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A generalized dual-input digital predistortion (DPD) methodology for supply-modulated (SM) power amplifiers (PAs) is proposed. Contrary to classical approaches, the SM and the radio frequency (RF) signals are treated as separate digital inputs to be jointly predistorted. A multi-objective optimization (MOO) strategy allows to explore the inherent PA performance tradeoffs to jointly optimize the DPD coefficients for improved power-added efficiency (PAE) and higher RF output power, yet guaranteeing a prescribed linearity performance. To avoid dealing with an unbearably high number of experimental acquisitions, MOO is here made feasible by fast simulation of an empirical surrogate model of the PA, which is progressively refined from a reduced set of iterative acquisitions. The proposed technique enables the adoption of a dynamic supply shaping function, and it automatically accounts for the signals' statistics. Eventually, the method outperforms classical SM approaches, yet using a DPD of the same order, as demonstrated by the experimental results on a gallium nitride (GaN) SM PA operating at 3.5 GHz in the presence of orthogonal-frequency-division-multiplexing (OFDM)-like high-peak-to-average power ratio (PAPR) modulated signals with 10- and 20-MHz bandwidths (BWs).

This article deals with the characterization of charge trapping dynamics in a novel 100-nm double-heterojunction AlN/GaN/AlGaN-on-Si radio frequency (RF) HEMT process. In order to study the detrapping mechanisms, we perform the wideband acquisitions of the transient behavior by sweeping the pulsed voltages to cover the entire device operating area. The fast acquisition also enables the characterization of the charge capture behavior, a key aspect for RF performance. From the analysis of the drain current transients, time constants are extracted, showing a fundamental release time constant in the order of 0.1-1 ms, and more than one capture constants, the fastest being in the order of 300 ns. To the best of authors' knowledge, this is the first time that trapping dynamics under large-signal regime are characterized for this type of process.

In this paper, we present a high-efficiency transmitter based on an integrated circuit (IC) supply modulator implemented in the same 0.15- $\mu \text{m}$ gallium nitride (GaN)-on-SiC RF process as the power amplifier (PA) monolithic microwave IC. The X-band 10-W two-stage PA is designed for stable operation with minimal drain capacitance, which enables fast supply modulation. The multilevel supply modulator provides eight voltage levels with 3-bit digital control [(power digital-to-analog converter (pDAC)], achieving a state-of-the-art slew rate of 5 kV/ $\mu \text{s}$ . Characterization of the dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ of the GaN switches allows the development of an efficiency model for the pDAC and an investigation of the effects of the pDAC internal resistance on the PA performance, resulting in a comprehensive efficiency model for the supply-modulated PA. The flexible compact transmitter consisting of the PA and pDAC ICs shows high efficiency in backoff for a variety of signals, both for radar and communications. Measured results for amplitude- and frequency-modulated radar pulses show a composite power-added efficiency (CPAE) of 44% with a peak power of 10 W at 9.57 GHz, with simultaneous spectral confinement and 52-dB improvement of the first time sidelobe. For a 20-MHz high peak-to-average ratio LTE signal, the CPAE increases from 11% to 32% compared to a fixed supply voltage transmitter, while linearity under dynamic supply operation is maintained through digital predistortion.

This paper describes the performance degradation of RF GaN-on-SiC HEMT switches due to the charge trapping, which is triggered by high voltages under operating regimes. A custom measurement setup is used for the characterization of the switching behavior under dynamic control and blocking voltages. It is shown that both small- [i.e., insertion loss (IL)] and large-signal (LS) performances (i.e., switch compression) are affected by traps. Depending on the applied voltages, an increase of the switch IL up to 50% and significant degradation of the switch compression characteristic were measured for a $4 \times 75\,\,\mu \text{m}^{2}$ RF switch in 0.25- $\mu \text{m}$ GaN-on-SiC technology. These mechanisms cannot be observed with conventional static characterization, and they are not described by standard RF switch models. A device model capable to account for the observed characteristics is identified and empirically validated under LS conditions at 10 GHz.

