Regular paper A new design procedure for wide band Doherty power amplifiers Mohammad Hadi Moradi Ardekani a , Habibollah Abiri b,⇑ a Electrical Engineering Department, Shiraz University, Shiraz 7134851154, Iran b Electrical Engineering Department, Faculty of Engineering, Shiraz University, Shiraz 7134851154, Iran article info Article history: Received 17 May 2018 Accepted 6 November 2018 Keywords: Doherty power amplifier (DPA) Frequency bandwidth Power output back-off (OBO) GaN Combiner abstract A generalized design procedure for Doherty power amplifier (DPA) with arbitrary output back-off power level (OBO) is proposed in this study. This method which covers many of previously reported configura- tions is employed to derive analytical formulas for a specific output combiner which is capable for absorbing parasitic elements of the main and auxiliary transistors. Combiner parameters can be adjusted to control operational frequency bandwidth at back-off and saturation. By considering parasitic elements in the design procedure, the needs for additional offset lines and CAD optimization are eliminated and efficiency does not degrade significantly at back-off. According to the proposed design methodology, a wide band DPA is designed, simulated and implemented with commercial packaged GaN transistor. Drain efficiency (DE) between 50 and 77% at 6 dB OBO and 57.5–80.4% at saturation in the frequency band of 1.05–2.35 (GHz) (76.4% fractional bandwidth) are achieved corresponding to the maximum out- put power and gain within 43–45.3 dBm and 11–13.3 dB, respectively. Gain fluctuation is lower than 0.6 dB for frequencies higher than 1.4 GHz. Combination of high efficiency, excellent power utilization factor and flat gain through an analytical design strategy introduces this method as a promising way to design wide band DPAs. Ó 2018 Elsevier GmbH. All rights reserved. 1. Introduction High order modulations in wide frequency bandwidth, employed in modern cellular communication systems to transmit high data rate signals, leads to high peak to average power ratio (PAPR). So, transmitters mostly operate in the range of average power. To avoid large cooling systems in base stations and improve battery life time in portable devices, power amplifiers with high efficiency at output back-off power level (OBO) over a wide fre- quency bandwidth are required [1,2]. To enhance efficiency at OBO, various techniques such as dynamic biasing [3], envelop tracking [4], tunable matching network [5] and Doherty power amplifier (DPA) [6] have been reported. Due to its simple imple- mentation, DPA that employs active load modulation to boost effi- ciency at OBO has attracted many interests in recent years. Operating principle of conventional DPA has been extensively investigated in different literatures [7–14]. A classical DPA consists of two class-B biased transistors, namely, main and auxiliary tran- sistors as shown in Fig. 1. In contrast with the main transistor which is connected to the combining load Ropt/2 by a k/4 line with characteristic impedance of Ropt, the auxiliary transistor is directly connected. Ropt is optimal load impedance seen by the transistors at saturation. At low power region, the auxiliary transistor is off and impedance seen by the main device is 2Ropt. While the main device saturates at half of its maximum output power, the auxiliary transistor starts contributing current to the combining load. Cur- rent injection of the auxiliary transistor with k/4 transformer line decreases output impedance seen by the main transistor. This active load modulation makes the main transistor to stay in satu- ration until the transistors attain their maximum current. This technique results in two efficiency peaks at 6 dB OBO and at max- imum output power. Due to the dispersion behaviour of k/4 line, DPA performance degrades as the operating frequency deviates from the designed frequency [15]. In practical designs, frequency bandwidth of the conventional DPA is less than 10% which is inappropriate for the new communication systems [16]. Many studies have been carried out to enhance frequency behaviour of the DPA. By eliminating k/4 transformer, a wide band DPA with 22.6% fractional bandwidth has been reported [17]. However, this method leads to efficiency degradation at OBO. Real frequency technique which usually results in bulky matching networks was adopted in [18] to broaden frequency bandwidth of DPA. A 35.3% fractional bandwidth has been achieved in [19] using a quasi lumped transformer and Klopfenstein tapered line to extend frequency bandwidth. Despite https://doi.org/10.1016/j.aeue.2018.11.021 1434-8411/Ó 2018 Elsevier GmbH. All rights reserved. ⇑ Corresponding author. E-mail address: abiri@shirazu.ac.ir (H. Abiri). Int. J. Electron. Commun. (AEÜ) 98 (2019) 181–190 Contents lists available at ScienceDirect International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue