20 research outputs found
Microwave class-E power amplifiers: a brief review of essential concepts in high-frequency class-E PAs and related circuits
Since Nathan Sokal's invention of the class-E power amplifier (PA), the vast majority of class-E results have been reported at kilohertz and millihertz frequencies, but the concept is increasingly applied in the ultrahigh-frequency (UHF) [1]-[13], microwave [14]-[20], and even millimeter-wave range [21]. The goal of this article is to briefly review some interesting concepts concerning high-frequency class-E PAs and related circuits. (The article on page 26 of this issue, "A History of Switching-Mode Class-E Techniques" by Andrei Grebennikov and Frederick H. Raab, provides a historical overview of class-E amplifier development.)We acknowledge support, in part, by a Lockheed Martin Endowed Chair at the University of Colorado and in part by the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO) through TEC2014-58341-C4-1-R and TEC2017-83343-C4-1-R projects, cofunded with FEDER
Caracterização, modelação e compensação de efeitos de memória lenta em amplificadores de potência baseados em GAN HEMTS
Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have
emerged as the most compelling technology for the transmission of highpower
radio-frequency (RF) signals for cellular mobile communications and
radar applications. However, despite their remarkable power capabilities, the
deployment of GaN HEMT-based RF power amplifiers (PAs) in the mobile
communications infrastructure is often ruled out in favor of alternative siliconbased
technologies. One of the main reasons for this is the pervasiveness of
nonlinear long-term memory effects in GaN HEMT technology caused by thermal
and charge-trapping phenomena. While these effects can be compensated
for using sophisticated digital predistortion algorithms, their implementation
and model-extraction complexity—as well as the power necessary for
their real-time execution—make them unsuitable for modern small cells and
large-scale multiple-input multiple-output transceivers, where the power necessary
for the linearization of each amplification element is of great concern.
In order to address these issues and further the deployment of high-powerdensity
high-efficiency GaN HEMT-based RF PAs in next-generation communications
and radar applications, in this thesis we propose novel methods for
the characterization, modeling, and compensation of long-term memory effects
in GaN HEMT-based RF PAs. More specifically, we propose a method
for the characterization of the dynamic self-biasing behavior of GaN HEMTbased
RF PAs; multiple behavioral models of charge trapping and their implementation
as analog electronic circuits for the accurate real-time prediction
of the dynamic variation of the threshold voltage of GaN HEMTs; a method
for the compensation of the pulse-to-pulse instability of GaN HEMT-based
RF PAs for radar applications; and a hybrid analog/digital scheme for the
linearization of GaN HEMT-based RF PAs for next-generation communications
applications.Os transístores de alta mobilidade eletrónica de nitreto de gálio (GaN HEMTs)
são considerados a tecnologia mais atrativa para a transmissão de sinais
de radiofrequência de alta potência para comunicações móveis celulares e
aplicações de radar. No entanto, apesar das suas notáveis capacidades de
transmissão de potência, a utilização de amplificadores de potência (PAs)
baseados em GaN HEMTs é frequentemente desconsiderada em favor de
tecnologias alternativas baseadas em transístores de silício. Uma das principais
razões disto acontecer é a existência pervasiva na tecnologia GaN
HEMT de efeitos de memória lenta causados por fenómenos térmicos e de
captura eletrónica. Apesar destes efeitos poderem ser compensados através
de algoritmos sofisticados de predistorção digital, estes algoritmos não são
adequados para transmissores modernos de células pequenas e interfaces
massivas de múltipla entrada e múltipla saída devido à sua complexidade
de implementação e extração de modelo, assim como a elevada potência
necessária para a sua execução em tempo real. De forma a promover a
utilização de PAs de alta densidade de potência e elevada eficiência baseados
em GaN HEMTs em aplicações de comunicação e radar de nova geração,
nesta tese propomos novos métodos de caracterização, modelação,
e compensação de efeitos de memória lenta em PAs baseados em GaN
HEMTs. Mais especificamente, nesta tese propomos um método de caracterização
do comportamento dinâmico de autopolarização de PAs baseados
em GaN HEMTs; vários modelos comportamentais de fenómenos de captura
eletrónica e a sua implementação como circuitos eletrónicos analógicos para
a previsão em tempo real da variação dinâmica da tensão de limiar de condução
de GaN HEMTs; um método de compensação da instabilidade entre
pulsos de PAs baseados em GaN HEMTs para aplicações de radar; e um
