Roberto Quaglia

3–3.6-GHz Wideband GaN Doherty Power Amplifier Exploiting Output Compensation Stages

We discuss the design, realization and experimental characterization of a GaN-based hybrid Doherty power amplifier for wideband operation in the 3-3.6-GHz frequency range. The design adopts a novel, simple approach based on wideband compensator networks. Second-harmonic tuning is exploited for the main amplifier at the upper limit of the frequency band, thus improving gain equalization over the amplifier bandwidth. The realized amplifier is based on a packaged GaN HEMT and shows, at 6 dB of output power back-off, a drain efficiency higher than 38% in the 3-3.6-GHz band, gain around 10 dB, and maximum power between 43 and 44 dBm, with saturated efficiency between 55% and 66%. With respect to the state of the art, we obtain, at a higher frequency, a wideband amplifier with similar performances in terms of bandwidth, output power, and efficiency, through a simpler approach. Moreover, the measured constant maximum output power of 20 W suggests that the power utilization factor of the 10-W (Class A) GaN HEMT is excellent over the amplifier band.

The Doherty Power Amplifier: Review of Recent Solutions and Trends

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.

Offset Lines in Doherty Power Amplifiers: Analytical Demonstration and Design

A theoretical validation of the offset lines method for design of Doherty power amplifiers is presented for the first time. The analysis is carried out considering the simplified unilateral active device model classically adopted in Doherty amplifier theory. In particular, it is proved that, in narrow band conditions, properly designed offset lines preserve the ideal load modulation across the full power sweep range. The demonstration is independent from the Doherty strategy adopted, e.g., in terms of output power back-off, even or uneven architecture. A simple analytical formula to calculate the proper length of the lines is given, and validated through a simulation example.

High-Efficiency 7 GHz Doherty GaN MMIC Power Amplifiers for Microwave Backhaul Radio Links

The potentialities of GaN monolithic technology for the growing microwave backhaul power amplifier market are discussed in this paper. To support this discussion, two GaN monolithic Doherty power amplifiers for 7 GHz backhaul applications are presented. They exhibit 5 W output power, with almost 10 dB gain and high efficiency at 7 dB output power back-off. In particular, one module has been optimized for maximum efficiency at center frequency (47% at 7 dB output power back-off), while the other for high efficiency on a larger bandwidth (15% fractional bandwidth).

K-Band GaAs MMIC Doherty Power Amplifier for Microwave Radio With Optimized Driver

In this paper, a Doherty power amplifier for K-band point-to-point microwave radio, developed in TriQuint GaAs 0.15-μm PWR pHEMT monolithic technology, is presented. Highly efficient driver stages on both the main and auxiliary branches have been designed and optimized to boost gain with minimal impact on power-added efficiency. The selected architecture enables a modular combination to reach higher power levels. Matching network structures have been designed, according to simple equivalent circuit approaches, to obtain the desired 10% fractional bandwidth. The fabricated power amplifier (PA) exhibits, at 24 GHz in continuous-wave conditions, an output power of 30.9 dBm, with a power-added efficiency of 38% at saturation and 20% at 6 dB of output power back-off, together with a gain of 12.5 dB. System-level characterization at 24 GHz, in very demanding conditions, with a 28-MHz channel 7.5-dB peak-to-average ratio modulated signal, showed full compliance with the standard emission mask, adopting a simple predistorter, with average output power of 23.5 dBm, and average efficiency above 14%. The measured performance favorably compare with other academic and commercial K-band PAs for similar applications.

Effect of Load Modulation on Phase Distortion in Doherty Power Amplifiers

The mechanisms leading to higher phase distortion of Doherty power amplifiers in comparison to standard architectures are theoretically investigated and experimentally verified. The analysis focuses on the Main amplifier contribution to AM/PM due to the load modulation, a key feature of the Doherty, and is based on a simplified active device circuit approach. Experimental characterization is carried out by source/load-pull measurements on a 4 W die GaN-HEMT at 7 GHz. Measured results confirm the role of the load modulation on the AM/PM distortion, showing that, even if the device nonlinearities are minimized by properly selecting the bias point, a significant residual AM/PM distortion (10° in the present case) appears when the output load is modulated. The effect of this phase distortion on a 14 MHz-256 QAM signal has been evaluated by system level simulations.

Linear GaN MMIC Combined Power Amplifiers for 7-GHz Microwave Backhaul

This paper presents the design of two combined linear power amplifiers for 7-GHz microwave backhaul, realized in 0.25- μm GaN on SiC monolithic technology. Both modules are based on a combined class-AB structure conceived for maximum back-off efficiency and reduced phase distortion, which are important requirements in backhaul systems. Different second harmonic loads are exploited in the two power amplifiers, leading to different performance in terms of output power, bandwidth and efficiency. The two stages exhibit a saturated output power in excess of 35 and 36 dBm on 16% and 26% of fractional bandwidth, respectively; moreover, the measured average efficiency in the presence of modulated signals with 7.4-dB peak-to-average power ratio is 18% and 25%. Simulations and experimental results demonstrate that the second-harmonic load has little influence on the linearity of the proposed amplifiers. Compliance with the spectrum emission mask defined for the targeted application has been achieved through low-order polynomial digital predistortion, thus demonstrating the high linearity of the stages. A comparison with a Doherty amplifier realized in the same technology and for the same application shows that the two proposed stages need a simpler predistorter to achieve the linearity required by standard specifications.

A K-band GaAs MMIC Doherty power amplifier for point-to-point microwave backhaul applications

This work reports the design of a GaAs monolithic K-band Doherty power amplifier for point-to-point microwave backhaul applications. The design of the module is described, from the choice of the architecture based on power budget and gain requirements, to the analysis of the solutions adopted. The MMIC is expected to achieve 32.5 dBm output power in the 20.8-24 GHz band, PAE higher than 32% at saturation (20% at 6 dB output back-off) and gain higher than 10 dB.

A 22W 65% efficiency GaN Doherty Power Amplifier at 3.5 GHz for WiMAX applications

The design, implementation and characterization of a Doherty Power Amplifier (DPA) for 3.5 GHz WiMAX applications are discussed. The DPA has been implemented using a commercial GaN HEMT from Cree inc., following a class AB and C scheme for the main and peak module, respectively. The measured maximum power of the DPA is 22W with a first peak efficiency of 57%, and maximum drain efficiency of 65% at the DPA saturation. Efficiency over the so-called Doherty region (where both the main and the peak amplifiers operate) does not drop below 55% from saturation to 6 dB input back-off. The gain at the onset of the Doherty region is 8 dB, with around 1 dB roll-off.

GaN-MMIC Doherty power amplifier with integrated reconfigurable input network for microwave backhaul applications

A 7 GHz GaN MMIC Doherty power amplifier with reconfigurable input network for backhaul applications, is presented. The reconfigurable solution is conceived to be simply implemented in MMIC technologies. To demonstrate the effectiveness of the strategy, a second Doherty with fixed input network, has been designed for comparison. Both prototypes have been realized adopting a commercial GaN HEMT process and characterized in large signal conditions. The experimental results prove the capability of the proposed solution to overcome the issues related to the inaccuracy of the active device nonlinear models especially for the Class-C bias condition required by the Peak amplifier. At 7 GHz both MMICs exhibit an output power of 40 dBm. However, at 6 dB of output power backoff, the reconfigurable Doherty efficiency is 43%, 10% higher than the one of the DPA with fixed input network.