Copyright © 2012 IEEE. Reprinted from IEEE Transactions on Microwave Theory and Techniques, VOLUME 60, ISSUE 8, JUNE 2012. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Cree’s products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertis- ing or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs- permissions@ieee.org By choosing to view this document, you agree to all provisions of the copyright laws protecting it. 3–3.6-GHz Wideband GaN Doherty Power Ampliﬁer Exploiting Output Compensation Stages Jorge Moreno Rubio, Jie Fang, Vittorio Camarchia, Member, IEEE, Roberto Quaglia, Marco Pirola, Member, IEEE, and Giovanni Ghione, Fellow, IEEE Abstract—We discuss the design, realization and experimental characterization of a GaN-based hybrid Doherty power ampliﬁer for wideband operation in the 3–3.6-GHz frequency range. The design adopts a novel, simple approach based on wideband com- pensator networks. Second-harmonic tuning is exploited for the main ampliﬁer at the upper limit of the frequency band, thus im- proving gain equalization over the ampliﬁer bandwidth. The re- alized ampliﬁer is based on a packaged GaN HEMT and shows, at 6 dB of output power back-off, a drain efﬁciency higher than 38% in the 3–3.6-GHz band, gain around 10 dB, and maximum power between 43 and 44 dBm, with saturated efﬁciency between 55% and 66%. With respect to the state of the art, we obtain, at a higher frequency, a wideband ampliﬁer with similar performances in terms of bandwidth, output power, and efﬁciency, through a sim- pler approach. Moreover, the measured constant maximum output power of 20 W suggests that the power utilization factor of the 10-W (Class A) GaN HEMT is excellent over the ampliﬁer band. Index Terms—Broadband matching networks, Doherty power ampliﬁers (PAs), GaN-based ﬁeld-effect transistors (FETs), wide- band microwave ampliﬁers, WiMAX. I. INTRODUCTION T HE success of the Doherty power ampliﬁer (PA) [1] for the implementation of wireless base-stations is mainly re- lated to its high efﬁciency in the presence of modulated signals with high ratio between peak and average power, i.e., noncon- stant envelope [2]. In fact, because of its high efﬁciency over a wide range of power levels, Doherty ampliﬁers can effec- tively handle nonconstant envelope signals without additional external controls [3], [4] that negatively impact on the overall system complexity, size, and efﬁciency. Limitations in linearity and bandwidth are recognized to be the most important Doherty Manuscript received February 09, 2012; revised May 03, 2012; accepted May 07, 2012. This work was supported by the Regione Piemonte NAMATECH project. J. M. Rubio was with the Department of Electronics and Telecommunica- tions, Politecnico di Torino, 10129 Torino, Italy. He is currently with the Elec- tronics Department, Universidad Pedagógica y Tecnológica de Colombia, Sog- amoso, Colombia (e-mail: jorgejulian.moreno@uptc.edu.co). J. Fang, R. Quaglia, M. Pirola, and G. Ghione are with the Department of Electronics and Telecommunications, Politecnico di Torino, 10129 Torino, Italy. V. Camarchia is with the Department of Electronics and Telecommunica- tions, Politecnico di Torino, 10129 Torino, Italy and also with the Center for Space Human Robotics, Istituto Italiano di Tecnologia, 10129 Torino, Italy (e-mvittorio.camarchia@polito.it). 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.2012.2201745 ampliﬁer issues; concerning the ﬁrst point, several lineariza- tion strategies, able to comply with the stringent communication system requirements, have been successfully reported [5]–[7]. Regarding instead bandwidth limitations, the rather low frac- tional bandwidth value (typically less than 10%) of the conven- tional Doherty PA prevents its exploitation in multiband, mul- tistandard base-stations. Techniques for wideband design have been discussed in the literature for frequencies up to 2.6 GHz: for example, in [8], a 20% fractional bandwidth extension is ob- tained through a modiﬁed Doherty topology requiring a driver module able to properly and separately feed the main and peak stages. A 35% fractional bandwidth is reported in [9], exploiting wideband ﬁlters; in this case, a standard topology is adopted, but the Doherty behavior is not clearly demonstrated, and the power utilization factor [4] is not constant in the declared band. Fre- quency reconﬁgurable matching networks, enabling for a frac- tional bandwidth of 20%, but requiring additional external con- trols, are proposed in [10]. In [11], the focus is on the use of non- conventional output combining stages, while the work in [12] presents a method applying to broadband matching the simpli- ﬁed real frequency technique. Finally, the work in [13] focuses on input direct coupling of main and peak branches and wide- band output matching. This paper proposes a wideband Doherty PA design approach for the 3–3.6-GHz frequency range (18% bandwidth), adopting a simple technique based on wideband compensators inserted at the output of the peak and main cells. Second-harmonic tuning of the main ampliﬁer [3], [14] has been implemented at the upper bandwidth limit to help gain equalization versus frequency. The active device exploited in the microstrip hybrid circuit implementation is a packaged GaN HEMT, with typical output power of 10 W in the selected band. The ampliﬁer CW characterization shows, at 6 dB of output power back-off, a drain efﬁciency between 38% and 56% in the 3–3.6-GHz band. In the same range, the ampliﬁer exhibits a maximum output power between 43 and 44 dBm, together with gain around 10 dB. Fig. 1 compares state-of-the-art results for wideband Doherty PAs: the present work shows high-power utilization factor [4], gain ﬂatness, and efﬁciency in a bandwidth similar to the other sources, but, for the ﬁrst time, to the best of the authors’ knowl- edge, it exhibits a frequency higher than 3 GHz. This paper is organized as follows. Section II presents the implemented Doherty design approach and highlights the spe- ciﬁc solutions implemented to enlarge the ampliﬁer bandwidth, while Section III illustrates the fabrication and presents the