This paper presents the first 28-/37-/39-GHz linear Doherty power amplifier (PA) in silicon for broadband fifth-generation (5G) applications. We introduce a new transformer-based on-chip Doherty power combiner that can reduce the impedance transformation ratio (ITR) in power back-off (PBO) and, thus, improve the bandwidth and power-combining efficiency. We also devise a “driver-PA co-design” method that creates power-dependent uneven feeding in the Doherty PA and enhances the Doherty operation without any hardware overhead or bandwidth compromise. For the proof of concept, we implement a 28-/37-/39-GHz PA fully integrated in a standard 130-nm SiGe BiCMOS process, which occupies 1.8 mm $^{\mathbf {2}}$ . The PA achieves a 52% −3-dB small-signal $\text{S}_{\mathbf {21}}$ bandwidth and a 40% −1-dB large-signal saturated output power ( $\text{P}_{\mathbf {sat}}$ ) bandwidth. At 28/37/39 GHz, the PA achieves +16.8−/+17.1−/+17-dBm $\text{P}_{\mathbf {sat}}$ , +15.2−/+15.5−/+15.4-dBm $\text{P}_{\mathbf {1\,dB}}$ , and superior 1.72/1.92/1.62 times efficiency enhancement over class-B operation at 5.9-/6-/6.7-dB PBO. Moreover, the PA demonstrates multi-gigabit-per-second data rates with excellent efficiency and linearity for 64-quadrature amplitude modulation (64-QAM) in three millimeter-wave (mm-wave) 5G bands. This PA advances the state of the art for Doherty, wideband, and 5G silicon PAs in mm-wave bands. It supports drop-in upgrade for current PAs in existing mm-wave systems and opens doors to compact system solutions for future multiband 5G massive multiple-input multiple-output (MIMO) and phased-array platforms.

A 28-/37-/39-GHz Linear Doherty Power Amplifier in Silicon for 5G Applications

This article reviews the state-of-the-art millimeter-wave (mm-wave) power amplifiers (PAs), focusing on broadband design techniques. An overview of the main solid-state technologies is provided, including Si, gallium arsenide (GaAs), GaN, and other III–V materials, and both field-effect and bipolar transistors. The most popular broadband design techniques are introduced, before critically comparing through the most relevant design examples found in the scientific literature. Given the wide breadth of applications that are foreseen to exploit the mm-wave spectrum, this contribution will represent a valuable guide for designers who need a single reference before adventuring in the challenging task of the mm-wave PA design.

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A Review of Technologies and Design Techniques of Millimeter-Wave Power Amplifiers

A review is presented of key power amplifier (PA) performance requirements for millimeter-wave 5G systems, along with a comparison of the potential of different semiconductor technologies for meeting those requirements. Output power, efficiency, and linearity considerations are highlighted, and related to semiconductor material characteristics. Prototype 5G PAs based on silicon technologies are then reviewed, with primary emphasis on CMOS-SOI. Stacked FET PAs based on nMOS and pMOS for 28-GHz operation are presented, along with outphasing and Doherty amplifiers. Peak power-added efficiency (PAE) up to 46% is demonstrated for a two-stack pMOS amplifier with saturation power (Psat) above 19 dBm. PAE at 6 dB backoff above 27% is shown for an nMOS Doherty PA with 22-dBm Psat. Operation with 64QAM OFDM modulation signals at 800-MHz bandwidth is reported, with up to 13-dBm output power and more than 17% PAE, without the use of digital predistortion. Future challenges for PA development are discussed.