This paper presents a dynamic characterization of a multilevel (ML) Chireix outphasing (ML-CO) power amplifier (PA) with modulated signals. The ML-CO technique combines the advantages of envelope tracking and outphasing architectures by limiting the supply modulation to discrete levels with an efficient power-DAC modulator and using outphasing for fine amplitude control. We describe an experimental test bench that supplies the required phase- and time-aligned modulated signals for outphasing and supply modulation simultaneously. Pulsed characterization is used to design an ML memoryless polynomial DPD. The linearized ML-CO GaN X-band MMIC PA is demonstrated with fixed input drive levels for 9.3-dB peak-to-average power ratio (PAPR), 1.4-MHz and 11.3-dB PAPR, 10-MHz LTE signals with a 9.7-GHz carrier. For both signals, the average total power consumption is reduced by a factor of two when supply modulation is used. An investigation of control signal generation strategies is presented, including combinations of discrete supply and continuous input modulation. A simple exponential function tracks the optimal efficiency trajectory of the ML-CO PA, enabling overall efficiency improvements within 9 and 27 percentage points. This control strategy is applied to radar pulses with amplitude shaping and frequency chirp, with the efficiency improvement of up to 11.8 percentage points compared with constant-supply operation, with 29-dB improvement in spectral confinement.

A pulsed characterization of gallium nitride (GaN) high-electron mobility transistors (HEMTs) under a controlled trap and thermal state is performed to evaluate the trapping-induced degradation in the actual operating conditions, corresponding to different classes of power amplifiers (PAs). Two state-of-the-art GaN-on-SiC technologies are evaluated: 0.15- ${\mu }\text{m}$ Qorvo and 0.25- ${\mu }\text{m}$ Wolfspeed HEMTs, with examples relative to class-AB, class-E, and supply modulated operation. It is shown that, in order to get an accurate device characterization, the measurements have to be performed by preconditioning the trap state consistently with the intended application.

This paper presents a supply modulated $X$ -band 12-W peak power transmitter that maintains an average efficiency greater than 50% for various shapes of amplitude-modulated pulses. The main power amplifier is a two-stage GaN-on-SiC MMIC with a peak efficiency of 65%, while the pulse envelope modulator is a 95% efficient hybrid 3-b power DAC implemented with GaN-on-Si transistor switches. Envelope shaping of a pulsed waveform results in improved spectral confinement of greater than 15 dB for the first sideband compared with constant-envelope pulses, with over 20 points improvement in total efficiency. The combination of supply modulation and digital predistortion is shown to result in high composite (total) efficiency of over 55%, with simultaneous high dynamic range and with flexible digitally programmable pulse shaping.

In this paper, we describe a rigorous theoretical approach to the circuit-level nonlinear design of an entire inductive resonant wireless power transfer (IR-WPT) system, including the transmitter and receiver nonlinear subsystems. Starting from a novel analytical characterization of the inductive resonant link, the system efficiency is parametrically computed as a function of a set of circuital parameters, including the power levels to be transferred. These quantities are then used as design goals inside the nonlinear optimization of the transmitter and receiver blocks. By adopting the last generation miniaturized enhanced-mode AlGaN/GaN-power field-effect transistor and fast Schottky diodes, a Class-D amplifier and a full-bridge rectifier followed by a switching dc–dc Buck converter that acts as load impedance transformer are designed in a single optimization process at 6.78 MHz. Thus, the transmitter and the receiver are directly connected by the IR two-port network, and the system is capable to adapt to variable distances between the resonators of the IR-WPT link. The choice of the Class-D topology for the transmitter and the adaptability of the active receiver enable to get rid of inter-stage matching networks, which can severely reduce the overall efficiency, especially in high power transfer environments. With the proposed IR-WPT system, up to 44 W of transferred power and a peak of 73% dc-to-dc efficiency were obtained with an input dc voltage V = 30 V at a link distance d = 5 cm. Numerical and experimental results are discussed, demonstrating the accuracy of the proposed design procedure.

