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 ΔV = VM - VO (maximum voltage VM, offset voltage Vo). This solution provides a better voltage resolution VS = ΔV/(2 ^N -1) with respect to typical multilevel switched-sources topologies (VS = ΔV/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.

This paper presents a broadband efficient power amplifier (PA) targeting sub-6-GHz 5G base station applications. Due to the demanding requirements in both peak-to-average power ratio (PAPR) and bandwidth in 5G systems, we employ a combination of both load and supply modulation for efficiency enhancement. Active matching, implemented using an RF-input load-modulated balanced amplifier (LMBA) architecture, enables efficient octave-bandwidth operation. Supply modulation, which is carrier frequency agnostic, is then used to further extend the back-off efficiency. This paper focuses on a study of supply modulation strategies for the load-modulated PA using an efficient GaN eight-level discrete supply modulator. To overcome the bandwidth limitations associated with discrete-level switching, a commutation rate reduction (CRR) filter is applied in digital baseband and its effects are analyzed theoretically and experimentally. The supply-modulated LMBA is characterized across 1.8-3.8 GHz with 100-MHz, 10-dB PAPR signals. An output power of 34 dBm with average composite (total) PAE ranging from 22.4% to 43.9% across the band is demonstrated, with an ACLR of about -50 dBc after digital predistortion.

GaN power switches provide remarkable performance in terms of power-density, reduced parasitics, and high-thermal handling capability that enable the realization of very efficient and compact dc/dc converters. Despite exhibiting state-of-the-art channel conductivity, GaN high electron mobility transistor (HEMT) devices are affected by the degradation of the dynamic ON-Resistance (R _ON ) at increasing OFF-state voltages and operative temperatures. In this paper, a novel laboratory setup and characterization procedure for the dynamic R _ON of GaN HEMT switches in the presence of thermal- and trapping-effects is presented. The proposed setup allows the study of R _ON transients after the switching event at variable off-state voltages and temperatures. The use of custom-designed differential amplification stages and a voltage-controlled current source enables the accurate characterization of R _ON even on large periphery devices. At first, the proposed setup is tested with a well-established and mature device technology such as a Si MOSFET. Degradation of the R _ON up to 120% due to temperature variation is observed with the presented setup. The setup is then used for the characterization of commercial-grade GaN-on-SiC and GaN-on-Si HEMTs. For both technologies dynamic R _ON degradations up to 75% and 20% are observed for temperature and OFF-state voltage variations, respectively. These characterization data are fundamental for the accurate estimation of conduction losses during the design of switching-mode power converters.

The asymmetry between capture and release time constants associated with charge-trapping phenomena observed in the electrical characteristics of microwave gallium–nitride (GaN) field-effect transistors (FETs) introduces distortion in GaN-based power amplifiers (PA). The PAs that operate with supply modulation to increase efficiency are particularly affected by this phenomenon, since the GaN FET trap state exhibits a nonlinear dependence on the voltage applied to the device terminals. In this paper, a measurement approach and the setup are presented for a large-signal characterization of GaN-based PAs operated with dynamic bias supply: a new prepulsing technique is introduced, which enables the characterization of the PA in controlled charge-trapping and thermal states. The characteristics obtained with this technique are shown to give an accurate description of the PA performance in the actual application working conditions. The proposed approach is validated by using the measured data for the direct computation of predistortion functions for the linearization of a 9.7-GHz envelope tracking 10-W GaN monolithic microwave integrated circuit PA for amplitude-modulated pulsed radar transmitters. Additional research on the PA trap-induced performance degradation is also presented and can be explored to predict the PA performance for different parameters of the operative regime or for the formulation of the PA behavioral models.

In this paper, we present a high-efficiency transmitter based on an integrated circuit (IC) supply modulator implemented in the same 0.15-μm 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/μs. Characterization of the dynamic Ron 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.