IEEE MICROWAVE AND WIRELESS COMPONENTSLETTERS, VOL. 20,NO. 2, FEBRUARY 2010 109 A Saturated Doherty Power Amplifier Based On Saturated Amplifier Jangheon Kim, Member, IEEE, Junghwan Son, Junghwan Moon, Student Member, IEEE, and Bumman Kim, Fellow, IEEE Abstract—A saturated Doherty power amplifier (PA) based on the saturated PA tuned by the self-generated harmonic currents is presented. When driven with mobile WiMAX 1FA signal, this Do- herty PA demonstrates high efficiency performance (average 57%) over the WiMAX band from 2.5 GHz to 2.7 GHz. Simulated and experimental results allow the evaluation for the load modulation behavior of the saturated PA and bandwidth correlation between the Doherty and saturated unit PAs. The linearity requirement of the WiMAX signal is met using the digital feedback predistortion linearization technique. Index Terms—Doherty amplifier, efficiency, linearity, power am- plifier (PA), predistortion (PD), worldwide interoperability for mi- crowave access (WiMAX). I. INTRODUCTION N EW wireless communication standards such as world- wide interoperability for microwave access (WiMAX) and long term evolution (LTE) offer high spectrum efficiency using the OFDM modulation scheme. Because these com- munication standards exhibit high peak-to-average power ratio (PAPR) and wide bandwidth, high efficiency and wide bandwidth characteristics at an average output power in power amplifier (PA) are required. Among various efficiency enhance- ment techniques, the Doherty PA has become one of the most promising and widely used because of its capability to enhance efficiency and linearity in complex modulation schemes and its simple topology [1]–[6]. To further improve efficiency of the Doherty PA, an advanced Doherty PA based on class-F or-F PAs, so called a saturated Doherty PA, has been developed, and high efficiency greater than 50% has been demonstrated [7], [8]. This good perfor- mance is accomplished by employing the waveform shaping of class-F or-F PAs using an additional termination circuit for harmonic loading at the current source. However, the Doherty PA cannot maintain the high efficiency performance over a wide Manuscript received July 30, 2009; revised November 09, 2009. First pub- lished January 19, 2010; current version published February 10, 2010. This work was supported by the Ministry of Knowledge Economy (MKE), Korea, under the Information Technology Research Center (ITRC) support program supervised by the National IT Industry Promotion Agency (NIPA) (NIPA-2009- C1090-0902-0037), and by Cree, Inc. J. Kim is with the Department of Electrical and Computer Engineering, Uni- versity of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail: rage3k@gmail. com). J. Son, J. Moon, and B. Kim are with the Department of Electrical Engi- neering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Korea (e-mail: bmkim@postech.ac.kr). Digital Object Identifier 10.1109/LMWC.2009.2038554 bandwidth because of the narrowband characteristic of the ter- mination circuit. This letter presents a new saturated Doherty PA based on the saturated PA, whose harmonics are tuned by self-generated har- monic currents. This Doherty PA consists of the only funda- mental matching circuit without any special harmonic termina- tions, providing a large bandwidth. From the experimental re- sults, it is demonstrated that the proposed Doherty PA delivers a high efficiency over approximately the 200 MHz wide band- width of the mobile WiMAX band, from 2.5 GHz to 2.7 GHz. II. SATURATED DOHERTY PA OPERATION BASED ON THE SELF-GENERATED HARMONIC CURRENTS In order to accomplish the class-F or- PAs operation at the linear region of I-V curve, the harmonic voltage component should be generated by a harmonic load with large impedance value at the harmonic frequency because the amount of each harmonic current component is rather small. Therefore, to realize the required harmonic termination, a special harmonic loading circuit with the large impedance is required. On the other hand, for the saturated operation through the knee region of I-V curve, resulting in highly nonlinear behavior, the PA generates a large amount of the harmonic currents. These har- monic currents depend on only the fundamental load impedance through the knee region, thus the harmonic current is named a self-generated harmonic current. The fundamental resistive load improves the power performance and has a larger load, compared to the conventional PA . We can expect that the current waveform is rectangle and the resulting voltage waveform should be shaped to half-sinusoidal for a high efficiency, similar to the waveforms of the class- PA. Moreover, we found that the harmonic termination impedance for the voltage waveform shaping is low enough that the termi- nation can be realized by without any additional harmonic load. As a result, the saturated PA based on the self-generated harmonic currents can achieve high efficiency performance over a wide bandwidth because the matching circuit is realized using only the fundamental load matching without the band- width limitation of the harmonic termination circuit [9]. To explore the load modulation behavior with the self-gen- erated harmonic currents, a new saturated Doherty PA based on the saturated PA is designed using the Cree CGH40010 GaN HEMT device model in the Agilent Advanced Design System (ADS) simulator. In addition, the carrier and peaking PAs are biased at class-AB and-C biases, respectively. Fig. 1 shows the simulated load lines of the carrier amplifier at the first and second peak efficiency regions. At the low power level, the fundamental load of the carrier amplifier is adjusted to 1531-1309/$26.00 © 2009 IEEE