Tommaso Cappello

Envelope Tracking of an RF High Power Amplifier With an 8-Level Digitally Controlled GaN-on-Si Supply Modulator

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 = 2N 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/(2N-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.

Pre-pulsing characterization of GaN PAs with dynamic supply

Nonlinear charge-trapping observed in the electrical characteristics of GaN FETs can introduce distortion in GaN-based power amplifiers (PA), especially in supply-modulated (envelope tracking) transmitters. A measurement approach is developed for large signal characterization of GaN-based PAs operated with dynamic bias supplies for efficiency enhancement. A new pre-pulsing technique is introduced which forces the active device to operate in trapping and thermal states close to those found in the actual application. The characteristics obtained with this technique are shown to give an accurate description of the PA performance. The measured data are used for the direct computation of pre-distortion functions for the linearization of a 10-GHz Envelope Tracking (ET) 12-W GaN MMIC PA for amplitude-modulated pulsed radar transmitters. The demonstrated measurement method can be also exploited for the identification of PA behavioral models, which take into account trapping effects.

X-Band GaN Multi-Level Chireix Outphasing PA with a Discrete Supply Modulator MMIC

This paper presents the characterization of a multi-level Chireix outphasing PA with a GaN discrete supply modulator MMIC at 9.7GHz. The internal PAs include class-F harmonic terminations, while the Chireix combiner determines the fundamental frequency load modulation. A power-DAC architecture provides 8 supply levels with 3 half-bridges, maintaining more than 85% efficiency. The combination of the two achieves a peak output power of 4.8W, and average total efficiencies for a 6dB PAR QPSK signal of 44.1% and 48.1% with and without considering the supply modulator consumption, respectively, demonstrating the feasibility of this PA architecture with a real discrete supply modulator.

Dynamic RON Characterization Technique for the Evaluation of Thermal and Off-State Voltage Stress of GaN Switches

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 <sc>on</sc>-Resistance (<italic>R</italic>_ON) at increasing <sc>off</sc>-state voltages and operative temperatures. In this paper, a novel laboratory setup and characterization procedure for the dynamic <italic>R</italic>_ON of GaN HEMT switches in the presence of thermal- and trapping-effects is presented. The proposed setup allows the study of <italic>R</italic>_ON transients after the switching event at variable <sc>off</sc>-state voltages and temperatures. The use of custom-designed differential amplification stages and a voltage-controlled current source enables the accurate characterization of <italic>R</italic>_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 <italic>R</italic>_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 <italic>R</italic>_ON degradations up to 75% and 20% are observed for temperature and <sc>off</sc>-state voltage variations, respectively. These characterization data are fundamental for the accurate estimation of conduction losses during the design of switching-mode power converters.

Multi-level supply-modulated Chireix outphasing for LTE signals

This work presents a dynamic characterization of a multi-level 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 (enabled by an efficient power DAC modulator) and using outphasing for fine amplitude control. We describe the development of an experimental test bench able to supply phase- and time-aligned modulated signals for outphasing and supply modulation simultaneously. Pulse characterization is used to design a multilevel memory-less polynomial DPD. The linearized ML-CO PA is demonstrated with 1.4 MHz and 10 MHz LTE signals at 9.7 GHz. For both signals, the average total power consumption is reduced by a factor of two when supply modulation is used. For the 9.3 dB PAR, 1.4 MHz signal the PA operates with 38% average drain efficiency at 0.54 W average output power.

A Prepulsing Technique for the Characterization of GaN Power Amplifiers With Dynamic Supply Under Controlled Thermal and Trapping States

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.

Efficient power amplifiers for amplitude-tapered pulses with improved spectral confinement

This paper reviews various techniques that allow efficient amplification of amplitude-modulated radar pulses through some type of supply modulation. Examples at S and X-band with GaN PAs will be shown with efficiencies above 50% for 10-W transmitters and greatly reduced spectral sidelobes.

An empirical behavioral model for RF PAs including self-heating

In this work, we report the formulation and an experimental validation of a behavioral electro-thermal model for radio-frequency (RF) Power Amplifiers (PA). Sensitivity functions with respect to an equivalent temperature are defined for the RF input-output and drain current characteristics, and then coupled with a quasi-static thermal equivalent network. The model is empirically identified with pulse-modulated RF signals and takes advantage of the possibility to impose and sense the case temperature of the PA. The presented model accurately predicts the influence of temperature on quasi-static and dynamic characteristics of an LDMOS PA.