Two C-band monolithic high power amplifiers (HPAs) have been designed and implemented exploiting a 0.25-μm AlGaN/GaN HEMT process on an SiC substrate. The circuits have been designed for use in transmit/recevie modules of satellite synthetic aperture radar antennas for Earth observation. The design was accurately focused on the HEMTs' electrical and thermal working conditions in order to guarantee the reliable operation required by space applications. The HPAs operate in pulsed conditions with typical pulsewidth of 50 μs and 10% duty cycle: in that regime, the circuits deliver about 40 W with more than 21-dB associated gain and 40% to 45% power-added efficiency in the 5-5.8-GHz band. The achieved performance clearly demonstrates the very high potentiality of this technology for the replacement of GaAs-based HPAs in new generations of this type of systems.

In this paper a new active bias network (ABN) for the technology- independent characterization of low-frequency (LF) dispersion in the output impedance and transadmittance of high-power microwave transistors is described. The proposed bias network is capable of synthesizing a high-impedance active DC-feed in the range of 10 Hz-1 MHz, where III-V microwave devices typically exhibit frequency response dispersion due to energy traps and/or self-heating. The input impedance values obtained in such a large bandwidth (five decades) are considerably higher than those that can be achieved with passive resistive and inductive solutions. In fact, these lead to severe limitations in terms of achievable impedance values, calibration accuracy, power handling capabilities, and physical dimensions. The ABN is particularly suitable for the characterization of high-voltage and high-current devices. In particular, here it is used, along with standard laboratory instrumentation, for the characterization of the LF dispersion of an AlGaN/GaN HEMT, suitable for microwave power amplifier applications. © 1963-2012 IEEE.

This paper presents an envelope tracking (ET) transmitter architecture based on the combination of a novel 3-bit $(N = 3)$ supply modulator and digital predistortion (DPD). The proposed power converter is based on a direct digital-to-analog conversion architecture that implements the binary-coded sum of $N$ isolated dc voltages, allowing the synthesis of an output waveform with $L = 2^{N}$ voltage levels, with a binary distribution in the range $\Delta V = V_{M} - V_{O}$ (maximum voltage $V_{M}$ , offset voltage $Vo$ ). This solution provides a better voltage resolution $V_{S} = \Delta V/(2^{N}-1)$ with respect to typical multilevel switched-sources topologies $(V_{S} = \Delta V/N)$ . The improved voltage resolution enables the correction of the residual discretization error in the ET transmitter by means of DPD of the RF signal without the need of an auxiliary linear envelope amplifier. The proposed ET solution has been tested with an L-band 30-W lateral-diffused MOS RF high power amplifier (RF HPA) with 1.4- and 10-MHz long-term-evolution signals. In these conditions the converter demonstrated 92% and 83% efficiency, respectively, whereas the congregate efficiency of the transmitter are 38.3% and 23.9% at 5.5 and 1.9 W of average RF output power, respectively. These performances correspond to an improvement of 17.2 and 17.9 points for the power-added efficiency of the RF HPA and to 13.4 and 13 points of improvement for the efficiency of the entire transmitter with respect to fixed bias operation.

A technology-independent, inherently nonlinear approach is proposed for the compact modelling of high-frequency noise in microwave transistors under large-signal operating conditions. For the compact nonlinear noise model formulation we assume that noise generation can be described by a suitable set of distributed stochastic processes perturbing a very general, non-quasi-static deterministic description of the electron device, in accordance with the strategies adopted in physics-based methods for the choice and exploitation of microscopic diffusion noise sources. The propagation of the internal distributed noise sources up to the intrinsic device terminals leads to a set of non-stationary, correlated equivalent noise generators, nonlinearly controlled by the instantaneous large-signal working point of the device. Starting from a first formulation for the generators, formally derived from a physics-based description of the noise generation mechanisms widely adopted in distributed numerical modeling, mild approximations provide a fully behavioral representation that can be empirically extracted on the basis of measurement data only, and can be easily implemented into commercial computer-aided design tools by means of conventional, uncorrelated noise sources. As far as small-signal (i.e., linear) bias-dependent operation is concerned, it is shown how well-known, widely applied compact models for high-frequency noise can be considered as linearized special cases of the proposed approach. For a full validation, experimental examples are provided, both in small-and large-signal operation, for a GaAs-pHEMT, by considering the case study of a broad-band low-noise amplifier progressively driven into nonlinear regime by an increasing power interferer. © 2014 IEEE.

