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

On the design of high-efficiency RF Doherty power amplifiers

Power amplifiers (PAs) are one of the most crucial elements in wireless standards becasue they are the most power hungry subsystems. These elements have to face an important issue, which is the power efficiency, a fact related with the output back-off (OBO). But the OBO depends on the kind of modulated signal, in proportion to the modulated signal peak-to-average power ratio (PAPR). The higuer is the data rate, the higer is the OBO, and consequently the lower is the efficiency. A low efficiency of PAs causes the waste of energy as heat. Furthermore, the trade-off between linearity and efficiency in PAs is another major issue. To cope with the undesired circumstances producing efficiency degradation, the Doherty power amplifier (DPA) is one of the useful techniques which provide high efficiency for high PAPR of modern communication signals. Nevertheless, the limited bandwidth (BW) of this kind of PAs (about 10% of fractional bandwidth) and its importance (in modern wireless systems such as LTE, WiMAX, Wi-Fi and satellite systems) have encouraged the researchers to improve this drawback in recent years. Some typical BW limiting factors effect on the performance of DPAs: i) quarter-wave length transformers, ii) phase compensation networks in/output matching circuits, iii) offset lines and device non-idealities; The quarter-wave length transformers performs as an inverter impedance in the load modulation technique of DPAs. The future objective in designing DPAs is to decrease the impact of these issues. In this context, this PhD-thesis is focused on improving fractional bandwidth of DPAs using the new methods that are related to impedance transformers instead of impedance inverters in the load modulation technique. This study is twofold. First, it is presented a novel DPA where a wideband GaN DPA in the 2.5 GHz band with an asymmetrical Wilkinson splitter. The impedance transformer of the proposed architecture is based on a matching network including a tapered line with multi-section transformer in the main stage. The BW of this DPA has ranged from 1.8 to 2.7 GHz. Plus, the obtained power efficiency (drain) is higher than 33% in the whole BW at both maximum and OBO power levels. Second, based on the benefits of the Klopfenstein taper, a promising DPA design is proposed where a Klopfenstein taper replaces the tapered line. In fact, this substitution results on reducing the reflection coefficient of the transformer. From a practical prototype realization of this novel Doherty-like PA in the 2.25 GHz band, this modification has demonstrated that the resulting DPA BW is increased in comparison to the conventional topology while keeping the efficiency figures. Moreover, this study also shows that the Klopfenstein taper based design allows an easy tuning of the group delay through the output reactance of the taper, resulting in a more straightforward adjustments than other recently published designs where the quarter-wave transformer is replaced by multi-section transmission lines (hybrid or similar). Experimental results have shown 43-54% of drain efficiency at 42 dBm output power, in the range of 1.7 to 2.75 GHz. Concretely, the results presented in this novel Doherty-like PA implies an specific load modulation technique that uses the mixed Klopfenstein tapered line together with a multi-section transformer in order to obtain high bandwidth with the usual efficiency in DPAs.Los amplificadores de potencia (PAs) son uno de los elementos más importantes para los transmisores inalámbricos desde el punto de vista del consumo energético. Un aspecto muy