esquema híbrido analógico/digital de linearização de PAs baseados em GaN
HEMTs para comunicações de nova geração.Programa Doutoral em Telecomunicaçõe
O impacto dos efeitos da memória de longo termo na linearizabilidade de amplificadores de potência baseados em AlGaN/GaN HEMT
AlGaN/GaN High Electron Mobility Transistor (HEMT)s are among the
preferred options for radio-frequency power amplification in cellular base
station transmitters and radar applications. However, despite their promising
outlook, the pervasiveness of trapping effects makes them resilient to
conventional digital predistortion schemes, which not only decrease their
current range of applications but could also preclude their integration in
future small cells and multiple-input multiple-output architectures where
simpler predistortion schemes are mandatory. So, this PhD thesis aims
at developing a meaningful link between the device physics and the linearizability
of the AlGaN/GaN HEMT-based Power Amplifier (PA). In order
to bridge this gap, this thesis begins with a clear explanation for the
mechanisms governing the dominant source of trapping effects in standard
AlGaN/GaN HEMTs, namely buffer traps. Based on this knowledge, we
explain why the best known physically-supported trapping models, used
to represent these devices, are insufficient and present a possible improvement
to what we consider to be the most accurate model, supported by
Technology Computer-Aided Design (TCAD) simulations. This has also
been corroborated through a novel double-pulse technique able to describe
experimentally both the capture and emission transients in a wide temporal
span under guaranteed isothermal conditions. The measured stretched
capture transients validated our understanding of the process while the temperature
dependence of the emission profiles confirmed buffer traps as the
dominant source of trapping effects. Finally, through both simulations and
experimental results, we elaborate here the relationship between the emission
time constant and the achievable linearity of GaN HEMT-based PAs,
showing that the worst-case scenario happens when the emission time constant
is on the order of the time between consecutive envelope peaks above
a certain amplitude threshold. This is the case in which we observed a more
pronounced hysteresis on the gain and phase-shift characteristics, and so,
a stronger impact of the memory effects. The main outcome of this thesis
suggests that the biggest linearizability concern in standard AlGaN/GaN
HEMT-based PAs lies on the large emission time constants of buffer traps.AlGaN/GaN HEMTs estão entre as opções preferidas para amplificação
de potência de radiofrequência em transmissores de estacão base celular
e aplicações de radar. No entanto, apesar de sua perspetiva promissora,
a influência dos efeitos de defeitos com níveis profundos torna-os imunes
aos esquemas convencionais de pre-distorção digital. Assim, esta tese de
doutoramento visa desenvolver uma ligação significativa entre a física do
dispositivo e a linearização de amplificadores de potência baseados em Al-
GaN/GaN HEMTs. Por forma a preencher esta lacuna, esta tese começa
com uma explicação clara dos mecanismos que governam a fonte dominante
de efeitos de defeitos com níveis profundos em AlGaN/GaN HEMTs standard,
especificamente defeitos no buffer. Com base neste conhecimento,
são aparentadas as falhas dos modelos físicos mais conhecidos de defeitos
de nível profundo usados para representar estes dispositivos, assim como
uma possível melhoria suportada em simulações de TCAD. Isto é também
corroborado por uma nova técnica de duplo-pulso capaz de descrever experimentalmente os transientes de captura e emissão num amplo intervalo
temporal sob condições isotérmicas. Os transientes de captura medidos
validam a nossa compreensão do processo, enquanto que a dependência da
temperatura nos perfis de emissão confirmou os defeitos no buffer como
a fonte dominante de efeitos de defeitos com níveis profundos. Por fim,
através de simulações e resultados experimentais, elabora-se aqui a relação
entre a constante de tempo de emissão e a linearizabilidade dos amplificadores
baseados em AlGaN/GaN HEMT, mostrando que o pior cenário
acontece quando a constante de tempo de emissão é da mesma ordem do
tempo entre picos consecutivos da envolvente acima de um certo limiar
de amplitude. Este é o caso para o qual se observa uma histerese mais
pronunciada nas características de ganho e fase e, consequentemente, um
impacto mais forte dos efeitos de memória. O resultado principal desta tese
sugere que a maior preocupação na linearização de amplificadores baseados
em AlGaN/GaN HEMTs standard está nas grandes constantes de tempo de
emissão dos defeitos no buffer.Programa Doutoral em Engenharia Eletrotécnic
Design and Characterization of Power Converters and Amplifiers for Supply-Modulation based Transmitter Architectures
