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Breaking Bandwidth Limit: a Review of Broadband Doherty Power Amplifier Design for 5G

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Breaking Bandwidth Limit: a Review of Broadband Doherty Power Amplifier Design for 5G 1 Breaking Bandwidth Limit: A Review of Broadband Doherty Power Amplifier Design for 5G Gholamreza Nikandish, Member, IEEE, Robert Bogdan Staszewski, Fellow, IEEE, and Anding Zhu, Senior Member, IEEE I. INTRODUCTION [6]–[11]. However, there is no complete review on broadband HE next generation wireless network, 5G, is expected to design techniques for the DPA, an essential subject for 5G T provide ubiquitous connections to billions of devices as wireless transmitters. well as to unlock many new services with multi-Gigabit-per- In this paper, we present a comprehensive review and second data transmission. To meet ever increasing demands for critical discussion on bandwidth enhancement techniques for higher data rates and larger capacity, new modulation schemes the DPA proposed in the literature, in order to provide a have been developed and wider frequency bands, e.g., at mm- thorough understating of broadband design of DPA for high- wave, have been designated to 5G [1], [2]. Massive multi-input efficiency 5G wireless transmitters. The paper is organized multi-output (MIMO), that uses a large number of antennas at as follows. In Section II, we discuss the main bandwidth the transmitter and receiver, has been considered as one of the limitation factors. In Section III, we review various bandwidth key enabling technologies in 5G to improve data throughput enhancement techniques for the DPA, including modified load and spectrum efficiency [3]. These new application scenarios modulation networks, frequency response optimization, para- pose stringent requirements on the wireless transceiver front- sitic compensation, post-matching, distributed DPA, and dual- ends and call for special considerations at both circuit and input digital DPA. This section also covers transformer-based system design levels. In the transmitter, power amplifiers (PAs) power combining PA and transformer-less load modulated should accommodate complex modulated signals, featuring PA architectures, which have a similar operation as in the high peak-to-average power ratio (PAPR) and wide modulation conventional DPAs. Challenges and design techniques for bandwidth. Moreover, in massive MIMO arrays, the PAs integrated circuit (IC) implementation of broadband DPA are should maintain high average efficiency to mitigate thermal discussed in Section IV. Finally, concluding remarks are given cooling issues. in Section V. Several PA architectures have been adopted to efficiently amplify signals with large PAPR. The popular architectures II. DPA BANDWIDTH LIMITATION include envelope tracking, outphasing and Doherty. Since its The basic DPA architecture is shown in Fig. 1, where the first introduction in 1936 [4], the Doherty power amplifier carrier amplifier is biased in a class-B mode while the peaking (DPA) has been extensively explored and it has become one of amplifier is biased in a class-C mode. At high input power the most widely used PA architectures in the existing cellular levels, both amplifiers are active and deliver power to the base stations. The DPA basically consists of two amplifiers load impedance. The characteristic impedances of the quarter having their output power combined through a load modulation wavelength (λ/4) transmission lines TL1 and TL2 are chosen network. It can maintain high efficiency over a large power such that both amplifiers see their optimum load resistance to range and it features a simple circuit implementation compared provide maximum power and efficiency. When the input power to the other architectures. Recent research also shows that it level is low, only the carrier amplifier is active in providing arXiv:1908.07755v1 [eess.SP] 21 Aug 2019 has a capability of operating at mm-wave frequencies [5]. the output power. The load resistance presented to the carrier Unfortunately, the classical DPA suffers intrinsic bandwidth amplifier is increased by the transmission line TL1, operating limitations, mainly due to narrow-band quarter-wavelength as an impedance inverter and serving to improve the DPA transmissions lines used for impedance transformation. Band- efficiency at lower output power levels. The λ/4 transmission width extension is thus an important consideration in modern line at the input of peaking amplifier provides a −90◦ phase DPA designs and it has received increasing attention in recent shift to ensure proper combining of output power from two research, especially for wideband 5G applications. There are amplifiers at the common output node. There are several number of review papers on DPAs published in the past years