Power Amplifiers for mm-Wave 5G Applications: Technology Comparisons and CMOS-SOI Demonstration Circuits

The fifth generation (5G) research and development has been fueled by many new breakthroughs in various areas. The recent progress in carrier aggregation (CA), licensed assisted access (LAA), massive MIMO (MaMi), beamforming techniques, cooperative spectrum sensing (CSS), compressive sensing (CS), machine learning, etc., has provided inspiring and promising approaches to address 5G and beyond challenges. However, at the user equipment (UE) end, limited design budget and hardware resources bring along a series of challenging implementation issues when delivering multi-standard and multi-functional wireless communications. In this paper, we first review recent advances in technical standards and critical enabling techniques, accompanied with several case studies of product developments. After the classification of typical 5G application and deployment scenarios, we propose and analyze a novel hardware reuse and multiplexing solution to facilitate cost-effective and energy-efficient UE design, followed by an investigation of state-of-the-art hardware development from the systems and circuits standpoint. Moreover, wireless UE hardware solutions, UE proof-of-concept (PoC) implementation and field test are proposed and discussed. Finally, the new trends of UE design and terahertz technologies for 5G and beyond applications are investigated and envisioned.

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Enabling Multi-Functional 5G and Beyond User Equipment: A Survey and Tutorial

This paper presents three millimeter-wave (mm-wave) continuous-mode power amplifiers (PAs) for fifth-generation (5G) MIMO applications, including a two-stage differential continuous-mode Class-F−1 PA in 130-nm SiGe process, a single-stage differential continuous-mode hybrid Class-F/F−1 PA in 45-nm SOI CMOS process, and a two-stage differential continuous-mode hybrid Class-F/F−1 PA in 45-nm SOI CMOS process. The first PA design covers 19–29.5 GHz, while the other two designs cover 23–41 GHz, all covering multiple 5G bands (24/28/37/39 GHz). All the presented PA designs are based on a proposed transformer-based continuous-mode harmonically tuned PA output network. This network provides proper harmonic impedance terminations for the fundamental, second-order, and third-order harmonic impedances, which supports continuous-mode Class-F−1 or hybrid Class-F/F−1 PA operations to achieve ultrawide bandwidth yet high efficiency. The first PA design achieves a wide $P_{\mathrm {sat}}~1$ -dB bandwidth of 19–29.5 GHz (43.3%) and high peak power-added efficiency (PAE) (43.5%). The second design achieves an ultrawide $P_{\mathrm {sat}}~1$ -dB bandwidth of 23.5–41 GHz (53.3%) and high peak PAE (46%). Moreover, it achieves 43.4% PAE and 18.6-dBm $P_{\mathrm {sat}}$ at 27 GHz, 40.2% PAE and 18.6-dBm $P_{\mathrm {sat}}$ at 37 GHz, and 41.2% PAE and 18.5-dBm $P_{\mathrm {sat}}$ at 39 GHz, respectively. The third PA design also achieves an ultrawide $P_{\mathrm {sat}}~1$ -dB bandwidth of 23.5–41 GHz (53.3%) and high peak PAE (43.2%). It achieves 43% PAE and 18.9-dBm $P_{\mathrm {sat}}$ at 27 GHz, 37% PAE and 19-dBm $P_{\mathrm {sat}}$ at 37 GHz, and 36% PAE and 18.9-dBm $P_{\mathrm {sat}}$ at 39 GHz, respectively. Extensive 64-/256-QAM modulation tests demonstrate the high PA linearity. Our proposed PA designs outperform the reported mm-wave silicon-based 5G PAs in terms of high efficiency over an ultrawide bandwidth.

Millimeter-Wave Continuous-Mode Power Amplifier for 5G MIMO Applications

This paper presents a packaged 28-GHz Chireix outphasing transmitter (TX) for power amplifier (PA) efficiency enhancement at both peak and back-off output power (PBO). The main objective is to investigate the possibility of employing a multi-feed antenna as a multi-port passive network to perform outphasing active load modulation. As a proof of concept, an outphasing TX architecture is proposed by leveraging a dual-feed loop antenna (DLA) to achieve high efficiency transmission of wideband complex modulated signals. Theoretical analysis and simulation results prove that the proposed DLA is electrically equivalent to an ideal differential series non-isolating power combiner, enabling superior on-antenna outphasing operation. Multi-functional signal processing is therefore achieved in a single loop-antenna footprint, including high-efficiency on-antenna power combining, outphasing active load modulation, and millimeter-wave (mm-Wave) signal radiation. Moreover, unlike conventional spatial vectorial combining techniques (e.g., spatial IQ or spatial outphasing), the antenna-level signal processing preserves the wide antenna unit field of view (FoV) and is particularly suitable for array or multiple-input-multiple-output (MIMO) applications. The proposed Chireix TX contains two phase-shifted sub-TXs simultaneously driving a DLA on package. The measurements demonstrate a 53%/43% PA drain/power-added efficiency (DE/PAE) at 17.1-dBm saturated output power ( $P_{\mathrm {sat}}$ ) and 36%/23% DE/PAE at 6-dB PBO. The high-efficiency Chireix TX supports up to 15 Gb/s 64-QAM with 10.4-dBm average output power and 34% average PA DE (19.2% average PAE) while maintaining EVM/ACLR better than −27.5 dBc/−28 dBc in a wireless radiation testing environment.