importante es su eficiencia energética, un concepto relacionado con el back-off de salida (OBO), que a su vez viene condicionadpo por el PAPR de la señal modulada a amplificar. Una baja eficiencia de los PA hace que la pérdida de energía se manifieste en forma de calor. De hecho, esta cuestión conduce al incremento de los costes y tamaño, esto último por los radiadores. Además, el compromiso entre la linealidad y la eficiencia en los PA es otro problema importante. Para hacer frente a las circunstancias que producen la degradación de la eficiencia, el amplificador de potencia tipo Doherty (DPA) es una de las técnicas más útiles que proporcionan una buena eficiencia incluso para los altos PAPR comunes en señales de comunicación modernos. Sin embargo, el limitado ancho de banda (BW) de este tipo de PA (alrededor del 10% del ancho de banda fraccional) y su importancia (en los sistemas inalámbricos modernos, tales como LTE, WiMAX, Wi-Fi y sistemas de satélites) han animado a los investigadores para mejorar este inconveniente en los últimos años. Algunos aspectos típicos que limitan el BW en los DPA son: i) transformadores de longitud de cuarto de onda, ii) redes de compensación de fase y circuitos de adaptación de salida, iii) compensación de las líneas y los dispositivos no ideales. Los transformadores de cuarto de onda actuan como un inversor de impedancia en la técnica de modulación de carga de la DPA "("load modulation"). Concretamente, el objetivo futuro de diseño de DPA es disminuir el impacto de estos problemas. En este contexto, esta tesis doctoral se centra en mejorar el ancho de banda fraccional de DPA utilizando los nuevos métodos que están relacionados con el uso de transformadores de impedancias en vez de inversores en el subcircuito de modulación de carga. Este estudio tiene dos niveles. En primer lugar, se presenta una novedosa estructura del DPA de banda ancha usándose dispositivos de GaN en la banda de 2,5 GHz con un divisor Wilkinson asimétrico. El transformador de impedancias de la arquitectura propuesta se basa en una red de adaptación, incluyendo una línea cónica con múltiples secciones del transformador en la etapa principal. El BW de este DPA ha sido de 1,8 a 2,7 GHz. Además, se obtiene una eficiencia de drenador de más del 33% en todo el BW, tanto a nivel de potencia máxima como a nivel del OBO. En segundo lugar, aprovechando los beneficios de un adaptador de Klopfenstein, se propone un nuevo diseño del DPA. Con la sustitución de la lina conica por el Klopfenstein se reduce el coeficiente de reflexión de transformador de impedancias. Sobre un prototipo práctico de esta nueva estructura del Doherty, en la banda de 2,25 GHz, se ha demostrado que el BW resultante se incrementa en comparación con la topología convencional mientras se mantienen las cifras de eficiencia. Por otra parte, en este estudio se demuestra que el diseño basado en el Klopfenstein permite una afinación fácil del retardo de grupo a través de la reactancia de salida del taper, lo que resulta en un ajuste más sencillo que otros diseños publicados recientemente en el que el transformador de cuarto de onda se sustituye por multi-líneas de transmisión de la sección (híbridos o similar). Los resultados experimentales han mostrado un 43-54% de eficiencia de drenador sobre 42 dBm de potencia de salida, en el intervalo de 1,7 a 2,75 GHz. Concretamente, los resultados presentados en esta nueva estructura tipo-Doherty implican una técnica de modulación de carga que utiliza una combinación de un Klopfenstein junto con un transformador de múltiples secciones con el fin de obtener un alto ancho de banda con la eficiencia habitual en DPAs.Els amplificadors de potència (PA) són un dels