The rapid evolution of telecommunication systems has strongly influenced our lives, and the way we communicate and exchange information. Nevertheless, much progress is expected to happen in the next years with the introduction of new generations of wireless communications standards, which require signals with large bandwidth and very high Peak-to-Average Power Ratio (PAPR) in order to enhance the spectral efficiency and maximize the data rate. However, such developments can only take place through the evolution of Radio-Frequency (RF) which should be capable of working at higher frequencies, higher bandwidth and with higher efficiencies than before. In order to meet these demanding specifications, transmitter architectures have to evolve from a single linear RF Power-Amplifier (PA) into more complex architectures. Envelope Tracking (ET) is one of the most promising solutions for the efficiency-enhancement of next generation transmitters. The research described in this thesis aims to provide solutions to enhance the efficiency of the RF PA by means of an ET architecture. To this purpose, a novel discrete level supply modulator is investigated, which is based on a direct digital-to-analog power conversion. This supply modulator is capable of synthesizing eight voltage steps by means of three isolated voltage sources, thus behaving like a Power Digital-to-Analog Converter (Power-DAC). A hybrid version of the Power-DAC exploiting very fast GaN devices is developed and tested with an L-band PA achieving efficiency improvement up to 13% with 10 MHz of bandwidth. Furthermore, a monolithic GaN version of the Power-DAC is prototyped and tested with an X-band PA achieving efficiency improvement up to 20% and bandwidth of 20 MHz. This supply modulator is tested with outphasing PAs showing promising results with modulated signals and efficiency improvement up to 9%. Finally, dispersive phenomena, which affect PAs and switches in supply modulators, are investigated, characterized and modeled
Supply modulated GaN HEMT power amplifiers - From transistor to system
Power amplifiers (PAs) for mobile communication applications are required to fulfil stringent requirements concerning linearity while keeping a high efficiency over a wide power range and bandwidth. To
achieve this, a number of advanced PA topologies have been developed, mostly based on either load modulation, such as Doherty PAs or load modulation balanced PAs, or on supply modulation such
as envelope tracking or envelope elimination and restoration. Supply modulation has an advantage
over other topologies as the power range of high efficiency can be realised over arbitrary bandwidths,
only limited by the bandwidth of the PA. This does, however, come at the cost of a significantly
more complicated voltage supply. Instead of a static supply voltage, the PA needs to be provided
with one which is rapidly changing, requiring a supply modulator capable of powering the PA while
modulating its supply voltage. This thesis investigates a number of challenges in supply modulated
power amplifiers, ranging from the transistor itself to circuit design and system level considerations
and focusses on power levels up to 10 W and frequencies between 1 GHz and 4 GHz.
Transistors, as the centre-piece of a PA, determine how well the PA reacts to a changing supply
voltage. In this work, the traits that make GaN HEMTs suitable for supply modulated PAs were
investigated, and gain variation with changing supply voltage was established as an important parameter. This gain variation is described in detail and its impacts on PA performance are discussed. By
comparing transistors in literature, gain variation has been demonstrated to be a prevalent characteristic in transistors with GaN HEMTs showing a very wide range of gain variation. Using a small-signal
model based on measurements, the voltage dependent behaviour of the feedback capacitance CGD is,
for the first time, identified as the origin of small-signal gain variation. This is traced down to the
gate field plate which is commonly used to combat surface trapping effects in GaN HEMTs. With
this in mind, two different ways of changing the transistor geometry to reduce the impact of gain
variation and thus optimise the transistor for operation in supply modulated PAs are discussed and
demonstrated using a 250 nm GaN HEMT.
As a result of the non-linearity of the feedback and gate-source capacitances, the input impedance
of GaN HEMTs changes with supply voltage and drive power. This prevents the transistor from being
matched at all supply voltages and input powers and introduces phase distortion. Using simulation and
measurement, the impact of input impedance on linearity and efficiency of supply modulated power
amplifiers is demonstrated on a 2.9 GHz 10 W PA. Careful selection of the input impedance allows
improvement of AM/PM distortion of a supply modulated PA with little cost in terms of AM/AM
and PAE.