A 28-GHz Flip-Chip Packaged Chireix Transmitter With On-Antenna Outphasing Active Load Modulation

This work introduces, for the first time, a millimeter-wave imaging system with a “synthetic” ultra-wide imaging bandwidth of 98 GHz to provide the ultra-high resolutions required for early-stage skin cancer detection. The proposed approach consists of splitting the required ultra-wide imaging bandwidth into four sub-bands, and assigning each sub-band to a separate imaging element, i.e., an antenna radiator. Each of the sub-band antennas transmits and receives signals only at its corresponding sub-band. The captured signals are then combined and processed to form the image of the target. For each sub-band, a Vivaldi tapered slot antenna fed with a combination of substrate-integrated waveguide and coplanar waveguide is designed and microfabricated. Design techniques are also provided for the four similarly-shaped sub-band antennas for achieving excellent impedance matches ( $S_{11}$ < –10 dB) and nearly constant gains of 10 dBi over the entire 12–110 GHz bandwidth. The design procedure is validated by comparing the simulated results with measurements performed on the fabricated prototypes. Excellent agreements are obtained between simulations and measurements. Finally, the feasibility of detecting early-stage skin tumors in three dimensions is experimentally verified by employing the sub-band antennas in a synthetic ultra-wideband imaging system with a bandwidth of 98 GHz. Two separate setups, each comprising a dispersive skin-mimicking phantom as well as two dispersive spherical tumors, are constructed for imaging experiments. Lateral and axial resolutions of 200 μm are confirmed, and a successful reconstruction of the spherical tumors is achieved in both cases.

Synthetic Ultra-High-Resolution Millimeter-Wave Imaging for Skin Cancer Detection

This paper presents a Doherty radiator architecture that explores multi-feed antennas to achieve an on-antenna Doherty load modulation network and demonstrates high-speed high-efficiency transmission of wideband modulated signals. On the passive circuits, we exploit the multi-feed antenna concept to realize compact and high-efficiency on-antenna active load modulation for close-to-ideal Doherty operation, on-antenna power combining, and millimeter-wave (mm-Wave) signal radiation. Moreover, we analyze the far-field transmission of the proposed Doherty radiator and demonstrate its wide field of view (FoV). On the active circuits, we employ a gigahertz-bandwidth adaptive biasing at the Doherty auxiliary power amplifier (PA) path to enhance the main/auxiliary Doherty cooperation and appropriate turning-on/-off of the auxiliary path. A proof-of-concept Doherty radiator implemented in a 45-nm CMOS silicon on insulator (SOI) process over 62–68 GHz exhibits a consistent 1.45 $\times $ –1.53 $\times $ PAE enhancement at 6-dB power backoff (PBO) over an idealistic class-B PA with the same PAE at $P_{\mathrm {1\,dB}}$ . The measured continuous-wave performance at 65 GHz demonstrates 19.4/19.2-dBm $P_{\mathrm {SAT}}/P_{\mathrm {1\,dB}}$ and achieves 27.5%/20.1% PAE at peak/6-dB PBO, respectively. For single-carrier 1 Gsym/s 64- quadratic-amplitude modulation (QAM), the Doherty radiator shows average output power of 14.2 dBm with an average 20.2% PAE and −26.7-dB error vector magnitude (EVM) without digital pre-distortion. Consistent EVMs are observed over the entire antenna FoV, demonstrating spatially undistorted transmission and constant Doherty PBO efficiency enhancement.