factors contributing to bandwidth limitation of the DPA which will be discussed in the following. We provide new insights This paper has been accepted for publication in IEEE Microwave Magazine. c 2019 IEEE. Personal use of this material is permitted. Permission from using theoretical derivation of the impedance presented to the IEEE must be obtained for all other uses, in any current or future media, carrier amplifier at the peak power and back-off. including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. A. Output Network The authors are with the School of Electrical and Electronic En- gineering, University College Dublin, Ireland (e-mail: [email protected], The main bandwidth-limiting element of the DPA is usually [email protected], [email protected]). the impedance inverter TL1. This can be shown by exploring 2 Power Carrier PA splitter TL1 Z1 , λ/4 TL2 in Zc out Peaking PA Z2 , λ/4 RL λ/4 Zp Fig. 1. The basic DPA architecture. Fig. 2. Real part of the normalized impedance presented to the carrier amplifier at peak power and 6-dB back-off. The DPA bandwidth is mainly Fig. 3. Real part of the normalized impedance presented to the carrier amplifier at peak power (top) and 6-dB back-off (bottom) for different values limited by the back-off impedance. p of m = Ropt=2RL (ideally, m = 1). The bandwidth at back-off can be improved using a proper value of m. the impedances presented to the carrier amplifier at the peak output power and back-off. The characteristic impedances of (k = 1 at peak power and k = 0:5 at back-off), it can be the λ/4 transmission lines, assuming a symmetric transistor shown that configuration (i.e., transistors in the carrier and peak amplifiers k + j tan( π f ) are identical), are chosen as 2 f0 Zc(f) = Ropt : (3) 1 + jk tan( π f ) 2 f0 Z1 = Ropt (1) The real part of Zc(f), normalized to Ropt, at the peak power and 6-dB back-off is depicted in Fig. 2. A narrower r1 bandwidth is observed at back-off. The fractional band- Z = R R ; (2) 2 2 L opt width for 20% reduction in the real part of the impedance, roughly corresponding to 1-dB drop in output power (Pout = 2 where Ropt is the optimum load impedance of the carrier and RefZLgIm=2), is 38% at back-off. peaking transistors [6]. The transmission line TL2 transforms The transmission line TL2 operates as an impedance 2 the load impedance RL into Z2 =RL = Ropt=2 at the common matching network that should transform RL to Ropt=2 over output node. At peak power, where the two amplifiers deliver the bandwidth. It can limit the DPA bandwidth when the identical output currents, each of them sees the optimum impedance transformation ratio, 2RL=Ropt, is excessive. To impedance, Ropt. The characteristic impedance of TL1 is investigate the bandwidth limitation effect of TL2, we derive chosen to match the optimum load resistance at peak power. the impedance presented to the carrier amplifier. It can be Thus, impedance Ropt is presented to both amplifiers. At shown that the impedance Zc(f) can be derived from (3) by 6-dB back-off, the peaking amplifier is turned off, and the replacing the parameter k with kc given by impedance presented to the carrier amplifier, assuming the f 2 1 π peaking amplifier presents an open circuit, is Z =(R =2) = m + j tan( 2 f ) 1 opt k = k 0 ; c π f (4) 2Ropt. It is noticed that the impedance transformation ratio of m + j tan( ) 2 f0 the impedance inverter TL1 is 1 at the peak power and 4 at p the 6-dB back-off. This limits the bandwidth of the DPA at where m = Ropt=2RL. The real part of the impedance back-off. In order to illustrate this bandwidth limitation, we at peak power and back-off is shown in Fig. 3. For m < compare the impedance presented to the carrier amplifier at 1, the bandwidth degrades compared to the ideal case m = the peak power and 6-dB back-off. Assuming that the load 1, while for m > 1 higher bandwidth is achieved, but with presented to the output of impedance inverter TL1 is kRopt additional peaks in the real part of the impedance at peak 3 Fig. 5. Conventional techniques to absorb output parasitic capacitances of transistors into the impedance inverter (a) reduced-length transmission line, (b) lumped-element transmission line. a higher-order network. A DPA with asymmetric (i.e., of unequal size) transistors is used to amplify signals with PAPR larger than 6 dB. Assuming the strength of peaking transistor is N times of the carrier transistor, the peak efficiency is achieved at 20 log(N + 1)-dB back-off. The characteristic impedance of TL1 is the same as in the symmetric DPA, i.e., Z1 = Ropt, while for TL2 it is given by Fig. 4. Real part of the impedance presented to the carrier amplifier at peak r power and 6-dB back-off by replacing TL with a two-section impedance 1 2 Z = R R : (5) transformer composed of two λ/4 transmission lines.
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