A Wideband High Efficiency Ka-Band MMIC Power Amplifier for 5G Wireless Communications Duy P.. 2SISLAB, VNU University of Engineering and Technology, Hanoi, Vietnam Abstract — A compact
Trang 1A Wideband High Efficiency Ka-Band MMIC Power Amplifier for 5G
Wireless Communications
Duy P Nguyen1, Xuan-Tu Tran2, Nguyen L K Nguyen1, Phat T Nguyen1, and Anh-Vu Pham1
1Department of Electrical and Computer Engineering, University of California, Davis, CA, USA
2SISLAB, VNU University of Engineering and Technology, Hanoi, Vietnam
Abstract — A compact wideband Ka-band monolithic
microwave/ millimeter-wave integrated circuit (MMIC)
power amplifier (PA) is demonstrated using a 0.15-µm
pseudomorphic high electron mobility transistor (pHEMT)
Gallium Arsenide (GaAs) process The proposed harmonic
load-pull and radial stub matching technique have been
employed to achieve wideband and high-efficiency
performance The fabricated PA exhibits a measured output
power of 24 dBm, gain of 20 dB and 35% power added
efficiency (PAE) The PA has a wideband performance in
which the power maintains higher than 23 dBm, and the PAE
is higher than 30% over a wide frequency range from 25 to
34 GHz
Index Terms — Gallium Arsenide, Ka-band, MMIC, Power
Amplifier, Radial stub, Wideband
I INTRODUCTION The demand for mobile data has seen tremendous growth
in recent years which makes the sub-3 GHz spectrum
become increasingly crowded The current mobile network
is reaching the limits in bandwidth and speed, requiring the
next generation of wireless communications to deploy a
millimeter-wave spectrum which has wider bandwidth and
higher data rate [1], [2] Examples of these capabilities
include very high achievable data rates, low latency,
ultra-high reliability, and the possibility to handle extreme device
densities Among millimeter-wave frequency spectrums,
Ka-band (28 GHz) is a major contender for 5G applications
However, the low efficiency of Ka-band power amplifiers
(PAs) imposes a major thermal issue in the base station
where hundreds of PAs are used in a small area [3], [4] To
alleviate the issue, it requires high-efficient amplifiers with
very compact size [5], [6]
Millimeter-wave PAs have been the focus of research for
the emerging 5G wireless communications in recent years
Medium-power MMIC Ka-band amplifiers have been
reported using different technologies including Gallium
Arsenide (GaAs) [5], [7]-[10], Gallium Nitride (GaN) [11],
and complementary metal–oxide–semiconductor (CMOS)
[12]-[14] However, low power added efficiency (PAE) and
(a) (b)
(c) (d) Fig 1 Load-pull contours for a 8 x 75 μm pHEMT device at 28 GHz: (a) Power contours, (b) PAE contours of fundamental impedance, (c) PAE contours of 2 nd harmonic impedance, and (d) PAE contours of 3 rd harmonic impedance
large chip size are still posing a challenge in those designs
In addition, narrow bandwidth makes the amplifiers less attractive to the 5G network applications
This paper reports a wideband, compact and high efficiency Ka-band PA using a 0.15-µm GaAs pseudomorphic high electron mobility transistor (pHEMT) process A two-stage class AB amplifier employs a proposed radial stub matching technique to achieve wideband, high-efficiency performance The fabricated PA demonstrates a measured output power of 24 dBm with an associated peak PAE of 35%, and gain of 20 dB Higher than 23 dBm output power and 30% PAE is observed over the very wide bandwidth from 25 to 34 GHz
II RADIAL STUB FOR WIDEBAND MATCHING AND
HARMONIC TERMINATION
To achieve the target output power at Ka-band, an 8 ×
75 µm device is chosen for the output stage Harmonic
978-1-7281-0397-6/19/$31.00 ©2019 IEEE
Trang 2Fig 2 Proposed radial stub versus conventional open stub
load-pull simulation of the device under a drain bias voltage
of 6 V and dc current of 60 mA is performed to find the
optimum impedances for highest output power and highest
PAE Fig 1(a) and (b) show the power and PAE contours
at the fundamental frequency 28 GHz A maximum output
