Millimeter-wave PA design techniques in ISSCC 2024

Yun Wang and Hongtao Xu

State Key Laboratory of Integrated Chips and Systems, School of Microelectronics, Fudan University, Shanghai 201203, China

Millimeter-wave systems with high integration level have been rapidly developed to enable modern wireless communication, sensing, and imaging.The millimeter-wave power amplifier (PA) is one of the most challenging and critical components in a millimeter-wave transceiver, it is the last active stage in a transmitter amplifying spectrally efficient complex modulation signal for radiation with high output power, high linearity, and high energy efficiency.Among the PA design techniques developed in recent years, power-back-off (PBO)efficiency-enhanced PA architecture, low-loss compact-area multi-way power combining topology, and silicon-process ultra-wideband sub-THz design technique are attractive in maximizing the system total efficiency, output power, and spectrum capacity.

Conventional linear PAs demonstrate low efficiency in modern wireless communication while amplifying complex modulation signals with high peak-to-average power ratio(PAPR), therefore, the Doherty PAs with intrinsic PBO efficiency enhancement are studied and reported in ISSCC 2024.In Ref.[1], a 47 GHz 4-way Doherty PA with high linearity and wide range PBO efficiency enhancement supporting 2000 MHz 64-QAM OFDM is proposed.It introduces a new Doherty network synthesis flow with fewer transformers, less loss, and compact area, it not only realizes 4-way Doherty load modulation but also achieves the desired impedance transformation from the 50 Ω antenna impedance to the optimal load impedance and absorbs the device parasitic capacitance.Besides, a compact high-speed adaptive-biasing circuit enabling wide modulation bandwidth is proposed.Power by 2 V/1 V supply voltages, the multi-way PA demonstrates 23.7 dBm P1dB, and 24 dBm PSATwith 1.6× and 2.0×efficiency enhancement over class-B at 6 dB and 12 dB PBO,which is attractive for high-power energy efficient 5G NR.In Ref.[2], a 24.25 to 29.5 GHz extremely compact Doherty PA is proposed with a core size of 0.14 mm2.It introduces a new inter-stage network by replacing the conventional differential mode matching network with a proposed differentialbreaking phase-offset (DBPO) single-ended network, therefore, it does not need the input power divider and consists of only one driver and three transformers, which greatly saves the area and simplifies the topology.Power by 2.2 V supply voltage, the PA achieves PAEpeakof 32.5% to 42.3%, PSATof 20.3 to 22.0 dBm, and the PAE6dB-BOof 22.1% to 27.8%.The achieved PAEavgand Pavgunder 5G NR 800 MHz 256-QAM signal are 17.1% to 23.8% and 12.0 to 13.4 dBm, which demonstrates competitive performance with a small area and low cost.In Ref.[3], an ultra-compact Doherty PA is realized in another approach by optimizing the output network.It proposes a single-transformer asymmetrically-coupled Dohertyoutput-load network, which features multiple functions of impedance-inverting network (IIN), impedance-scaling network (ISN) and wideband balun.By vertically stacking three coils, the coupling factors for IIN and ISN can be independently designed by sharing the center coil.The PA occupies a core area of 0.154 mm2, it achieves a PSATof 21 dBm, 38%PAE at an OP1dB of 20.4 dBm, 29.3% PAE at 6 dB PBO, and 20.3% PAE at 9 dB PBO.The achieved high modulation efficiency and small area make it suitable for use in large-scale phased arrays.Load-modulated balanced PA (LMBA) is another PA architecture that can achieve wideband efficiency enhancement, in ISSCC 2024 work, Ref.[4] proposes a 27.8-to-38.7 GHz LMBA with a scalable 7-to-1 load-modulated power-combine network.To extend more PBO efficiency enhancement points, the number of cascaded 90°hybrids can be increased with additional isolation ports and extended high-efficiency PBO range.A general architecture for scalable N-way power-combined and load-modulated balanced PA is introduced.The LMBA achieves a 27.8-to-38.7 GHz small-signal bandwidth with 25.4 dBm PSAT, 22%PAEpeak, 23.2% PAE6dB-BO, 16.3% PAE9dB-BOand 11.9%PAE12dB-BO.The measured 4.5 Gbps 64QAM modulation signal demonstrates that the LMBA is well suited for broadband high-speed wireless communication.

High output power PA using multi-way power combining techniques are also reported in ISSCC 2024.In Ref.[5], a 71 to 86 GHz E-band four-stage eight-way PA in the GaN process is introduced.The PA employs a low-loss distributed matching network (DMN) at the input and pre-transformed DMN at the output.It produces a peak PSATof 36.4 dBm and PAEpeakof 18.5%, which is suitable for potential 5G backhaul applications.In Ref.[6], a 35 to 43 GHz VSWR-resilient balanced PA (BPA) is introduced, it features: 1) An eight-way power combined with high output power, gain, and linearity,2) chain-weaver balanced power combiner for VSWR resilience, and 3) integrated power/impedance sensor.The BPA achieves a PSATof 25.19 dBm and PAE of 16.18%, the detected VSWR magnitude error is 0.106 and the worst angle error is 12.3°.The BPA is a promising solution for implementing a 5G antenna array.

Moreover, an ultra-wideband 67.8-to-108.2 GHz PA is implemented for maximizing the spectrum capacity and multiple bands coverage in Ref.[7].A complementary-gain-boosting (CGB) technique is proposed to overcome wideband flatgain design issues.The PA achieves a measured 3-dB small-signal bandwidth from 67.8 to 108.2 GHz, a PSATof ＞14.2 dBm,an OP1dB of ＞11.1 dBm and PAEmaxof ＞19%.The superior bandwidth and GBW with competitive output power, and a PAE demonstrate a good candidate for enabling various mm-wave band services.

In summary, these millimeter-wave PAs in ISSCC 2024 show several recent research trends, including improving PBO efficiency by using the Doherty technique and LMBA architecture, increasing output power and linearity by using power combining, VSWR detection and VSWR resilient design, ultra-wideband sub-THz PA as shown in Fig.1.The millimeter-wave communication enabled by using these PAs and techniques can achieve ever higher speeds and longer distances than before.The authors believe that the millimeterwave PA design techniques and research trends are not limited to the works in above summary, for example: 1) The directly-digital-modulated RF-DAC with high operation frequency, wide bandwidth high-order modulation, and high DAC linearity for digital-intensive energy-efficient transmitter.2) Ultra-wideband highly-linear PA supporting multi-hundred-Gbps data rate.3) Highly-precise VSWR detection and healing technique for large scale phased-array system.4) High performance THz and sub-THz PA in silicon.

Fig.1.(Color online) (a) PBO efficiency enhancement technique, (b) multi-way power combining techniques, (c) complementary-gain-boosting ultra-wideband technique.