elements més importants per els sistemes ràdio ja que sone ls principals consumidors d'energía. Un aspecte molt important és l'eficiència de l'amplificador, aspecte relacionat amb el back-off de sortida (OBO) que a la seva vegada ve condicionat pel PAPR del senyal modulat. Una baixa eficiència dels PA fa que la pèrdua d'energia en manifesti en forma de calor. De fet, aquesta qüestió porta a l'increment dels costos i grandària, degut als dissipadors de calor. A més, el compromís entre la linealitat i l'eficiència en els PA es un altre problema important. Per fer front a les circumstàncies que porten a la degradació de l'eficiència, l'amplificador de potència Doherty (DPA) és una de les tècniques més útils i que proporcionen una bona eficiència per als alts PAPR comuns en senyals de comunicació moderns. No obstant això, l'ample de banda limitat (BW) d'aquest tipus de PA (al voltant del 10% de l'ample de banda fraccional) i la seva importància (en els sistemes moderns, com ara LTE, WiMAX, Wi-Fi i sistemes de satèl·lits) han animat els investigadors per millorar aquest inconvenient en els últims anys. Alguns aspectes tipicament limitadors del BW en els DPA son: i) transformadors de longitud d'quart d'ona, ii) xarxes de compensació de fase en circuits / adaptacions de sortida, iii) compensació de les línies i els dispositius no ideals. Els transformadors de quart d'ona s'utilitzen com a inversors d'impedàncies en la tècnica de modulació de càrrega del DPA ("load modulation"). Concretament, l'objectiu futur de disseny d'DPA és disminuir l'impacte d'aquests problemes. En aquest context, aquesta tesi doctoral es centra en millorar l'ample de banda fraccional dels DPA utilitzant nous mètodes que estan relacionats amb l'ús de transformadors d'impedàncies, en comptes d'inversors, en el subcircuit de modulació de càrrega. Aquest treball té dos nivells. En primer lloc, es presenta un DPA novedós que fa servir dispositus GaN DPA a la banda de 2,5 GHz amb un divisor Wilkinson asimètric. El transformador d'impedàncies de l'arquitectura proposada es basa en una xarxa d'adaptació, incloent una línia cònica amb múltiples seccions del transformador en l'etapa principal. El BW d'aquest DPA ha mostrat ser d'1,8 a a 2,7 GHz. A més, s'obté una eficiència de drenador de més del 33% en tot el BW, tant a nivell de potència màxima com de OBO. En segon lloc, sobre la base dels beneficis del adaptador de Klopfenstein, un proposa un nou disseny on un Klopfenstein substitueix la anterior línia cònica. Aquesta substitució repercuteix en la reducció del coeficient de reflexió de transformador d'impedàncies.Des d'una realització pràctica (prototipus) d'aquest nou amplificador tipus Doherty a la banda de 2,25 GHz, s'ha demostrat que el BW resultant s'incrementa en comparació amb la topologia convencional mentre es mantenen les xifres d'eficiència. D'altra banda, en aquest estudi es demostra que el disseny basat en el Klopfenstein permet una afinació fàcil del retard de grup a través de la reactància de sortida de la forma cònica, el que resulta en un ajust més senzill que altres dissenys publicats recentment en què el transformador de quart d'ona es substitueix per multi-línies de transmissió de la secció (híbrids o similar). Els resultats experimentals han mostrat un 43-54% d'eficiència de drenador en 42 dBm de potència de sortida, en l'interval de 1,7-2,75 GHz. Concretament, els resultats presentats en aquest nou amplificador tipus Doherty impliquen una tècnica de modulació de càrrega específic que utilitza una combinació del Klopfenstein juntament amb un transformador de múltiples seccions per tal d'obtenir un alt ample de banda amb la usual eficiència en DPAs.Postprint (published version

