes are provided for a heat sink to be mounted to the
ground plane on the bottom side of the evaluation
ISBN: 978-1-61804-014-5
Figure 3: Trajectories of input impedance during the various
phases of matching design
The trajectories of the input match can be
systematically verified as shown in the Figure 3. The
curve marked as “1” represents the initial impedance at
the position of the first matching component, C1.
Subsequently, the addition of C1 moves the input-side
impedance along the constant resistance circle to “2”.
The shunt capacitor, C2, shifts point “2” to the final
position “3” near to the Smith chart centre whilst
traveling along the constant admittance circle.
The trajectories of the output match are shown in
Figure 5. The curve marked as “1” represents the initial
impedance at the position of the first output matching
component, C3. Subsequently, the addition of C3 moves
the trace along the constant resistance circle to “2”. The
last matching component, C4 nudges the curve along
the constant admittance circle to position “3” in the
vicinity of the chart centre.
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Computational Engineering in Systems Applications (Volume II)
Small Signal Gain and Reverse Isolation (dB)
20
Figure 4: ADS Simulation of Output Matching Network
Small Signal Gain
Reverse Isolation
0
-20
-40
-60
-80
-100
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Frequency (GHz)
Figure 7: Small Signal Gain and Reverse Isolation Performance
The gain was approximately 12.5 dB in the middle
of the pass-band. Slightly more gain can be obtained at
the expense of higher cost by using high Q inductors
and/or a PCB substrate with lower loss. 22dB of reverse
isolation will be a good figure when cascading the ATF50189 into the transmitter line-up. Any tuning or
optimization on the amplifier’s output will not
significantly affect the input of the amplifier and viceversa.
Figure 5: Trajectories of output impedance during the various
phases of matching design
3
RESULT AND DISCUSSIONS
3.1
Measured Performance
The evaluation board performance was measured
under the following test conditions: Vds of 4.5V, Ids of
280mA and Operating Frequency fc of 2.5GHz.
48
0
44
OP1dB & OIP3 (dBm)
Return Loss (dB)
-5
-10
-15
-20
40
36
OIP3
OP1dB
32
28
24
-25
Input Retun Loss
Output Return Loss
-30
0.0
20
2.45
2.46
2.47
2.48
2.49
2.50
2.51
2.52
2.53
2.54
2.55
Frequency (GHz)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Frequency (GHz)
Figure 8: OP1dB and OIP3 Performance
Figure 6: Input and Output Return Loss Performance
The 1 dB gain compression point, P1dB, indicates
the upper limit of either the input or the output power
level at which saturation has started to occur. Non-linear
effects become increasingly prominent as the amplifier
is driven to this limit. Linear modulation schemes like
64QAM OFDMA in Mobile WiMAX requires the
power to be backed off several dBs from this limit [3].
The P1dB is measured by progressively increasing the
input power while noting the point when the gain
became compressed by 1 dB. In transmit environment,
P1dB is usually referred to the output. The evaluation
board nominal output P1dB is approximately 27dBm as
can be seen from Figure 8.
The intercept point is another measure of
The ATF-50189 evaluation board amplifier exhibits
good input and output return losses. Return losses of
greater than 10dB of return losses as shown in Figure 6
will sufficiently minimizes detuning effects when the
amplifier is cascaded with other stages in WiMAX
transmitter chain. As for example, filters and antennas
are commonly vulnerable to the adverse effects of
reflective terminations. In view of that, designing the
amplifier’s input and output for a close match to 50Ω
over the operating bandwidth, prevents unpredictable
shift in the cascaded frequency response [8].
ISBN: 978-1-61804-014-5
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Computational Engineering in Systems Applications (Volume II)
demonstrates increasing gain corresponding with
decreasing frequency. If this phenomenon is not tamed
with the appropriate countermeasures, the amplifier can
break into self or parasitics oscillation below its
operating frequency - in the tens of MHz range. In order
to assess the effectiveness of the low frequency circuit
stabilization described previously, the Rollett stability
criterion was calculated from the measurement of the
evaluation board’s S-parameters. The ATF-50189
evaluation board exhibits unconditional stability (K > 1)
up to 20GHz which is sufficient to take the third
harmonics into considerations.
amplifier’s linearity. The intercept point is a theoretical
point when the fundamental signal and the third order
Intermodulation Distortion (IMD) are of equal
amplitude is the third-order intercept point, IP3. The
distortion level at other power levels can be
conveniently calculated from the amplifier’s IP3
specification.
