A Wideband High Efficiency KaBand MMIC Power Amplifier for 5G Wireless Communications44889

A Wideband High Efficiency Ka-Band MMIC Power Amplifier for 5G Wireless Communications Duy P Nguyen1, Xuan-Tu Tran2, Nguyen L K Nguyen1, Phat T Nguyen1, and Anh-Vu Pham1 Department of Electrical and Computer Engineering, University of California, Davis, CA, USA SISLAB, VNU University of Engineering and Technology, Hanoi, Vietnam Abstract — A compact wideband Ka-band monolithic microwave/ millimeter-wave integrated circuit (MMIC) power amplifier (PA) is demonstrated using a 0.15-µm pseudomorphic high electron mobility transistor (pHEMT) Gallium Arsenide (GaAs) process The proposed harmonic load-pull and radial stub matching technique have been employed to achieve wideband and high-efficiency performance The fabricated PA exhibits a measured output power of 24 dBm, gain of 20 dB and 35% power added efficiency (PAE) The PA has a wideband performance in which the power maintains higher than 23 dBm, and the PAE is higher than 30% over a wide frequency range from 25 to 34 GHz (a) (b) (c) (d) Index Terms — Gallium Arsenide, Ka-band, MMIC, Power Amplifier, Radial stub, Wideband I INTRODUCTION The demand for mobile data has seen tremendous growth in recent years which makes the sub-3 GHz spectrum become increasingly crowded The current mobile network is reaching the limits in bandwidth and speed, requiring the next generation of wireless communications to deploy a millimeter-wave spectrum which has wider bandwidth and higher data rate [1], [2] Examples of these capabilities include very high achievable data rates, low latency, ultrahigh reliability, and the possibility to handle extreme device densities Among millimeter-wave frequency spectrums, Ka-band (28 GHz) is a major contender for 5G applications However, the low efficiency of Ka-band power amplifiers (PAs) imposes a major thermal issue in the base station where hundreds of PAs are used in a small area [3], [4] To alleviate the issue, it requires high-efficient amplifiers with very compact size [5], [6] Millimeter-wave PAs have been the focus of research for the emerging 5G wireless communications in recent years Medium-power MMIC Ka-band amplifiers have been reported using different technologies including Gallium Arsenide (GaAs) [5], [7]-[10], Gallium Nitride (GaN) [11], and complementary metal–oxide–semiconductor (CMOS) [12]-[14] However, low power added efficiency (PAE) and 978-1-7281-0397-6/19/$31.00 ©2019 IEEE Fig Load-pull contours for a x 75 μm pHEMT device at 28 GHz: (a) Power contours, (b) PAE contours of fundamental impedance, (c) PAE contours of 2nd harmonic impedance, and (d) PAE contours of 3rd harmonic impedance large chip size are still posing a challenge in those designs In addition, narrow bandwidth makes the amplifiers less attractive to the 5G network applications This paper reports a wideband, compact and high efficiency Ka-band PA using a 0.15-µm GaAs pseudomorphic high electron mobility transistor (pHEMT) process A two-stage class AB amplifier employs a proposed radial stub matching technique to achieve wideband, high-efficiency performance The fabricated PA demonstrates a measured output power of 24 dBm with an associated peak PAE of 35%, and gain of 20 dB Higher than 23 dBm output power and 30% PAE is observed over the very wide bandwidth from 25 to 34 GHz II RADIAL STUB FOR WIDEBAND MATCHING AND HARMONIC TERMINATION To achieve the target output power at Ka-band, an ì 75 àm device is chosen for the output stage Harmonic Fig Proposed radial stub versus conventional open stub load-pull simulation of the device under a drain bias voltage of V and dc current of 60 mA is performed to find the optimum impedances for highest output power and highest PAE Fig 1(a) and (b) show the power and PAE contours at the fundamental frequency 28 GHz A maximum output power of 24.5 dBm and 34% maximum PAE can be achieved at the impedance around Zopt = 8.5 + j10 (Ω) To further enhance the PAE, optimum impedances at the 2nd harmonic and 3rd harmonic frequencies can be found from the harmonic load-pull According to Fig 1(c) and (d), by terminating the 2nd harmonic close to open circuit, the PAE can reach a maximum value of 40% and is increased another 2% by terminating the 3rd harmonic at its optimum value In this process, at