A state-space empirical nonlinear model for GaN-based field-effect transistors (FETs) is defined, along with the associated identification procedures based on a recently published double pulse measurement technique. Charge trapping phenomena are dealt with in terms of a nonlinear state equation, which describes the rate of change of the trap state as a function of its actual distance from the corresponding steady state. Model experimental validation is carried out, after on-wafer characterization of a 1-mm AlGaN-GaN on SiC FET, both under strong and mild nonlinear operation.

A laboratory setup, along with a set of measurement and identification procedures, have been developed expressly for the characterization of the thermal behavior of AlGaN/GaN HEMTs, suitable for microwave high power amplifier (HPA) design. The setup allows the measurement of the drain current time-domain dynamic response to positive drain bias pulses, performed at different temperatures and different dissipated power densities. The proposed measurement conditions discriminate thermal phenomena from electrical dispersive effects for this particular technology. Both the thermal resistance and the 'transient thermal resistance' are identified for a single-cell 1-mm device and for a 4-mm power-bar composed of four devices, designed to be used as the final stage of a monolithic $C$-band HPA for pulsed radar application. Transient data allow to compute the device operative channel temperature as a function of the pulsewidth and duty cycle, which is a crucial feature for pulsed HPA applications, typical for the GaN technology. The measured thermal data point out the nonlinearity of the thermal resistance versus dissipated power and base-plate temperature and the consequent critical thermal issue inherent in physically packing together such devices. © 1963-2012 IEEE.

The chip-set for the transmitting power lineup of satellite SAR antenna T/R modules has been designed and implemented exploiting a 2- μm GaInP-GaAs heterojunction bipolar transistor (HBT) technology suitable for space applications. The HBT technology features an integrated emitter ballast resistor that enables high-power density operation without suffering thermal runaway phenomena. Two monolithic microwave integrated circuit (MMIC) driver amplifiers and a MMIC HPA are described: the drivers exhibit small-signal gains exceeding 21 dB and P1 dB output power of about 28 and 29 dBm, respectively, in a 2-GHz bandwidth and CW condition. The HPA delivers more than 40-dBm power at about 2.5-dB gain compression and power-added efficiency (PAE) exceeding 36% in a 700-MHz bandwidth in pulsed operation. Its peak performance at the center of the band are 40.9-dBm output power and 45% PAE. These performance are obtained within tight de-rating conditions for space applications.

In this paper, a new cyclostationary nonlinear low-frequency (LF) noise model for field-effect microwave transistors is presented based on the general theory of the technology-independent Charge-Controlled Nonlinear Noise modeling approach. The model definition, experimental extraction, and validation are described. For model parameter identification, the characterization of the device in terms of both LF noise in quiescent operation and its up-conversion into phase noise under large-signal RF oscillating conditions was performed using in-house developed measurement setups. The model is exploited in the design of a C-band monolithic microwave integrated circuit (MMIC) voltage-controlled oscillator (VCO) developed for space applications. The selected technology is a space-qualified GaAs 0.25-μm pseudomorphic HEMT (pHEMT) process. The large-signal technique adopted for the VCO design is also highlighted. Comparisons between measurements and simulations are provided, which show the validity of the design methodology and demonstrate the accuracy of the proposed cyclostationary noise modeling approach for phase-noise large-signal analyses of pHEMT-based circuits. The MMIC exhibits 350-MHz bandwidth at 7.3 GHz, with 14-dBm output power and -86-dBc/Hz single-sideband phase noise at 100 kHz from the carrier.

This paper addresses both experimentally and through simulations the effects of a microwave tone on the lowfrequency noise properties of GaInP/GaAs heterojunction bipolar transistors. The aim is to contribute to the still unsolved controversy on which modulation strategy should be adopted for the low-frequency noise sources when they should face large signal conditions, as in the cases of oscillators and mixers. An approach similar but not equal to the modulated stationary noise model has been adopted for three different kinds of noise source modulation strategy in the frame of a bias-dependent compact low-frequency noise model.