I
Supply modulators have a significant impact on efficiency and linearity of the ET system. One
supply modulator topology with the potential to generate a supply voltage with a high modulation
bandwidth is the RF modulator in which the input DC voltage is turned into an RF signal and
rectified, resulting in an output voltage which depends on the excitation of the PA. While PAs are
well understood in every detail, there are gaps in the understanding of RF rectifiers. Using active
load-pull/source-pull measurements, intrinsic gate and drain waveforms of a GaN HEMT operated as
a rectifier are demonstrated for the first time. This allows in-detail evaluation of the impact of the
gate termination in self-synchronous rectifiers. It also allows detailed analysis of the loss mechanisms
in rectifiers and formulation of the required impedances to realise efficient self-synchronous operation,
resulting in efficiencies exceeding 90% over wide power ranges. Using waveform engineering, a new type
of RF modulator, with potentially very high bandwidths, based on even harmonic generation/injection
is proposed. The necessary operating conditions of the rectifier part of the modulator are emulated
using an active load-pull/source-pull system to successfully demonstrate that the rectifier behaves
as predicted. Using a simple demonstrator, preliminary measurements were conducted and the RF
modulator was shown to work, reaching efficiencies up to 78%.
As PA and supply modulator are combined, they present impedances to each other. These
impedances have a significant impact on the behaviour of both sub-systems. A simple way to characterise both the impedance presented to the PA by the modulator and the impedance presented to the
modulator by the PA is described. Using a state-of-the-art modulator, these impedances are measured,
the modulator impedance is demonstrated to be close to the simulated value. These measurements
also demonstrate that the impedances change significantly with the operating conditions
Recommended from our members
Assessment of Outphasing Power Amplifiers and Their MMIC Implementations
Power amplifier (PA) applications consistently require increased performance for bandwidth, power, and efficiency. Modern communication and telemetry systems are required to operate across multiple bands that can span octaves while scaling down unit power amplifier cells to fit on the back-end of phased array systems. Phased array implementation scales branch complexity for both signal generation and linearization. This creates problems with conventional approaches for back-off efficiency enhancement techniques as well as increasing the importance of linearity optimization. This reduces the complexity of the digital pre-distortion (DPD) required to operate the system while maintaining high average efficiency for large peak to average power ratio (PAPR) signals.
This thesis presents advanced outphasing PA architectures. Each of the architectures answers a different design challenge for modern communication and telemetry systems. Analog signal separation reduces the need for a digital signal split removing cost, size, weight, and power (C-SWaP). Improved modeling and understanding of the design of the inphasing splitter allows for optimization of linearity and efficiency. This work looks at utilizing novel power combining implementations with reconfigurable matching networks to operate outphasing PA’s across multiple bands spanning an octave allowing for functional flexibility andreduced system complexity.
The thesis works on two distinct outphasing implementations: the RF-input outphasing power amplifier (RFIO PA) and the dual-input reconfigurable outphasing power amplifier (DIRO PA). The designs presented in this work are as follows:
• Hybrid X-band RFIO PA utilizing a previously designed dual-input X-band outphasing monolithic microwave integrated circuit (MMIC) in an unreleased Qorvo GaN 150 nm process
• Hybrid X-band RFIO PA utilizing three different non-linear elements (PIN diode, diode-connected transistor, and mesa resistor) in the signal splitter design to optimize for linearity and efficiency in conjunction with a custom designed dual-input outphasing PA MMIC in Qorvo 150 nm GaN es process
• MMIC RFIO PA utilizing diode-connected transistors for integrated signal splitter design in Qorvo 150 nm GaN es process
• MMIC 6-12 GHz reconfigurable outphasing combiner with a switchable shunt line to control reactive loading in Qorvo’s GaAs 250 nm process
• MMIC DIRO PA which is reconfigurable from 18-38 GHz with >2 GHz of instantaneous bandwidth in the WIN semiconductor PIH-110 process</p
The auxiliary envelope tracking RF power amplifier system
