This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS 1 Envelope Tracked Pulse Gate Modulated GaN HEMT Power Ampliﬁer for Wireless Transmitters Maryam Jouzdani, Student Member, IEEE, Mohammad Mojtaba Ebrahimi, Member, IEEE, Karun Rawat, Senior Member, IEEE, Mohamed Helaoui, Member, IEEE, and Fadhel M. Ghannouchi, Fellow, IEEE Abstract—This paper proposes a complete transmitter prototype for wireless applications using envelope tracked pulsed gate mod- ulated power ampliﬁer (PA). The proposed transmitter architec- ture is developed using two high power 10 W gate modulated PAs combined in a fashion to operate as a switched voltage source for the range of duty cycles of pulses driving the gates of power am- pliﬁers. These PAs are designed and implemented using packaged GaN HEMT transistors from CREE to operate at the carrier fre- quency of 2.35 GHz. For a 5 MHz bandwidth WiMAX 802.16e down-link signal with the PAPR of 7.9 dB and the oversampling ratio of 100, the average drain efﬁciency of 46.2% is achieved at the average output power of 35.8 dBm. Using a 5 MHz bandwidth LTE down-link signal with 11 dB PAPR and centered at 2.35 GHz, the power ampliﬁer delivers the average output power of 33.2 dBm with the average drain efﬁciency of 46%. The adjacent channel leakage ratio (ACLR) measured for this signal is less than 36.85 dBc at 10 MHz offset from the center frequency of 2.35 GHz. Index Terms—Delta-sigma modulation, Doherty power ampli- ﬁer, ﬁeld-programmable gate array (FPGA), GaN HEMT, Long Term Evolution (LTE), peak-to-average power ratio, power ampli- ﬁer, quantization noise, Worldwide Interoperability for Microwave Access (WiMAX). I. INTRODUCTION R ECENT advances in modern wireless communications have introduced mobile standards such as 3GPP LTE, 4G LTE-Advanced, and WiMAX to provide high speed data ac- cess services for all users [1]. These standards utilize orthog- onal frequency division multiplexing (OFDM) as a modulation scheme. Transmitting OFDM modulated signals with non-con- stant envelopes and high PAPRs necessitates using power am- pliﬁers with high power efﬁciency at the output power back-off (OPBO) region. Techniques such as envelope elimination and restoration (EER) [2]–[4], envelope tracking (ET) [5]–[7], Do- herty [8]–[11] and Chireix outphasing [12]–[15] have been re- ported for the purpose of improving the power ampliﬁer’s efﬁ- ciency while maintaining the required linearity. Well-known PA conﬁgurations such as Doherty PA (DPA) and the Chireix out- phasing ampliﬁer use load modulation technique and some other structures like EER and ET transmitters beneﬁt from the supply Manuscript received May 28, 2014; revised September 08, 2014; accepted September 27, 2014. This paper was recommended by Associate Editor N. M. Neihart. The authors are with iRadio Laboratory, Department of Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, Calgary AB T2N 1N4, Canada (e-mail: mjouzdan@ucalgary.ca; mm.ebrahimi@ucalgary.ca; krawat@ucalgary.ca; mhelaoui@ucalgary.ca; fghannou@ucalgary.ca). 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/TCSI.2014.2362311 modulation techniques to improve the power efﬁciency. In Do- herty PA design, for instance, the main ampliﬁer’s output load impedance is modulated at the back-off operation by applying a class-C ampliﬁer (peaking ampliﬁer) as a current source. In the EER and the envelope tracking transmitter conﬁgura- tions, the average efﬁciency of the power ampliﬁer improves by modulating the supply voltage of the RF PA. The operating principle of the EER transmitter is to split the phase and en- velope information of the input modulated signal by sending the input signal to a limiter and an envelope detector, respec- tively. The supply voltage of the ampliﬁer is then modulated by the envelope of the modulated signal through an efﬁcient enve- lope ampliﬁer so that the power ampliﬁer operates close to the saturation region for all envelope amplitudes. Although these techniques promise optimal efﬁciency over any power levels, design of linear, high efﬁcient envelope ampliﬁer is challenging for EER and ET transmitters. Recently, pulsed modulated mod- iﬁed polar transmitters have been reported in [16]–[18] to re- move the envelope ampliﬁer from the drain supply path of the RF ampliﬁer. In these architectures, the signal’s envelope is en- coded to a pulse train, using a pulse width modulator (PWM), and then the envelope pulse train controls a mixer or a switch at the gates of nonlinear, high efﬁcient RF power ampliﬁers. The supply modulation of the power ampliﬁer can be in terms of the drain or the gate bias. In particular, if the gate bias of PA is driven by digital pulses, the PA with pulse gate modulation must operate like a switched voltage source with different duty cycles of pulses, in order to amplify efﬁciently. One such scheme, is based on the digital load modulation technique named pulsed load modulation (PLM) was recently introduced to improve the power efﬁciency of PAs at the 6 dB OPBO region [19]. In 2010, a PLM power ampliﬁer was designed based on GaAs pHEMT devices for the maximum output power of 27.1 dBm [20]. This paper presents a complete transmitter architecture in- spired by the PLM technique, where, the two high power GaN HEMT PAs are connected as in Doherty PA fashion to operate in envelope gate modulation (EGM) scheme. The block diagram of power ampliﬁers used for the proposed transmitter architecture is shown in Fig. 1. The main and auxiliary ampliﬁers are con- trolled through their gate biases by digital pulses generated from a one bit envelope modulator such as the envelope delta-sigma modulator (DSM). The input signal of the EGM ampliﬁer is the carrier modulated phase of the signal with a constant envelope. As shown in Fig. 1, the main and auxiliary ampliﬁers are con- nected together by a 90 transmission line and they are switched at the same time by the modulated envelope of the signal be- tween the class-B (the on state) and pinch-off (the off state). 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