A Linear High-Efficiency Millimeter-Wave CMOS Doherty Radiator Leveraging Multi-Feed On-Antenna Active Load Modulation

Since its invention in early 2000s, Distributed Active Transformer (DAT) has been a popular power-combiner technique for high-power high-efficiency Power Amplifiers (PAs) in voltage-limited Silicon processes at GHz frequencies [1, 2]. However, successful DAT implementations at high mm-wave frequencies still remain elusive. Substantial inter-winding capacitance causes asymmetry in impedance transformation among the DAT sections, resulting in mismatched and non-optimum loads for high-frequency mm-wave DAT PAs, including those using transformer-based DAT combiners [3]. Moreover, the DAT physical size at mm-wave frequencies is comparable to a mm-wave signal wavelength, which leads to significant physical phase delays between DAT sections. As a result, the combined vectoral signal at the DAT output is substantially mismatched, causing appreciable degradation in the DAT output power and efficiency [4].

4.9 A 60GHz CMOS Power Amplifier with Cascaded Asymmetric Distributed-Active-Transformer Achieving Watt-Level Peak Output Power with 20.8% PAE and Supporting 2Gsym/s 64-QAM Modulation

Designing Doherty power amplifiers (PAs) at high millimeter-wave (mm-Wave) range remains a major challenge because most Doherty combiners at this frequency band are inefficient. To resolve this bottleneck, we analyze balun response under load variation and subsequently propose the Doherty PA architecture with impedance inverting and scaling baluns that perform active load modulation on differential PAs and delivers combined power to a single-ended load. At the PA output/input stage, instead of utilizing the transformer-based balun commonly used at RF and low mm-Wave frequencies, we explore the coupler-based balun to resonate out the device parasitic capacitance and achieve well-balanced differential to single-ended conversion at high mm-Wave frequencies. Based on the coupled-line theory and our analysis, we derive the [Y] matrix of the coupler-based balun and originate various solutions for baluns that can invert or scale impedance. From this derivation, we conceive a canonical form of impedance inverting baluns when the electrical length of coupled lines equals 45°. We also propose a close-formed solution for the coupler-based impedance scaling balun that is electrically equivalent to an idealistic transformer in series with a small inductance. We then combine the proposed impedance inverting and scaling balun structures to construct a Doherty network that exhibits desirable Doherty active load modulation and occupies a single-transformer footprint. On the active circuit design, we employ the adaptive biasing circuit for both driver and PA stages of the auxiliary PA to enhance the main and auxiliary cooperation. Using a 45-nm GlobalFoundries CMOS SOI process, we fabricate a proof-of-concept Doherty PA that demonstrates 20.1-dBm $P_{\mathrm {sat}}$ , 19.3dBm OP1,dB, 26% peak power-added efficiency (PAE), and 16.6% PAE at 7-dB power back-off (PBO) from OP1 dB at 60 GHz. Our results indicate a substantial PAE enhancement at PBO and a state-of-the-art modulation efficiency achieved when transmitting digital pre-distortion (DPD)-free 64-QAM-modulated signals.

Huy Thong Nguyen

Papers

A 28-GHz Flip-Chip Packaged Chireix Transmitter With On-Antenna Outphasing Active Load Modulation

Synthetic Ultra-High-Resolution Millimeter-Wave Imaging for Skin Cancer Detection

A Linear High-Efficiency Millimeter-Wave CMOS Doherty Radiator Leveraging Multi-Feed On-Antenna Active Load Modulation

4.9 A 60GHz CMOS Power Amplifier with Cascaded Asymmetric Distributed-Active-Transformer Achieving Watt-Level Peak Output Power with 20.8% PAE and Supporting 2Gsym/s 64-QAM Modulation

A Coupler-Based Differential mm-Wave Doherty Power Amplifier With Impedance Inverting and Scaling Baluns