Two test signals spaced 5MHz apart with the power
of -5dBm per signal were used for evaluating the ATF50189 evaluation board. The large dynamic range
between the fundamental tones and the intermodulation
products meant that the latter is barely above the
spectrum analyzer’s noise floor. In order to measure the
3rd order product amplitude accurately, a narrow sweep
span need to be used to improve the signal to noise ratio
performance of the analyzer. As a tradeoff from the
narrow sweep span, only one fundamental and one 3rd
order intermodulation output signals can be practically
displayed on the same graph. The IP3, referenced to the
output, can be calculated from:
10
Small Signal Gain (dB)
∆IM
OIP3 = Pout +
2
20
(1)
0
-10
-20
-30
-40
-50
-60
0
2
4
6
8
10
12
14
16
18
20
Frequency (GHz)
where
Figure 10: Wideband Gain Sweep
Pfund = Pin + SSGain
(2)
Unintentional coupling between the input and
output of amplifier in transmitter chain plus component
parasitic can lead to instability in the upper microwave
region. If there are pronounced gain peaks above its
operating frequency, the amplifier may oscillate under
certain operating conditions [7]. In a wideband sweep of
the ATF-50189 evaluation board up to 18 GHz, no
abnormal peak was observed in the frequency response
as shown in Figure 10. A reasonable 42% of PAE at
P1dB was obtained as shown in Figure 11.
and Pout is the amplitude of either one of the
fundamental outputs and ∆IM is the amplitude
difference between the fundamental tones (Pout) and the
third intermodulation products [4, 6 & 10]. Pout is the
function of ATF-50189’s small signal gain and the input
power applied during the two-tone test. Based on this
formula, the output intercept point, OIP3, as illustrated
is approximately 44dBm.
10
9
32
48
Pout
PAE
SSGain
28
7
42
24
36
20
30
4
16
24
3
12
18
8
12
4
6
6
5
2
1
0
0
2
4
6
8
10
12
14
16
18
20
0
-12
Frequency (GHz)
-9
-6
-3
0
3
6
9
12
15
18
0
21
Input Power (dBm)
Figure 9: Rollet Stability Factor, K
Figure 11: PAE, Output Power and SSGain over Input
Power at 2.5GHz
Like all microwave transistors, the ATF-50189
ISBN: 978-1-61804-014-5
169
PAE (%)
Rollet Stability Factor, K
8
Computational Engineering in Systems Applications (Volume II)
4
[9] Saleh A, Salz J. Adaptive Linearization of Power
Amplifier in Digital Radio Systems. Bell Systems
Technical Journal 1983; April; 62: 1019 – 33.
[10] Vendelin, G. D., A. M. Pavio, and U. L. Rohde,
Microwave Circuit Design Using Linear and Nonlinear
Techniques, New York: John Wiley & Sons, 1990.
CONCLUSION
A 2.5GHz high linearity driver amplifier design for
IEEE 802.16e Mobile WiMAX applications was
demonstrated using a systematic Small-Signal SParameters design approach without relying to a typical
large-signal i.e. load-pull data. A Small-Signal (linear)
simulation, crucial factors that affects the driver
amplifier performance i.e. component Q until evaluation
board layout was thoroughly discussed. With a GaAs EpHEMT transistor chosen, the Class A driver amplifier
able to produce Input and Output Return Loss (IRL &
ORL) that are better than 10dB across the 2.5GHz
WiMAX band, a Small Signal Gain (SSGain) of 12dB,
Third-Order Intercept Point (OIP3) of 44dBm and
Output 1-dB Gain Compression (OP1B) of 27dBm. The
amplifier not only having a high linearity performance
but with low power (5V, 300mA) consumption, it
should be able to cope for the high efficiency demanded
in Mobile WiMAX i.e. CPE applications. In addition, a
practical measurement method on the critical driver
amplifier parameters such as P1dB and P1dB were
methodically discussed.
References
[1] Application Note 1281, “A High IIP3 Balanced Low
Noise Amplifier for Cellular Base Station Applications,”
Agilent Technologies, February 2002.
[2] Bailey, M. J., “Intermodulation Distortion in
Pseudomorphic HEMTs and an Extension of the
Classical Theory,” IEEE Trans. on Microwave Theory
and Techniques, Vol. 48, No. 1, January 2000, pp. 104–
110.
[3] Crescenzi, E.J. Wood, S.M. Prejs, A. Pengelly,
R.S. Pribble, W. “A 2.5 Watt 3.3-3.9 GHz Power
Amplifier for WiMAX Applications using a GaN
HEMT in a Small Surface-Mount Package”, IEEE
International Microwave Symposium, 2007.
[4] Cripps, S., RF Power Amplifiers for Wireless
Communications, Norwood, MA: Artech House, 1999.
[5] De Carvalho, N. B., and J. C. Pedro, “Compact
Formulas to Relate ACPR and NPR to Two-Tone IMR
and IP3,” Microwave Journal, December 1999, pp. 70–
84.
[6] Gonzalez, G., Microwave Transistor Amplifiers
Analysis and Design, 2nd ed., Englewood Cliffs, NJ:
Prentice Hall, 1997
[7] Ingruber, B., et al., “Rectangularly Driven Class-A
Harmonic-Control Amplifier,” IEEE Trans. on
Microwave Theory and Techniques, Vol. 46, No. 11,
November 1998, pp. 1667–1672.
[8] Leenaerts, D., J. van der Tang, and C. Vaucher,
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Kluwer Academic Publishers, 2001.
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