A complete theoretical analysis is presented for a highly efficient power amplifier (PA) architecture based on a non-reciprocal combiner. The architecture consists of two amplifier branches and a microwave circulator acting as the output combiner, referred to as the circulator load-modulated amplifier (CLMA). The continuous-mode (CM) design technique has been further exploited for the CLMA to demonstrate the wideband capabilities of the architecture. The analysis reveals that the CLMA performs active load modulation to maintain high PA efficiency performance across a wide bandwidth and large output power dynamic range. As a proof of concept, a CLMA demonstrator circuit based on gallium nitride (GaN) transistors and a microwave circulator, is designed and characterized with continuous-wave and modulated-signal measurements. The implemented CLMA prototype circuit experimentally demonstrates Doherty-like efficiency enhancement over a large bandwidth covering 2.1 to 3.5 GHz. In measurements, the prototype exhibits a drain efficiency of 46.7-57.5% at peak output power and 35.6-50.6% at 7-dB output power back-off level, within the design bandwidth. When tested with a 60-MHz multi-carrier orthogonal frequency-division multiplexing (OFDM) signal having a 7-dB peak-to-average power ratio (PAPR), an average efficiency of 48.5% with better than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 47.5-dBc adjacent channel leakage ratio (ACLR) is achieved after applying digital pre-distortion (DPD).

A Wideband and Highly Efficient Circulator Load Modulated Power Amplifier Architecture

In the emerging 5G and beyond 5G (B5G) era, the spotlight is sharply focused on the power amplifier, a critical component with stringent specification requirements that dictates the performance of the transmitter. The gallium nitride (GaN) device, with its superior inherent properties, is surfacing as a front-runner for power amplifier applications. The increasing demand for high frequency, high linearity, and cost-effective GaN power amplifiers is driven by anticipated traffic surges and the need for extensive 5G deployment. This paper offers a thorough review and future perspective on research developments in RF GaN device technology. It encompasses critical issues in advanced device and circuit technology, with a focus on high frequency, high linearity, cost-effective GaN-on-Si high electron mobility transistors (HEMTs), and compact modeling. This work aims to serve as a guide for the utilization of GaN HEMTs in 5G communication applications. 

A Review of GaN RF Devices and Power Amplifiers for 5G Communication Applications

In this article, the theory and design of high-efficiency broadband continuous class-B/J power amplifiers (PAs) with active second harmonic injection technique have been presented. By injecting the second harmonic component generated by the auxiliary PA, the optimal phase shift parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $</tex-math> </inline-formula> between the drain voltage and current can be selected properly, leading to an efficiency and power improvement of the conventional class-B/J PA. Detailed design guidelines and step-by-step procedure are also suggested. To verify the feasibility of the proposed circuit, a wideband PA operated from 1.1 to 1.7 GHz (fractional bandwidth of 42.9%) has been designed and fabricated. Experimental results show an output power of 40.9–42.6 dBm, drain efficiency of 70.0%–80.9%, and average efficiency (AE) of 74.4%. To the best of our knowledge, this is the highest AE of class-B/J PAs over such bandwidth.

Theory and Design of High-Efficiency Broadband Class-B/J Power Amplifiers With Active Second Harmonic Injection

In this article, an efficiency-improved inverse class-F2,3 (class-F2,3-1) power amplifier (PA) is designed and implemented using a novel harmonic control circuit (HCC) based on a metamaterial structure. It is proposed to utilize a bi-band, compact microstrip resonant cell (CMRC) inspired by the split ring resonator (SRR) that behaves like an HCC, to control concurrently the second and third harmonics. The main objective is to demonstrate the potentiality and versatility of SRRs for suppression of unwanted harmonics in microwave PAs, shaping voltage and current waveforms, miniaturization (due to subwavelength operation of SRRs), as well as improving the performance of the PA. The recommended architecture is validated by the design, fabrication and testing of a metamaterial-based class-F2,3-1 PA utilizing a commercial 10 W GaN-HEMT device. The proposed PA yields a power added efficiency (PAE) of 78 %, a drain efficiency (DE) of 83.3 %, a saturated output power of 41 dBm, and a gain of 16.8 dB at 3.5 GHz. 

An efficiency-improved inverse class-F2,3 power amplifier design using a novel metamaterial-based harmonic control circuit

The design of a multi-octave power amplifier (PA) is presented based on extended continuous Class-B/J (ECB/J) modes in this paper. The wide design space provided by the ECB/J modes has been used as the goal for the design. A unique bandwidth extended matching network that can achieve a broadband impedance matching in the appropriate impedance region by simultaneously manipulating six frequency points is proposed to compensate for the impedance dispersion effect caused by the change of the optimal impedance with the operating frequency in the design of the matching network. To verify the proposed methodology, a wideband PA is designed and fabricated by using a commercial 10-W gallium nitride (GaN) device. The implemented PA achieves a bandwidth of 163% from 0.4 to 3.9 GHz while exhibiting excellent performance of 38-42.3 dBm output power, 57-68% drain efficiency, and 10-13 dB gain. The measured adjacent channel power ratio (ACPR) is lower than -26 dBc at 0.8, 1.5, 2.0, 2.5, 3.0, and 3.5 GHz when the output power is lower than 36 dBm. 