power of 24.5 dBm and 34% maximum PAE can be
achieved at the impedance around Zopt = 8.5 + j10 (Ω) To
further enhance the PAE, optimum impedances at the 2nd
harmonic and 3rd harmonic frequencies can be found from
the harmonic load-pull According to Fig 1(c) and (d), by
terminating the 2nd harmonic close to open circuit, the PAE
can reach a maximum value of 40% and is increased
another 2% by terminating the 3rd harmonic at its optimum
value In this process, at Ka-band frequencies, the 3rd
harmonic termination only contributes very little to the
efficiency enhancement [15] Therefore, to reduce the size
and losses of the matching network, only the 2nd harmonic
termination will be considered in the proposed design
The design of a class-F and class-J amplifiers to achieve
high PAE by terminating the 2nd and 3rd harmonic
impedances has been well presented in [16]-[18] However,
the output matching networks of the class-F and class-J
amplifiers requires several matching sections, thus are quite
bulky and lossy In our design, we propose to use microstrip
radial stubs in the matching network The high-quality
factor of the radial stubs can significantly reduce the
insertion loss and size of the matching network, as well as
satisfy the harmonic load conditions Fig 2 presents the
proposed radial stub and conventional open stub Both of
which has the same equivalent capacitance value at the
center frequency of 28 GHz The use of the radial stub can
reduce the length by 53%, making the matching network
more compact and lower loss
Fig 3 compares the simulated return loss and insertion
loss of the two output matching networks using the radial
stub versus conventional open stub The proposed radial
stub matching significantly improves the bandwidth and
has much lower loss In other words, the reactance of the
radial stub varies much less than that of the conventional
open stub Moreover, the radial stub has three independent
Fig 3 Return loss S11 and insertion loss S21 of the radial stub matching and conventional open stub matching
Fig 4 Fundamental impedance and 2 nd harmonic impedance of the radial stub and conventional open stub matching networks
parameters that can be optimized separately: radius (R o),
width (W), and angle (θ) [19] Therefore, the 2nd harmonic impedance can be easily tuned to achieve the optimum load-pull value that gives the highest PAE Fig 4 illustrates the fundamental and 2nd harmonic matching of the two output matching networks (radial stub versus open stub) Both matching networks provide good 50 Ω match at fundamental frequencies However, the radial stub provides the 2nd harmonic impedance close to the optimum value found in Fig 1(c), resulting in higher PAE
III POWER AMPLIFIER DESIGN The proposed power amplifier has two amplifying stages and is fully matched to 50 Ω system at input and output The schematic of the PA is showed in Fig 5 The MMIC
PA employs a 1:2 gate periphery drive ratios to provide sufficient power margins between stages while preserving high gain and efficiency The peripheries of the pHEMTs are 300 µm (4×75 µm) and 600 µm (8×75 µm) for the driver stage (Q1) and the output stage (Q2), respectively Both
Trang 3Fig 5 Circuit schematic of the Ka-band power amplifier
stages are biased in deep class AB in which the bias voltage
of the power stage Q2 is Vd2 = 6 V; I d2 = 60 mA and that of
the driver stage Q1 is Vd1 = 6 V and I d1 = 32 mA
The matching networks consist of radial stubs,
transmission lines, and metal-insulator-metal (MIM)
capacitors The proposed radial stubs are used in the both
the output and inter-stage matching networks to help
enhance the bandwidth, PAE, and reduce the chip size
Another benefit from using radial stubs is that we can avoid
the discontinuity from a large tee junction The inter-stage
matching employs a dual symmetrical radial stub, a dual
shunt stub, and two series transmission lines Compared to
the conventional matching topology, dual stubs provide
totally symmetrical design and enhances performance at
