In this paper, an extensive review of the most up-to-date papers on microwave Doherty power amplifiers is presented. The main applications are discussed, together with the employed semiconductor technologies. The different research trends, all aimed to improve the advantages of the Doherty scheme and to solve its inherent drawbacks, are presented. The first considered topic is the maximization of efficiency and/or linearity, where analog and digital techniques are exploited. Another important trend is the bandwidth enlargement of the Doherty architecture, that involves a large number of papers. Multi-band, multi-mode solutions are also considered, using either fixed or reconfigurable solutions. The final section is dedicated to the most significant Doherty integrated implementations.

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The Doherty Power Amplifier: Review of Recent Solutions and Trends

A wideband, pattern-reconfigurable antenna is reported that is, for example, a good candidate for ceiling-mounted indoor wireless systems. Switchable linearly polarized broadside and conical radiation patterns are achieved by systematically integrating a wideband low-profile monopolar patch antenna with a wideband L-probe fed patch antenna. The monopolar patch acts as the ground for the L-probe fed patch, which is fed with a coaxial cable that replaces one shorting via of the monopolar patch to avoid deterioration of the conical-beam pattern. A simple switching feed network facilitates the pattern reconfigurability. A prototype was fabricated and tested. The measured results confirm the predicted wideband radiation performance. The operational impedance bandwidth, i.e., |S
 11
| ≤ -10 dB, is obtained as the overlap of the bands associated with both pattern modalities. It is wide, from 2.25 to 2.85 GHz (23.5%). Switchable broadside and conical radiation patterns are observed across this entire operating bandwidth. The peak measured gain was 8.2 dBi for the broadside mode and 6.9 dBi for the conical mode. The overall profile of this antenna is 0.13λ 0 at its lowest operating frequency.

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Wideband Pattern-Reconfigurable Antenna with Switchable Broadside and Conical Beams

This paper presents a high-efficiency gallium nitride Doherty power amplifier (DPA) using an integrated compensating reactance (CR) for broadband operation. With an additional quarter-wavelength transmission line integrated in the peaking amplifier output, a CR is generated to compensate the load impedance of the carrier amplifier in the low-power region and thus enhance the back-off efficiency over a wide frequency range without affecting the Doherty load modulation at saturation. For this purpose, a peaking output matching network (OMN) is employed to convert the output impedance of the peaking device into quasi-short circuit when it is off and achieve proper impedance matching when it is on. A two-point matching technique using the transmission ( $ABCD$ ) matrix is employed to design such desired OMN. Measurement results show that the DPA has a 6-dB back-off efficiency of 50%–55% and a saturated efficiency of 57%–71% over the frequency band of 1.7–2.8 GHz (49% fractional bandwidth). When driven by a 20-MHz long term evolution modulated signal at 6.5-dB back-off power, the DPA can achieve an average efficiency of more than 50% with high linearity after linearization over the design frequency band.

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A Broadband High-Efficiency Doherty Power Amplifier With Integrated Compensating Reactance

This paper presents an $8\times 8$ -element slot array antenna with single-layered corporate-feed based on the ridge gap waveguide (RGW) technology in the 60 GHz band. As is well known, a corporate-feed slot array antenna usually has backed cavities to increase the bandwidth and provides a space for its distribution network, and therefore three layers in total: one layer for radiating slots and two layers for feed network with one layer of back cavities and the other one of power dividers. The antenna in this paper is designed by utilizing only two separate metallic layers–a corporate-feed network layer and a radiating slot layer. Compared with the conventional three-layered slot array antennas, the proposed antenna avoids the utilization of the backed cavity layer so that its complexity and manufacture cost decrease. In order to solve the problem of the narrow bandwidth caused by taking away the backed cavities, we utilize double-ridged radiating slots instead of the conventional rectangular ones. A compact transition power divider from standard waveguide WR-15 to the RGW is introduced to excite the proposed array antenna. The $8\times 8$ -element slot array antenna has been fabricated by computerized numerical control machining technique. The measured results demonstrate that the −10 dB reflection coefficient has around 17% bandwidth covering 56.5–67 GHz frequency range, and the measured gain is better than 26 dBi with more than 70% antenna efficiency over 58–66 GHz.

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A Slot Array Antenna With Single-Layered Corporate-Feed Based on Ridge Gap Waveguide in the 60 GHz Band

GaN technology is not only gaining traction in power and RF electronics but is also rapidly expanding into other application areas including digital and quantum computing electronics. This paper provides a glimpse of future GaN device technologies and advanced modeling approaches that can push the boundaries of these applications in terms of performance and reliability. While GaN power devices have recently been commercialized in the 15–900 V classes, new GaN devices are greatly desirable to explore both higher-voltage and ultra-low-voltage power applications. Moving into the RF domain, ultra-high frequency GaN devices are being used to implement digitized power amplifier circuits, and further advances using the hardware–software co-design approach can be expected. On the horizon is the GaN CMOS technology, a key missing piece to realize the full-GaN platform with integrated digital, power, and RF electronics technologies. Although currently a challenge, high-performance p-type GaN technology will be crucial to realize high-performance GaN CMOS circuits. Due to its excellent transport characteristics and ability to generate free carriers via polarization doping, GaN is expected to be an important technology for ultra-low temperature and quantum computing electronics. Finally, given the increasing cost of hardware prototyping of new devices and circuits, the use of high-fidelity device models and data-driven modeling approaches for technology-circuit co-design are projected to be the trends of the future. In this regard, physically inspired, mathematically robust, less computationally taxing, and predictive modeling approaches are indispensable. With all these and future efforts, we envision GaN to become the next Si for electronics.