The advancement of the mobile communication industry increases the need for RF power amplifier (RFPA) to be more efficient and linear. The communication network that is shifting towards smaller micro-cell or nano-cell network has also motivated the design of the RF power amplifier to be simple, compact and cost efficient. In this research work, a novel technique for efficiency and linearity improvement of the RFPA is presented. A simplistic approach in the technique called ‘Auxiliary Envelope Tracking' (AET) system has promoted the design for small and straightforward AET tracking generator, a key component in the system. The use of low cost components in the AET tracking generator has made the technique commercially attractive. The AET technique proposes a separation in generating DC and AC components of the AET signal that biases the drain of the RFPA. The separation eases the generation of the signals resulting in low power consumption that leads to efficiency improvement. The investigation of the gain characteristic of gallium nitride (GaN) RFPA has shown an important RFPA attribute where the gain varied substantially as the drain voltage increases. By using the AET technique, the gain characteristic is harnessed to get linearity improvement. In order to validate the technique, AET measurement systems for two-carrier and WCDMA signals were developed and experimented. A special Class AB RFPA is designed and implemented to use dedicatedly for this investigation. In two-carrier signal measurement, a tracking generator is developed that consists of an envelope amplifier (EA) and a diplexer. The RFPA and the tracking generator are then combined to be an integrated AET block. In order to accommodate the high peak-toaverage ratio (PAR) and high bandwidth WCDMA signal, a broadband RF transformer was designed as part of the AET tracking generator to replace the diplexer. The two-carrier and WCDMA signals measurement results have proven that the AET technique is a valid technique for efficiency and linearity improvement. The improvements were achieved with simple, compact and cost-effective implementation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
The auxiliary envelope tracking RF power amplifier system
The advancement of the mobile communication industry increases the need
for RF power amplifier (RFPA) to be more efficient and linear. The communication
network that is shifting towards smaller micro-cell or nano-cell network has also
motivated the design of the RF power amplifier to be simple, compact and cost
efficient. In this research work, a novel technique for efficiency and linearity
improvement of the RFPA is presented. A simplistic approach in the technique called
‘Auxiliary Envelope Tracking' (AET) system has promoted the design for small and
straightforward AET tracking generator, a key component in the system. The use of
low cost components in the AET tracking generator has made the technique
commercially attractive.
The AET technique proposes a separation in generating DC and AC
components of the AET signal that biases the drain of the RFPA. The separation
eases the generation of the signals resulting in low power consumption that leads to
efficiency improvement. The investigation of the gain characteristic of gallium
nitride (GaN) RFPA has shown an important RFPA attribute where the gain varied
substantially as the drain voltage increases. By using the AET technique, the gain
characteristic is harnessed to get linearity improvement.
In order to validate the technique, AET measurement systems for two-carrier
and WCDMA signals were developed and experimented. A special Class AB RFPA
is designed and implemented to use dedicatedly for this investigation. In two-carrier
signal measurement, a tracking generator is developed that consists of an envelope
amplifier (EA) and a diplexer. The RFPA and the tracking generator are then
combined to be an integrated AET block. In order to accommodate the high peak-toaverage
ratio (PAR) and high bandwidth WCDMA signal, a broadband RF
transformer was designed as part of the AET tracking generator to replace the
diplexer.
The two-carrier and WCDMA signals measurement results have proven that
the AET technique is a valid technique for efficiency and linearity improvement. The
improvements were achieved with simple, compact and cost-effective
implementation
The design of a multilevel envelope tracking amplifier based on a multiphase buck converter
Envelope Tracking (ET) and Envelope Elimination and Restoration (EER) are techniques that have gained in importance in the last decade in order to obtain highly efficient Radio Frequency Power Amplifier (RFPA) that transmits signals with high Peak to Average Power Ratio (PAPR). In this work a multilevel multiphase buck converter is presented as a solution for the envelope amplifier used in ET and EER. The presented multiphase buck converter generates multilevel voltage using “node” duty cycles and non-linear control. In this way the multilevel is implemented using only one simple power stage. However, the complexity of the multilevel converter implementation has been shifted from complicated power topologies to complicated digital control. Detailed discussion regarding the influence of the design parameters (switching frequency, output filter, time resolution of the digital control) on the performance of the proposed envelope amplifier is presented. The design of the output filter is conducted fulfilling the constraints of the envelope slew rate and minimum driver pulse that can be reproduced. In the cases when these two constraints cannot be fulfilled, they may be relieved by the modified control that is presented and experimentally validated. Finally, in order to validate the concept, a prototype has been designed and integrated with a nonlinear class F amplifier. Efficiency measurements showed that by employing EER it is possible to save up to 15% of power losses, comparing to the case when it is supplied by a constant voltage. Additionally, Adjacent Channel Power Ratio (ACPR) has been measured. The obtained results showed the value higher than 30dB for signals up to 5 MHz of bandwidth, without using predistortion technique