IEEE Circuits and Systems Magazine - Q4 2020 - 62

It is expected that even more Doherty power amplifiers will be used
in the design of mmW mmic power amplifiers. A demand
foreseen in massive mimo applications.
In the authors' opinion, the advantages of these three
technologies are different. For CMOS, when considering
its maximum operating frequency (fmax), it is suitable for
high frequency, up to 60 GHz or even higher frequency
into the terahertz region. For GaAs, it is a good solution for millimeter wave PAs but the output power and
efficiency is limited. Hence, its application is mainly focused for use in handset devices. While for GaN, it has
good capability for high power and efficiency but the
operating frequency is limited to sub 6 GHz. In recent
two years, the 100 nm GaN process have been proposed,
which has pushed GaN PA to the millimeter waves.

62â

From October of 2014 to June of 2015, he was a research assistant with the SYSU-CMU shunde international joint research institute. From October of 2018 to
August of 2020, he was a postdoctoral fellow with Department of Electrical Engineering at the City University of Hong Kong, Kowloon, Hong Kong. He is currently
a postdoctoral research associate with the Department of Electrical Engineering at the Princeton University, New Jersey, USA. His current research interests
include broadband high-efficiency and high-linearity
power amplifier in RF and millimeter-wave, and microwave passive circuits.
Dr. Zhou was a recipient of First Place Award of the
High Efficiency Power Amplifier Student Design Competition, IEEE Microwave Theory and Techniques Society (IEEE MTT-S) International Microwave Symposium
(IMS) in 2017 and a recipient of Second Place Award of
the same Student Design Competition in 2018.

VI. Conclusion
In this work, we provide an overview of Doherty PAs,
which are widely adopted in modern base stations and
are expected to gain further popularity. Operating principles of the conventional Doherty PA have first been discussed. Broadband architectures were then introduced,
followed by a discussion of three recent broadband
post-matching Doherty PA implementations. We have
observed that, based on the post-matching topology
transformer topology, the overall (including bandwidth,
efficiency, and power) performance of Doherty PAs can
be enhanced significantly at Sub-6 GHz and at mm-wave.
Current technical challenges of broadband Doherty PAs
and the applications of Doherty PAs in 5G have been
highlighted. Finally, recent process technologies for
broadband Doherty PAs are compared. CMOS processes
are generally suitable for high frequency operations, extending from 60 GHz up to the terahertz. GaAs is typically
chosen for millimeter wave PAs but with limited output
power and efficiency. Hence, its application is mainly in
handsets. GaN has good capability at high power and is
power efficient, but the operating frequency is often limited to below 6 GHz. However, recent 100 nm GaN process
enables operation up to the millimeter wave range. Finally, a comparison among state-of-the-art wideband sub-6
GHz and mm-wave Doherty PAs are presented in Table II
and Table III.

Wing Shing Chan (M' 94) received the
B.Sc (eng) in Electronic Engineering in
1982 from Queen Mary College, University of London and the Ph.D. from City
University of Hong Kong in 1995.
From 1982 to 1984, he worked for
Plessey RADAR in the Solid-State Techniques Department
as an Engineer as part of a team that produced the world's
first solid-state RADAR transmitter in s-band. From 1984
to 1988 he worked for Microwave Engineering Designs
Limited, Newport, UK, as a Senior Design Engineer in RF/
Microwave amplifiers. In 1988 he joined the Department
of Electronic Engineering at the City University of Hong
Kong as a Lecturer. He is now an Associate Professor in
the same department.
Dr. Chan is a Chartered Engineer of the Engineering
Council, UK, and a member (MIEE) of the IEE since 1991.
He was the past Chairman of the IEEE AP/MTT Chapter,
HK Section. He has previously served as a member of
the Radio Spectrum Advisory Committee (RSAC) in the
Office of the Telecommunications Authority.

Xin Yu Zhou (S'15-M'18) was born in Tsingtao, Shandong Province, China. He
received the M.Sc. and Ph.D. degrees in
electronic engineering from City University of Hong Kong, Kowloon, Hong
Kong, in 2014 and 2018, respectively.

Shichang Chen was born in Zhejiang,
China, in 1987. He received the B.S and
Ph.D. degrees from the Nanjing University of Science and Technology and City
University of Hong Kong in 2009 and
2013, all in electronic engineering.

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