A survey on RF and microwave doherty power amplifier for mobile handset applications

This survey addresses the cutting-edge load modulation microwave and radio frequency power amplifiers for next-generation wireless communication standards. The basic operational principle of the Doherty amplifier and its defective behavior that has been originated by transistor characteristics will be presented. Moreover, advance design architectures for enhancing the Doherty power amplifier’s performance in terms of higher efficiency and wider bandwidth characteristics, as well as the compact design techniques of Doherty amplifier that meets the requirements of legacy 5G handset applications, will be discussed.Agencia Estatal de Investigación | Ref. TEC2017-88242-C3-2-RFundação para a Ciência e a Tecnologia | Ref. UIDP/50008/201

Development of High-Efficiency Switch-Mode Concurrent Dual-Band RF Power Amplifiers

For the past ten years, we’ve seen the rapid development of wireless communication along with the number of frequency bands to be supported in a wireless device. RF power amplifiers (PAs), as the last and most important stage in a transmitter, then have to support the increasing number of frequency bands and operation modes. The solution that has been used in industry is simply to increase the number of PAs with each of them covers several adjacent frequency bands. It might be feasible from 2G to 4G since the frequency range is confined within low GHz (\u3c3 GHz), however, as 5G comes, not only the number of frequency band will keep increasing, but the frequency range will expand to much higher ranges (~6 GHz, 30 GHz, 60 GHz, etc.). Higher frequency range will need much more PAs and thus make the traditional solution impractical due to constrained cost and area. In addition, carrier aggregation technique used in 4G and future 5G requires additional filters (diplexer/triplexer) to combines different single-band PAs which will introduce extra power loss. Multi-band PAs that are able to operate at several frequency bands (not adjacent to each other) simultaneous could potentially reduce the number of PAs and filters thus making the increasingly complicate RF front end feasible in terms of area and cost with reduced power loss. Such PAs are defined as concurrent multi-band PAs. Unfortunately, traditional multi-band PAs were designed for operate one band at a time and thus experienced significant efficiency and output power drop when operate concurrently. A few concurrent dual-band PAs were designed in recent years targeting concurrent operation. However, the drop in both efficiency and output power were still too much to make those PAs useful in actual applications. The performance of existing concurrent dual-band PAs are mainly limited by their linear-type topology. In this thesis, a switch-mode concurrent dual-band PAs was developed for the first time which, as expected, could achieve higher efficiency with minimum drop in both efficiency and output power. A concurrent dual-band current-switching class-D PA was proposed in this thesis, and developed from fundamental theories, design methodology, to actual implementation and finally measurement results. The theoretical analysis showed that, the proposed PA could provide a concurrent-mode (two carriers simultaneously) drain efficiency of 87% at 6 dB over drive which was only 5% lower than single-mode operation (one carrier at a time). A concurrent dual-band class-B PA (one of linear-type PAs) on the other hand only have a maximum concurrent-mode drain efficiency 62.5%, 16% lower than single-mode case. The output power drop was also reduced from 3 dB in linear-type PAs to 1.2 dB in the proposed PA. The design of the proposed PA however was complicated due to a large number of harmonics and intermodulation components (IMs) to be properly terminated at the output. To reduce the design complexity, the tradeoff between number of harmonics/IMs to be properly terminated and efficiency was discussed based on ADS simulation. It was found out that, the 2nd harmonics and IM2 were critical to maintain high efficiency, 3rd harmonics and IM3 however had smaller effect on efficiency thus can be neglected (partially) to greatly reduce design complexity with tolerable efficiency degradation. The bias effect was also explored and was suggested that the PA should be bias into triode (defined in Chapter 4) or in other words, bias above class A, in order to achieve high efficiency. The proposed PA was implemented in a push-pull structure which need a balun at both output and input. The design equations of balun were derived in this thesis together with some parameter optimization for minimum imbalance and largest bandwidth. The output balun provides not only differential to single-ended conversion but also a 1:4 impedance transformation. The final PA was fabricated and measured in lab. A drain efficiency of 60% was achieve when operating concurrently at 880 MHz and 1.49 GHz with less than 0.5 dB output power drop compared single-mode operation. The performance was among the best concurrent dual-band PAs. Measurement results together with simulation results show that the proposed PA has the ability to achieve much higher efficiency than linear-type concurrent dual-band PAs with minimum efficiency and output power drop, and thus is capable to make increasingly complicated RF FEM feasibl

0.5 GHz-1.5 GHz Bandwidth 10W GaN HEMT RF Power Amplifier Design

With the current development in wireless communication technology, the need for a wide bandwith in RF power amplifier (RF PA) is an essential. In this paper, the design and simulation of 10W GaN HEMT wideband RF PA will be presented. The Source-Pull and Load-Pull technique was used to design the input and output matching network of the RF PA. From the simulation, the RF PA achieved a flat gain between 15dB to 17dB from 0.5GHz to 1.5GHz. At 1.5GHz, the drain efficiency is simulated to achieve 36% at the output power of 40 dBm while the power added efficiency (PAE) was found to be 28.2%

Optimisation of a Doherty power amplifier based on dual-input characterisation

The success of the Doherty architecture compared to other efficiency enhancement techniques derives mainly from its simple design and full-RF nature, not requiring complex digital signal processing to achieve high back-off efficiency. In this work we propose a design strategy for the optimisation of a Doherty power amplifier to mitigate the typical practical issues of this architecture related to inaccuracy of the non-linear model and of the manufacturing. The approach is based on the experimental characterisation of a dual-input Doherty prototype without input section. This test structure is obtained from a single-input Doherty amplifier, designed only through non-linear simulations, by removing the input section and allowing for separate control of the two RF inputs. From the collected data, approximated functions for the phase shift and power splitting versus frequency are identified to be realizable in hardware with RF networks. Compared to the reference single-input Doherty stage, a significantly improved behavior is registered in terms of output power (up to 2.7 dB), efficiency at saturation and back-off (30 % and 15 % respectively) and power gain (2 dB)