Design Scheme for Broadband Doherty Power Amplifier Using Broadband Load Combiner Karun Rawat, 1 Gowrish B, 2 Girish Ajmera, 2 Ananjan Basu, 2 Shiban K. Koul 2 1 Department of Electronics and Communication Engineering, Indian Institute of Technology Roorkee, Uttarakhand, India 2 Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, Delhi, India Received 16 October 2014; accepted 25 February 2015 ABSTRACT: This article proposes a design strategy for broadband Doherty power ampli- fier (PA) using broadband load combiner. The bandwidth of the Doherty PA based on the proposed combiner using packaged transistor is about 2.5 times the bandwidth of conven- tional Doherty PA using a quarter-wave transformer. An easy to implement analytical design methodology has been presented for the proposed load-combiner while describing the bandwidth enhancement strategy. The design methodology is validated with the design of a broadband Doherty PA based on CREE 10 W packaged GaN high electron mobility transistor devices using the proposed load combiner. Measurement results show more than 45% drain efficiency at 6 dB output power back-off (OPBO) over 400 MHz frequency range, centred around 1.95 GHz. The peak drain efficiency at saturation is better than 60% over this band of operation. At 6 dB OPBO, the maximum improvement of 18.5% in drain efficiency is achieved as compared to the balanced mode PA. Measurement with single car- rier wideband code division multiple access modulated signal shows the average drain effi- ciency of more than 44% at 36.6 dBm average output power at center frequency of operation. The adjacent channel power ratio is better than 245 dBc after applying digital predistortion. The circuit is realized with microstrip technology, which can be easily fabri- cated using conventional printed circuit processes. V C 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:655–674, 2015. Keywords: Doherty power amplifier; load modulation; broadband; load combiner I. INTRODUCTION The rapid evolution of communication standards demands multiband/multistandard software defined radio (SDR) for smooth network migration and upgrades. Moreover, the upcoming standards are targeting high data rates which require higher bandwidth and hence broad-band transmis- sion. These requirements of high data rates and high spec- tral efficiency also demand better modulation schemes with large peak to average power ratio (PAPR). To accommodate these high PAPR signals, the power ampli- fier (PA) in the transmitter is operated at certain output power back-off (OPBO), where, the DC to radio fre- quency conversion efficiency is low. This results into high energy consumption in the wireless transmitters. The Doh- erty PA has been widely accepted as an effective solution to such problems in base stations [1–7]. Based on the principle of load modulation, the Doherty PA provides high average efficiency at certain OPBO depending on the choice of load modulation factor [2, 8, 9]. Thus, multi- band/broadband Doherty PA can be considered as an opti- mum solution for energy efficient multiband/multistandard SDR based transmitters for wireless communication. A conventional Doherty PA is inherently narrow-band due to the presence of quarter-wave transformer used as load inverter in the Doherty load combiner [10]. Extend- ing the Doherty PA for multiband operation might be con- sidered a good approach, but such PA operates with narrow-bandwidth at certain selected frequencies which can be center frequencies of commercial wireless commu- nication bands [11–14]. In addition to multifrequency/ multistandard applications, the bandwidth requirement Correspondence to: K. Rawat; e-mail: karun.rawat.in@ieee.org DOI: 10.1002/mmce.20906 Published online 9 April 2015 in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Wiley Periodicals, Inc. 655