IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 3, MARCH 2011 699
Two-Stage GaN HEMT Amplifier With Gate–Source
Voltage Shaping for Efficiency Versus
Bandwidth Enhancements
Alaaeddine Ramadan, Tibault Reveyrand, Member, IEEE, Audrey Martin, Jean-Michel Nebus,
Philippe Bouysse, Luc Lapierre, Jean-François Villemazet, and Stéphane Forestier
Abstract—In this paper a two-stage 2-GHz GaN HEMT ampli-
fier with 15-W output power, 28-dB power gain, and 70% power-
added efficiency (PAE) is presented.
The power stage is designed to operate under class F conditions.
The driver stage operates under class F conditions and feeds the
power stage with both fundamental and second harmonic compo-
nents. The inter stage matching is designed to target a quasi-half
sine voltage shape at the intrinsic gate port of the power stage. The
goal is to reduce aperture angle of the power stage and get PAE
improvements over a wide frequency bandwidth.
In addition to the amplifier design description, this paper
reports original time-domain waveform measurements at internal
nodes of the designed two-stage power amplifier using cali-
brated high-impedance probes and large signal network analyzer.
Furthermore, waveform measurements recorded at different
frequencies show that aperture angle remains reduced over large
frequency bandwidth. In this study, a PAE greater than 60% is
reached over 20% frequency bandwidth.
Index Terms—Aperture angle, classes F and F , GaN power
amplifier (PA), power-added efficiency (PAE), waveform measure-
ments.
I. INTRODUCTION
T HE POWER amplifier (PA) remains a critical componentin radio communication systems because it is an important
energy consumer that impacts significantly the overall power
consumption budget of a radio transmitter. Consequently, de-
signing high-efficiency amplifiers is of prime importance to re-
duce power consumption, cooling requirements, and cost in RF
transmit subsystem and to improve reliability aspects.
AlGaN/GaN HEMT technology has demonstrated a strong
potential for high-efficiency high-power microwave amplifica-
tion because of high electron mobility, high power density, and
Manuscript received October 05, 2010; revised October 11, 2010; accepted
October 30, 2010. Date of publication December 17, 2010; date of current ver-
sion March 16, 2011. This work was supported by the French Space Agency
and by Thales Alenia Space.
A. Ramadan, T. Reveyrand, A.Martin, J.-M. Nebus, and P. Bouyssee are with
the XLIM Laboratory, UMR 6172, University of Limoges, 87060 Limoges,
France (e-mail: alaaeddine.ramadan@xlim.fr; tibault.reveyrand@xlim.fr; au-
drey.martin@xlim.fr; jean-michel.nebus@xlim.fr; philippe.bouysse@xlim.fr).
L. Lapierre is with the French Space Agency (CNES), Toulouse 31055,
France (e-mail: Luc.Lapierre@cnes.fr).
J.-F. Villemazet and S. Forestier are with Thales Alenia Space, Toulouse
31100, France (e-mail: jean-francois.villemazet@thalesaleniaspace.com;
stephane.forestier@thalesaleniaspace.com).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMTT.2010.2095033
high breakdown voltage. High power-added efficiency (PAE)
performances of microwave PAs are reached by implementing
appropriate source and load matching conditions at harmonics.
A lot of stuides have been carried out to investigate and demon-
strate optimization strategies and procedures to enhance PAE
performances ofmicrowave PAs [1]–[16]. During the past years,
many high-efficiency class F, class F , class J, class E, and
class D GaN amplifiers have been reported [17]–[19]. High-
efficiency performances and limitations of microwave PAs can
be appreciated by observing the shape of voltage and current
waveforms at transistor ports. Minimized overlapping between
drain–current and drain–source voltage indicates a minimized
dissipated power. Recently we have reported in [20] and [21]
an experimental study and calibrated time-domain waveform
measurements concerning PAE enhancement conditions of GaN
HEMTs with active second harmonic injection at the gate port.
Following these studies, in this paper we present a two-stage
GaN HEMT PA design. The driver stage provides appropriate
second harmonic injection to the input of the power stage. An
appropriate inter stage matching network has been designed
to get a quasi-half-sine-wave gate–source voltage at the input
of the power stage. In addition to the design of the PA, the
main focus of this paper is to examine the aspect of enhanced
PAE performances when the frequency is shifted away from
the center frequency. This aspect concerning PAE versus
bandwidth is not so frequently reported to authors’ knowledge.
Furthermore, an original point of the paper stands in accurate
gate–source voltage waveform measurements at internal nodes
of the amplifier at different operating frequencies to highlight
appropriate operating conditions favorable to maintain very
good PAE performances over a wide bandwidth.
This paper is organized as follows. In Section II, the prin-
ciple used in this work to improve the PAE of a class F power
transistor driven by a half-sine gate–source voltage is recapitu-
lated and illustrated with simulation results. In Section III, the
proposed two-stage PA design is presented and discussed. In
Section IV, measurement results are given to validate the de-
signmethodology. A particular emphasis is laid on time-domain
measurements at internal nodes of the PA using calibrated high-
impedance probes and a large signal network analyzer (LSNA).
Measurements of the gate–source voltage of the power stage
are performed at different operating frequencies and are com-
pared to the measurements of the gate–source voltage wave-
form of a single-stage class F amplifier having a conventional
quarter-wavelength line topology at the gate port used either
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