Emerging GaN technologies for power, RF, digital, and quantum computing applications: Recent advances and prospects

The postmatching topology is an effective approach for broadening the bandwidth of a Doherty power amplifier (DPA). Its efficiency can be enhanced using a second-harmonic short-circuit network (SHSN), but at the expense of bandwidth. In this paper, the SHSNs with mutual coupling are proposed to achieve efficiency enhancement without sacrificing bandwidth. A broadband Doherty amplifier was designed and fabricated based on commercially available gallium nitride HEMT (Cree CGH 40006P) devices to validate the proposed technique. Under continuous-wave excitation, the measured results of the proposed DPA demonstrated that a wide bandwidth of 40% (1.8–2.7 GHz) can be maintained, with the 6-dB back-off drain efficiency between 47.5% and 54%. Using a wideband code-division multiple access 3GPP test signal with the peak-to-average power ratio of 6.6 dB, the adjacent channel power ratio over the entire frequency band is below −30 and −25 dBc at a 6-dB back-off and saturation, respectively.

Broadband Efficiency-Enhanced Mutually Coupled Harmonic Postmatching Doherty Power Amplifier

A novel beam-steering reconfigurable substrate integrated waveguide (SIW) horn antenna is presented. By employing p-i-n diodes as tuning mechanisms, the states of copper posts of the SIW horn can be electronically controlled. Accordingly, the aperture direction of the horn can be switched, and a 360° beam-scanning ability is realized. A pair of bowl-shaped reflectors is introduced for achieving a wide impedance bandwidth and good directional radiation patterns. A fully functional prototype with a 26.2% impedance bandwidth is developed and tested, demonstrating the antenna with a measured gain varying between 8 and 10 dBi and a radiation efficiency of approximately 75% over the impedance band. Moreover, owning to its symmetrical structure, the proposed antenna is capable of steering the radiation beam every 22.5° with identical radiation patterns in the H-plane.

360° Beam-Steering Reconfigurable Wideband Substrate Integrated Waveguide Horn Antenna

This letter presents a novel harmonic control circuit (HCC) for class-F power amplifiers (PAs). A compact microstrip resonant cell (CMRC) is inserted between the transistor and the output fundamental matching network. Third harmonic open circuit condition and size reduction are achieved simultaneously. This scheme provides a straightforward and simple approach to design HCC. A class-F PA is designed and fabricated for demonstration. The measured maximum power-added efficiency (PAE) is 74% with 10 dB power gain near the 1 dB compression point.

A Class-F Power Amplifier With CMRC

This letter introduces a broadband and compact output combiner for Doherty power amplifier (DPA). A grounded quarter Wavelength $(\lambda/4)$ line is added to the output of the peaking amplifier, which helps to compensate for the bandwidth limiting effect that happens in the conventional combiner. Frequency response analysis indicates that stable impedance is achieved across a wide band for the carrier PA. For demonstration, a DPA prototype based on the proposed structure is implemented using two 10 W GaN HEMT transistors. At 6 dB back-off power, over 42% drain efficiency is obtained from 1.5 GHz to 2.5 GHz, accounting for 50% fractional bandwidth. In addition, a minimum of 55% drain efficiency is maintained at saturation in the same frequency band.

A Bandwidth Enhanced Doherty Power Amplifier With a Compact Output Combiner

This letter presents a novel high efficiency power amplifier (PA) with good bandpass responses. By designing a bandpass filter (BPF) with different load impedances to meet the requirement of PA, good bandpass responses can be exhibited. Compared with the classical cascaded PA and filter, the total size of the proposed design is greatly reduced. Meanwhile, the bandwidth is extended. Two transmission zeros are generated at the passband edges, resulting in high selectivity. For demonstration, a filter-integrated PA prototype is implemented. Both theoretical analysis and measured results are presented for validation.

Shichang Chen

Papers

Broadband Efficiency-Enhanced Mutually Coupled Harmonic Postmatching Doherty Power Amplifier

360° Beam-Steering Reconfigurable Wideband Substrate Integrated Waveguide Horn Antenna

A Class-F Power Amplifier With CMRC

A Bandwidth Enhanced Doherty Power Amplifier With a Compact Output Combiner

Compact Power Amplifier With Bandpass Response and High Efficiency