Design of a 0.4-3.9-GHz Wideband High-Efficiency Power Amplifier Based on a Novel Bandwidth Extended Matching Network

We present an ultra-broadband and low-modal-crosstalk mode multiplexer based on cascaded vertical directional couplers formed by adiabatic-tapered waveguides without mode conversion. The input and output ports of the mode multiplexer are designed in the same planar layer to meet the standard package requirement. We design and fabricate a three-mode mode multiplexer, which serves to spatially combine and separate the E 11 , E 21 and E 12 modes in the few-mode-core outputs without mode conversion with polymer material, where adiabatic tapers are utilized in the design of directional coupler to extend the operation wavelength and two pairs of directional couplers serve to suppress the modal crosstalks. We fabricate a typical device to demonstrate the idea. The fabricated device has a length of 29 mm, which provides coupling ratios higher than 97% and modal crosstalks lower than – 19.6 dB over the C+L band and beyond with weak polarization dependence. The proposed mode multiplexer is fiber capable and allows to connect with the few-mode fibers directly by the use of fan-in and fan-out technology. The proposed mode multiplexer is simple, scalable, and effective to extend the operation wavelength as well as suppressing the modal crosstalks, which provides a powerful device to manipulate the modes in a mode division multiplexing system and an effective method to realize an all-optical chip for simplifying the Multi-Input Multi-Output signal processing with low power consumption.

Ultra-broadband and low-modal-crosstalk mode multiplexer based on cascaded vertical directional couplers formed by adiabatic-tapered waveguides without mode conversion

This article presents a modeling approach and its implementation to study the impact of the top-side diamond integration on the <inline-formula> <tex-math notation="LaTeX">${f}_{\text {T}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${f}_{\text {max}}$ </tex-math></inline-formula> performance of a millimeter-wave (mm-wave) N-polar gallium nitride (GaN) high-electron-mobility transistor (HEMT). This approach uses a co-simulation model formed by an equivalent small-signal circuit model of the device implemented in PathWave advanced design system (ADS) and full-wave simulations of 3-D modeling diamond passivation from Ansys high frequency simulation software (HFSS). Thin-film diamond as a passivation layer and a heat spreader on top of the device channel is explored as a function of the diamond’s dielectric constant and its thickness to understand how it affects the device’s <inline-formula> <tex-math notation="LaTeX">${f}_{\text {T}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${f}_{\text {max}}$ </tex-math></inline-formula>. The simulation results serve as a guide to the optimization of the radio frequency (RF) performance of mm-wave HEMT devices, aiding the device design of the diamond passivation. The designed methodology was applied to other passivation, such as benzocyclobutene (BCB) for benchmarking. This method allowed us to estimate the tradeoff in electrical performance for anticipated thermal benefits. A maximum reduction of 23.6% in <inline-formula> <tex-math notation="LaTeX">${f}_{\text {T}}$ </tex-math></inline-formula> and 21.8% in <inline-formula> <tex-math notation="LaTeX">${f}_{\text {max}}$ </tex-math></inline-formula> was obtained when the diamond passivation thickness is <inline-formula> <tex-math notation="LaTeX">$2.7 \mu \text{m}$ </tex-math></inline-formula> with a dielectric constant of 4.

Impact of Diamond Passivation on fT and fmax of mm-wave N-Polar GaN HEMTs

With the fast development of next-generation wireless communication and radar sensing, the millimeter-wave frequency band is becoming more and more important due to its rich spectrum, wide bandwidth, and ability to support the miniaturization of antennas and circuits <xref ref-type="bibr" rid="ref1">[1]</xref>, <xref ref-type="bibr" rid="ref2">[2]</xref>, <xref ref-type="bibr" rid="ref3">[3]</xref>, <xref ref-type="bibr" rid="ref4">[4]</xref>. High-performance, low-loss, and low-cost millimeter-wave circuits and modules are the key elements for wireless front-end systems. For planar transmission lines, such as microstrip lines and coplanar waveguides, they are very easy to integrate with other circuits for low-cost and low-profile physical designs. However, the high insertion loss and radiation loss from planar transmission lines limit their applications at the millimeter-waveband. As for rectangular and cylindrical waveguides, their main application is in the areas of low loss and high power handling, but their 3D structure is difficult to integrate with other planar passive and active circuits. The substrate-integrated waveguide (SIW) is a better way to merge the advantages of planar transmission lines and rectangular waveguides, but it still suffers from dielectric losses at the millimeter-wave frequency range.

Gap Waveguide Technology: An Overview of Millimeter-Wave Circuits Based on Gap Waveguide Technology Using Different Fabrication Technologies

In this paper, two types of balanced-to-unbalanced quadrature couplers with six hybrid ports and arbitrary power division ratio are proposed. For Type 1, the quadrature coupler can realize two power division functions, including balanced/unbalanced to balanced-unbalanced-hybrid, and for Type 2, the quadrature coupler contains balanced -to-unbalanced and unbalanced-to-balanced power division functions. To reduce the circuit size, double-sided parallel-strip line (DSPSL) swap structure is used to replace the half-wavelength transmission line, and compact circuit size can be easily achieved. To validate the performance of the quadrature couplers, two types of the balanced-to-unbalanced quadrature couplers centered at 1 GHz with power division ratios 1:1 and 3:1 are fabricated and measured, respectively. The common mode suppression higher than 10 dB (bandwidths over than 176%) can be realized, the isolation and in-band matching level can be also improved.

Balanced-to-Unbalanced Quadrature Couplers With Wide-Band Common-Mode Suppression

A dual-band high efficiency power amplifier (PA) at Ka-band (28/39 GHz) by using 0.15 um GaN on SiC process is proposed. The PA utilizes two-stage two-way combined structure with dual-band matching networks which are composed of a series transmission line and impedance buffer element, to fulfil matching conditions at both fundamental and harmonic termination. The proposed dual-band PA achieves the small signal gain of 26/15 dB, 35 dBm saturation output power, 38%/29 % peak power added efficiency (PAE) at 28 and 39 GHz, respectively. The proposed GaN MMIC PA achieves in a small size (1.6 mm* 1.2 mm) while still remaining the comparable performance in other parameters.

Design of Dual-Band 28/39GHz Power Amplifier MMIC in 0.15-μm GaN HEMT Technology

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