high frequencies In addition, it also allows balanced
biasing in which the dc current can be provided from either
side of the chip or both sides concurrently This additional
feature improves the PA reliability, especially at the drain
bias where high current is needed The input matching
employs a series MIM capacitor and two shunt stubs to
achieve a compact size
The design layout and full electromagnetic (EM)
simulation were carefully handled to ensure a balanced and
symmetrical design Bias pads were placed symmetrically
to allow current supplied from both sides This technique
guarantees current and heat are distributed equally, which
improves reliability All matching networks are designed to
guarantee unconditional stability from dc to 80 GHz
IV EXPERIMENTAL RESULTS
The process selected for our Ka-band PA is a 0.15-µm
GaAs pHEMT transistor This process has an f max of
185 GHz and f t of 90 GHz which make it attractive to
Ka-band applications The process is designed to operate with
a peak drain voltage of up to 6 V and provide relatively
high-power density The 0.15-µm PHEMT device can
deliver 870 mW/mm at 29 GHz in which I dmax= 620 mA/
mm and the breakdown voltage of V gd is 16.5 V Fig 6
shows a photograph of the fabricated MMIC PA The size
of the chip is 2.1 mm × 1.4 mm × 0.01 mm
Fig 6 Chip photo of the fabricated amplifier (2.1 mm × 1.4 mm)
Fig 7 Measured small signal gain and return loss at V d1 = 6 V;
Id1 = 32 mA; V d2 = 6 V; I d2 = 60 mA.
Fig 8 Measured output power, gain, and PAE at 28.5 GHz
Fig 7 shows the measured versus simulated small signal
performance of the MMIC PA at V d1 = 6 V; I d1 = 32 mA;
Vd2 = 6 V; I d2 = 60 mA The maximum small signal gain is
20 dB and maintains above 17 dB from 27 to 32 GHz The
PA achieves good input and output return loss in the frequency range The measured results are correlated with the simulation Fig 8 presents the output power, gain and efficiency of the PA at 28.5 GHz as a function of the input
Trang 4Fig 9 Measured output power and PAE as a function of frequency
power with the bias condition as mentioned above The
input power is swept from -15 dBm to +13 dBm with 1 dB
step The output power at 1dB compression point (OP1dB)
is 21 dBm, and the saturation power is 24 dBm The
maximum PAE 35% is achieved at 23.5 dBm output power
Maximum power and PAE across the frequency range
from 25 to 35 GHz is illustrated in Fig 9 The output power
maintains higher than 23 dBm from 25 to 34 GHz while the
PAE is higher than 30% in that frequency range The PA
exhibits excellent power and PAE flatness over a wide
bandwidth Fig 10 shows the measured output power, gain,
relative third order intermodulation (IM3) and third-order
intercept point OIP3 as a function of input power The
linearity data was obtained by a two-tone measurement
with equal amplitude and 10 MHz tone spacing at
28.5 GHz The amplifier demonstrates good linearity in
which 35 dBm OIP3 is observed in Fig 10 Table I
summarizes the proposed PA performance and compares
with other previously published Ka-band MMIC PAs Our
proposed PA achieves among the highest efficiency over a
wide bandwidth
Fig 10 Two-tone measurement at center frequency 28.5 GHz and
10 MHz tone spacing.
V CONCLUSION
We have demonstrated a wideband, compact MMIC
Ka-band PA The 2.0 × 1.4 mm2 amplifier exhibits a measured gain of 20 dB, output power of 24 dBm and peak PAE of 35% at 28.5 GHz Employing the proposed matching topology, the PA achieves a very wide band performance in which measured output power and PAE are flat from 25 to 34 GHz In addition, good linearity and compact chip size are observed, which makes the PA more attractive to the 5G millimeter-wave wideband wireless communication systems
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TABLE I
COMPARISON TO OTHER PUBLISHED MILLIMETER-WAVE POWER AMPLIFIERS Reference Frequency
(GHz)
Power (dBm)
Gain (dB)
Supply voltage (V)
PAE (%)
Die size (mm 2 )
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