José Ángel García

Wireless Power Transmission: R&D Activities Within Europe

Wireless power transmission (WPT) is an emerging technology that is gaining increased visibility in recent years. Efficient WPT circuits, systems and strategies can address a large group of applications spanning from batteryless systems, battery-free sensors, passive RF identification, near-field communications, and many others. WPT is a fundamental enabling technology of the Internet of Things concept, as well as machine-to-machine communications, since it minimizes the use of batteries and eliminates wired power connections. WPT technology brings together RF and dc circuit and system designers with different backgrounds on circuit design, novel materials and applications, and regulatory issues, forming a cross disciplinary team in order to achieve an efficient transmission of power over the air interface. This paper aims to present WPT technology in an integrated way, addressing state-of-the-art and challenges, and to discuss future R&D perspectives summarizing recent activities in Europe.

Characterizing the gate to source nonlinear capacitor role on FET IMD performance

This paper discusses the gate to source nonlinear capacitor contribution on small signal intermodulation distortion (IMD) performance of FET devices. The second and third order coefficients for the Cgs(Vgs) Taylor-series expansion, experimentally extracted with a simplified one-sided version of our previously proposed test set-up, are shown to be responsible for some detected differences on IMD behaviour at high frequencies.

Resistive FET mixer conversion loss and IMD optimization by selective drain bias

This paper describes a dedicated nonlinear MESFET model extraction technique, which was used to accurately characterize the devices channel resistance nonlinearity. Plotting Ids(Vgs,Vds) Taylor series expansion coefficients across V/sub GS/ and V/sub DS/ revealed not only the presence of important minimum conversion loss bias, but also of in-band IMD sweet spots that were then used to optimize a FET resistive mixer performance.

GaN HEMT Class

In this paper, the design and performance of class E2 resonant topologies for dc/dc power conversion at UHFs are considered. Combining the use of RF gallium-nitride HEMT devices, both for the inverter and synchronous rectifier, with high-Q lumped-element terminating networks, peak efficiency values over 70% may be obtained. Control strategies based on carrier bursting, switching frequency modulation, or outphasing are also shown to be feasible. Taking advantage of their improved dynamic response, when compared to low-frequency more traditional switched-mode converters, a class E3 polar transmitter for the EDGE standard has been designed and tested at 770 MHz, offering an average global efficiency over 46% at 4.3 W of output power, through RF-based amplitude and phase constituting branches. Finally, the potential of such a high frequency of operation in terms of power density is explored, absorbing undesired coil parasitics for the original LC series interconnecting network in a 1-GHz design methodology.

{\hbox{E}}^{2}

This paper discusses the gate to source nonlinear capacitor contribution on small signal intermodulation distortion (IMD) performance of FET devices. The second and third order coefficients for the Cgs(Vgs) Taylor-series expansion, experimentally extracted with a simplified one-sided version of our previously proposed test set-up, are shown to be responsible for some detected differences on IMD behaviour at high frequencies.

Resonant Topologies for UHF DC/DC Power Conversion

Spectrally efficient wireless communication standards impose stringent linearity specifications, which would require traditional IQ transmitters to operate with back-offed and power inefficient linear RF power amplifiers (PAs). In order to overcome such a significant limitation, alternative architectures have been proposed, as those based on the envelope elimination and restoration technique. An example of the application of this technique is the polar transmitter. In this paper, a UHF polar transmitter is presented, combining switching and linear stages in the envelope amplifier as to achieve both wide bandwidth and high efficiency, when drain modulating a GaN HEMT Class E RF PA. Several tests, using EDGE, TETRA, and WCDMA standards have been performed with good results.

Characterizing the gate-to-source nonlinear capacitor role on GaAs FET IMD performance

Abstract In this paper we use a generalized radial basis function (GRBF) network to model the intermodulation properties of microwave GaAs MESFET transistors under dynamic operation. The proposed model receives as input the bias voltages of the transistor and provides as output the derivatives of the drain-to-source current, which are responsible for the intermodulation properties. The GRBF network is a generalization of the RBF network, which allows different variances for each dimension of the input space. This modification allows to take advantage of the soft nonlinear dependence of the output derivatives with the drain-to-source bias voltage. The learning algorithm chooses the GRBF centers one by one in order to minimize the output error. After selecting each new center from the training set, the centers and variances of the global network are optimized by applying gradient descent techniques. Finally, the amplitudes are obtained by solving a least-squares problem. The effectiveness of the proposed GRBF model is validated through load-pull intermodulation prediction based on the experimental nonlinear characterization of an NE72084 MESFET device.

A Linear Assisted Switching Envelope Amplifier for a UHF Polar Transmitter

In this work, a continuous nonlinear model for the I/sub ds/ current source in recessed gate pseudomorphic HEMT heterostructures is proposed. A careful characterization of the DC, pulsed, and small-signal nonlinear distortion behaviour of this predominant nonlinearity has been employed in order to extract the model parameters. Being able to reproduce the current-voltage (I-V) behaviour as well as the higher order derivatives of the transconductance and output conductance, the equation is valid for an accurate control of the critical nonlinear distortion phenomena in HEMT applications. Comparisons between measured and simulated results prove its validity under both static and dynamic conditions for either large- or small-signal operation.

A nonlinear MESFET model for intermodulation analysis using a generalized radial basis function network

In this paper, the design of a class E synchronous rectifier, working in the 900 MHz frequency band and based on an Enhancement-mode Pseudomorphic High Electron Mobility Transistor (E-PHEMT), is proposed. Thanks to its small on-state resistance and its slightly positive threshold voltage, this type of device may offer an excellent performance when operated as a switch without biasing its gate terminal. After properly extracting its model parameters, the optimum drain terminations, required for guaranteeing zero-voltage and zero-derivative switching conditions (ZVS and ZDS), were estimated. Taking advantage of the time-reversal duality, a lumped-element class E amplifier was first designed, to then introduce a drain-to-gate feedback and operate it in the desired synchronous rectifying mode. An efficiency peak of 83% has been measured at 17 dBm, staying above 70% for a 14 dB input power range, a distinguishing characteristic when compared to Schottky diode based alternatives. The verified AM-AM conversion linearity would also allow using the rectifier for the efficient extraction of a time-varying excitation envelope without significant distortion.

Accurately modeling the drain to source current in recessed gate P-HEMT devices

This paper presents the design of an adaptive digital predistorter (DPD) for power amplifier (PA) linearization whose implementation and real time adaptation can be fully performed in a field programmable gate array (FPGA). The distinctive characteristic of this adaptive DPD is its straightforward deduction from a nonlinear auto regressive moving average (NARMA) PA model and the possibility to be completely implemented in a FPGA without the need of an additional digital signal processor performing the DPD adaptation. The adaptive DPD presents a NARMA structure that can be implemented by means of look-up tables (LUTs). This configuration results in a Multi-LUT implementation where LUT contents are directly updated by means of an LMS algorithm. Details on the internal adaptive DPD organization as well as its linearization capabilities are provided, taking into account memory effects compensation.