Ka-band frequencies, the 3rd harmonic termination only contributes very little to the efficiency enhancement [15] Therefore, to reduce the size and losses of the matching network, only the 2nd harmonic termination will be considered in the proposed design The design of a class-F and class-J amplifiers to achieve high PAE by terminating the 2nd and 3rd harmonic impedances has been well presented in [16]-[18] However, the output matching networks of the class-F and class-J amplifiers requires several matching sections, thus are quite bulky and lossy In our design, we propose to use microstrip radial stubs in the matching network The high-quality factor of the radial stubs can significantly reduce the insertion loss and size of the matching network, as well as satisfy the harmonic load conditions Fig presents the proposed radial stub and conventional open stub Both of which has the same equivalent capacitance value at the center frequency of 28 GHz The use of the radial stub can reduce the length by 53%, making the matching network more compact and lower loss Fig compares the simulated return loss and insertion loss of the two output matching networks using the radial stub versus conventional open stub The proposed radial stub matching significantly improves the bandwidth and has much lower loss In other words, the reactance of the radial stub varies much less than that of the conventional open stub Moreover, the radial stub has three independent Fig Return loss S11 and insertion loss S21 of the radial stub matching and conventional open stub matching Fig Fundamental impedance and 2nd harmonic impedance of the radial stub and conventional open stub matching networks parameters that can be optimized separately: radius (Ro), width (W), and angle (θ) [19] Therefore, the 2nd harmonic impedance can be easily tuned to achieve the optimum load-pull value that gives the highest PAE Fig illustrates the fundamental and 2nd harmonic matching of the two output matching networks (radial stub versus open stub) Both matching networks provide good 50 Ω match at fundamental frequencies However, the radial stub provides the 2nd harmonic impedance close to the optimum value found in Fig 1(c), resulting in higher PAE III POWER AMPLIFIER DESIGN The proposed power amplifier has two amplifying stages and is fully matched to 50 Ω system at input and output The schematic of the PA is showed in Fig The MMIC PA employs a 1:2 gate periphery drive ratios to provide sufficient power margins between stages while preserving high gain and efficiency The peripheries of the pHEMTs are 300 àm (4ì75 àm) and 600 µm (8×75 µm) for the driver stage (Q1) and the output stage (Q2), respectively Both Fig Circuit schematic of the Ka-band power amplifier stages are biased in deep class AB in which the bias voltage of the power stage Q2 is Vd2 = V; Id2 = 60 mA and that of the driver stage Q1 is Vd1 = V and Id1 = 32 mA The matching networks consist of radial stubs, transmission lines, and metal-insulator-metal (MIM) capacitors The proposed radial stubs are used in the both the output and inter-stage matching networks to help enhance the bandwidth, PAE, and reduce the chip size Another benefit from using radial stubs is that we can avoid the discontinuity from a large tee junction The inter-stage matching employs a dual symmetrical radial stub, a dual shunt stub, and two series transmission lines Compared to the conventional matching topology, dual stubs provide totally symmetrical design and enhances performance at high frequencies In addition, it also allows balanced biasing in which the dc current can be provided from either side of the chip or both sides concurrently This additional feature improves the PA reliability, especially at the drain bias where high current is needed The input matching employs a series MIM capacitor and two shunt stubs to achieve a compact size The design layout and full electromagnetic (EM) simulation were carefully handled to ensure a balanced and symmetrical design Bias pads were placed symmetrically to allow current supplied from both sides This technique guarantees current and heat are distributed equally, which improves reliability All matching networks are designed to guarantee unconditional stability from dc to 80 GHz Fig Chip photo of the fabricated amplifier (2.1 mm × 1.4 mm) Fig Measured small signal gain and return loss at Vd1 = V; Id1 = 32 mA; Vd2 = V; Id2 = 60 mA IV EXPERIMENTAL RESULTS The process selected for our Ka-band PA is a 0.15-µm GaAs pHEMT transistor This process has an fmax of 185 GHz and ft of 90 GHz which make it attractive to Kaband applications The process is designed to operate with a peak drain voltage of up to V and provide relatively high-power density The 0.15-µm PHEMT device can deliver 870 mW/mm at 29 GHz in which Idmax= 620 mA/ mm and the breakdown voltage of Vgd is 16.5 V Fig shows a photograph of the fabricated MMIC PA The size of the chip is 2.1 mm × 1.4 mm × 0.01 mm Fig Measured output power, gain, and PAE at 28.5 GHz Fig shows the measured versus simulated small signal performance of the MMIC PA at Vd1 = V; Id1 = 32 mA; Vd2 = V; Id2 = 60 mA The maximum small signal gain is 20 dB and maintains above 17 dB from 27 to 32 GHz The PA achieves good input and output return loss in the frequency range The measured results are correlated with the simulation Fig presents the output power, gain and efficiency of the PA at 28.5 GHz as a function of the input TABLE I COMPARISON TO OTHER PUBLISHED MILLIMETER-WAVE POWER AMPLIFIERS Reference [7] [5] [8] [11] [12] [13] [14] This work Frequency (GHz) 29 - 31.8 26 - 31 18 26.5 34 28 28 25 - 34 Power (dBm) 26.3 31.5 33.7 29 19.9 14 19.8 24 Gain (dB) 14 16.7 14 19.5 13.8 22 13.6 20 Supply voltage (V) 4.5 12 12 20 2.4 0.9 2.2 Fig Measured output power and PAE as a function of frequency power with the bias condition as mentioned above The input power is swept from -15 dBm to +13 dBm with dB step The output power at 1dB compression point (OP1dB) is 21 dBm, and the saturation power is 24 dBm The maximum PAE 35% is achieved at 23.5 dBm output power Maximum power and PAE across the frequency range from 25 to 35 GHz is illustrated in Fig The output power maintains higher than 23 dBm from 25 to 34 GHz while the PAE is higher than 30% in that frequency range The PA exhibits excellent power and PAE flatness over a wide bandwidth Fig 10 shows the measured output power, gain, relative third order intermodulation (IM3) and third-order intercept point OIP3 as a function of input power The linearity data was obtained by a two-tone measurement with equal amplitude and 10 MHz tone spacing at 28.5 GHz The amplifier demonstrates good linearity in which 35 dBm OIP3 is observed in Fig 10 Table I summarizes the proposed PA performance and compares with other previously published Ka-band MMIC PAs Our proposed PA achieves among the highest efficiency over a wide bandwidth PAE (%) 35 33 29.5 14.5 25.8 21.8 28.5 35 Die size (mm2) 3.57 11.2 0.365 0.19 0.28 2.94 Number of stages Doherty Dual-mode 2-stack Technology 0.15-um GaAs 0.15-µm E-mode GaAs 0.15-µm GaAs 0.15-μm AlGaN/GaN-on-Si 65-nm CMOS 65-nm CMOS 28-nm CMOS 0.15-µm GaAs Fig 10 Two-tone measurement at center frequency 28.5 GHz and 10 MHz tone spacing V CONCLUSION We have demonstrated a wideband, compact MMIC Ka-band PA The 2.0 × 1.4 mm2 amplifier exhibits a measured gain of 20 dB, output power of 24 dBm and peak PAE of 35% at 28.5 GHz Employing the proposed matching topology, the PA achieves a very wide band performance in which measured output power and PAE are flat from 25 to 34 GHz In addition, good linearity and compact chip size are observed, which makes the PA more attractive to the 5G 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PowerCombining and Inverse Class-F GaN Power Amplifier at XBand," IEEE Trans Microw Theory Techn., vol 61, no 3, pp 1291-1300, 2013 [19] W Zhebin and P Chan-Wang, "Novel wideband GaN HEMT power amplifier using microstrip radial stub to suppress harmonics," in IEEE MTT-S Int Microw Symp Dig., 2012, pp 1-3  ... power and highest PAE Fig 1 (a) and (b) show the power and PAE contours at the fundamental frequency 28 GHz A maximum output power of 24.5 dBm and 34% maximum PAE can be achieved at the impedance... and 10 MHz tone spacing V CONCLUSION We have demonstrated a wideband, compact MMIC Ka-band PA The 2.0 × 1.4 mm2 amplifier exhibits a measured gain of 20 dB, output power of 24 dBm and peak PAE... back-off PAE using GaAs PHEMTS," IET Microwaves, Antennas & Propagation, vol 10, no 10, pp 1101-1105, 2016 D P Nguyen and A Pham, "An Ultra Compact Watt-Level Ka-Band Stacked-FET Power Amplifier, "