Advanced High Efficiency and Broadband Power Amplifiers Based on GaN HEMT for Wireless Applications

In advanced wireless communication systems, a rapid increase in the mobile data traffic and broad information bandwidth requirement can lead to the use of complex spectrally efficient modulation schemes such as orthogonal frequency-division multiplexing (OFDM). Generally, complex non-constant envelope modulated signals have very high peak-to-average ratios (PAPR). Doherty Power Amplifier (DPA) is the most commonly used power amplifier (PA) architecture for meeting high efficiency requirement in advanced communication systems, in the presence of high PAPR signals. However, limited bandwidth of the conventional DPA is often identified as a bottleneck for widespread deployment in base-station application for multi-standard communication signals. The research in this thesis focuses on the development of new designs to overcome the bandwidth limitations of a conventional PA. In particular, the bandwidth limitation factors of a conventional DPA architecture are studied. Moreover, a novel design technique is proposed for DPA’s bandwidth extension. In the first PA design, limited bandwidth and linearity problems are addressed simultaneously. For this purpose, a new Class-AB PA with extended bandwidth and improved linearity is presented for LTE 5 W pico-cell base-station over a frequency range of 1.9–2.5 GHz. A two-tone load/source-pull and bias point optimization techniques are used to extract the sweet spots for optimum efficiency and linearity from the 6 W Cree GaN HEMT device for the whole frequency band. The realized prototype presented saturated PAE higher than 60%, a power gain of 13 dB and an average output power of 36.5 dBm over the desired bandwidth. The proposed PA is also characterized by QAM-256 and LTE input communication signals for linearity characterization. Measured ACPRs are lower than -40 dBc for an input power of 17 dBm. The documented results indicate that the proposed Class-AB architecture is suitable for pico-cell base-station application. In the second PA design, an inherent bandwidth limitation of Class-F power amplifier forced by the improper load harmonics terminations at multiple harmonics is investigated and analyzed. It is demonstrated that the impedance tuning of the second and third harmonics at the drain terminal of a transistor is crucial to achieve a broadband performance. The effect of harmonics terminations on power amplifier’s bandwidth up to fourth harmonics is investigated. The implemented broadband Class-F PA achieved maximum saturated drain efficiency 60-77%, and 10 W output power throughout (1.1-2.1 GHz) band. The simulated and measured results verify that the presented Class-F PA is suitable for a high-efficiency system application in wireless communications over a wide range of frequencies. In the third PA design, a single- and dual-input DPA for LTE application in the 3.5 GHz frequency band are presented and compared. The main goal of this study is to improve the performance of gallium–nitride (GaN) Doherty transmitters over a wide bandwidth in the 3.5 GHz frequency band. For this purpose, the linearity-efficiency trade-off for the two proposed architectures is discussed in detail. Simulated results demonstrate that the single- and dual-input DPA exhibited a peak drain efficiency (DE) of 72.4% and 77%, respectively. Both the circuits showed saturated output power more than 42.9 dBm throughout the designed band. Saturated efficiency, gain and bandwidth of dual-input DPA are higher than that of the single-input DPA. On the other side, dual-input DPA linearity is worse as compared to the single-input DPA. In the last PA design, a novel design methodology for ultra-wide band DPA is presented. The bandwidth limitation factors of the conventional Doherty amplifier are discussed on the ground of broadband matching with impedance variation. To extend the DPA bandwidth, three different methods are used such as post-matching, low impedance transformation ratio and the optimization of offset line for wide bandwidth in the proposed design. The proposed Doherty power amplifier was designed and realized based on two 10 W GaN HEMT devices from Cree Inc. The measured results exhibited 42-57% of efficiency at the 6-dB back-off and saturated output power ranges from 41.5 to 43.1 dBm in the frequency range of 1.15 to 2.35 GHz (68.5% fractional bandwidth). Moreover, less than -25 dBc ACPRs are measured at 42 dBm peak output power throughout the designed band. In a nutshell, all power amplifiers presented in this thesis are suitable for wideband operation and their performances are satisfying the required operational standard. Therefore, this thesis has a significant contribution in the domain of high efficiency and broadband power amplifiers

GaN HEMT based Class-F Power Amplifier with Broad Bandwidth and High Efficiency

This paper presents the design and realization of a highly efficient broadband class-F power amplifier (PA) with a multi-harmonic controlled output network. Optimum performance in terms of bandwidth and efficiency is targeted over the frequency band 1.1 - 2.1 GHz. The design is developed in the Keysight Advanced Design System (ADS) environment and verified experimentally through small- and large-signal characterization. The optimum load and source impedances are determined by performing load-pull and source-pull simulations. The output matching network is designed including harmonic resonators up to the fourth harmonic. In order to achieve broadband operation, the load impedances at harmonics are optimized. The realized PA exhibits state-of-the-art performance, with a power gain of 10-15 dB, a saturated drain efficiency of 60-73% and 10 W output power throughout the selected frequency band (1.1-2.1 GHz). Experimental results show remarkably good agreement with the simulation results

A 5W Class-AB Power Amplifier Based on a GaN HEMT for LTE Communication Band

In this paper, the design and characterization of a GaN HEMT based class-AB power amplifier (PA) operating in LTE communication band (1.9 - 2.5 GHz) are presented. A model-based design procedure is adopted. Source and load impedances for optimum linearity, power and efficiency are extracted through source- and load-pull simulation. The implemented PA shows excellent performance on the whole frequency band from both the efficiency and linearity points of view. It provides a saturated output power of 36.9 dBm and a gain of 19 dB at the design frequency of 2.3 GHz, over a 600 MHz bandwidth. Measurements report a saturated power-added efficiency (PAE) of 68 % and a PAE at 1dB compression of 54.3 %. System level characterization with a 7 MHz-bandwidth QAM-256 input signal is also carried out on the fabricated PA

Hot-electron reliability improvement using perhydropolysilazane spin-on-dielectric passivation buffer layers for AlGaN/GaN HEMTs

We investigate the effects of perhydropolysilazane spin-on-dielectric (SOD) buffer layer adopted prior to Si3N4 passivation on the dc drain current level and degradation after the electrical stress in the AlGaNGaN high electron mobility transistors (HEMTs). The SOD-buffered HEMTs show ~1.6 times greater drain current densities (~257 mA/mm) than those of the devices with conventional-Si3N4 passivations (~155 mA/mm). After the hot electron stresses (step-wise and constant) applied to the devices, it is also found that the SOD-buffered structure produces greatly improved device reliability in terms of the dc current collapse (15% for step-stress and constant stress) compared to the conventional structure (25% for each case). We propose that the enhancement of SOD-buffered structure in dc current collapse is due to the reduction in surface state density at the passivation interface and the suppressed electron trapping

A miniaturized wilkinson power divider for ultra wide-band operation

This paper presents a new miniaturized multi-section Wilkinson power divider based on coupled-line for ultra-wideband operation. The proposed divider is composed of three-sections. An even- and odd-mode analysis is used to calculate the characteristics impedances at the designed frequency, and the performance is optimized for ultra-wideband operation from 0.9 to 4.1 GHz. The realized power divider has return loss lower than -10 dB and insertion loss better than -3.8 dB for the frequency band 0.9-4.1 GHz. Moreover, it occupies more than 60% smaller circuit area compared to conventional ring structure Wilkinson power divider. The measurement results show the good agreement with simulated ones