Daniel Niessen

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.

A Double-Pulse Technique for the Dynamic I/V Characterization of GaN FETs

Standard dynamic characterization methods based on periodic narrow-pulse low duty-cycle excitation waveforms provide suboptimal I/V curves when used along with GaN field effect transistors (FETs), due to complex nonlinear charge trapping effects. Thus, a double-pulse technique for the dynamic characterization of GaN FETs is here presented. The double-pulsed I/V characteristics are shown to be not only isothermal but also corresponding to a fixed charge trapping state.

New pulsed measurement setup for GaN and GaAs FETs characterization

A new setup is proposed for the measurement of current–voltage pulsed characteristics of electron devices. The main advantages of the system consist in: shorter pulse widths through generation in a 50-Ω environment, simple average current monitoring through separation of the direct and alternate current paths, setting of average voltage values independently of pulse amplitudes and duty cycle, and stability of the setup guaranteed by wide-band dissipative terminations. The system is used for the characterization of dispersive effects due to carrier energy traps and thermal phenomena in GaAs and GaN on SiC field effect transistors. The basic differences between the two technologies are highlighted in the paper.

GaN FET Nonlinear Modeling Based on Double Pulse

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.

{ I}/{ V}

A C-Band MMIC high power amplifier (HPA) has been designed exploiting a 0.25 μm HEMT GaN process on SiC substrate. The HPA is designed for future synthetic aperture radar (SAR) antenna applications. The HPA delivers 16 W output power with PAE over 38% at 6 dB gain compression within a 900 MHz bandwidth around 5.75 GHz. Up to 20 W output power and 40% PAE are obtained at higher gain compression. A comparison with another amplifier, differing only for the layout of the devices in the final stage, points out that the transistor thermal conditions represent the main limitation for this high power density technology.

Characteristics

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.

A C-Band AlGaN-GaN MMIC HPA for SAR

A large-signal measurement setup with sinusoidal excitation is used for the characterization of low-frequency dispersive phenomena in III–V FET devices. This low-cost setup operates in the MHz frequency range and its components are easily available in most research laboratories. A dispersive model of the dynamic drain current, taking into account the non linear behaviour of charge trapping phenomena, is identified for an AlGaN/GaN HEMT on the basis of the proposed characterization setup.

Pre-pulsing characterization of GaN PAs with dynamic supply

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.

Large-signal characterization of GaN-based transistors for accurate nonlinear modelling of dispersive effects

A fast and simple method for the direct characterization of nonlinear charge functions of electron devices is presented. The input and output transistor ports are simultaneously excited through single-tone sources at different frequencies and calibrated large signal waveforms are measured by means of an advanced NVNA-based setup. Proper choice of the two frequencies guarantees an almost complete coverage of the voltages domain in a single and very fast measurement set and allows the extraction of the charge functions by direct integration of currents in the frequency domain, since, contrary to other methods, the measured waveforms are both iso-thermal and iso-dynamic (i.e. at fixed charge trapping status). The method is validated by characterizing the gate charge function of a 5W 8×125μm GaN FET and implementing a simple table-based model of the transistor input port. Very good results are achieved by comparison with large-signal measurements under conditions different than the ones used for the characterization.

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

We propose an efficient procedure for the extraction of a charge-controlled nonlinear model of a 1-mm gallium nitride on silicon carbide field-effect transistor (L = 0.25 μm) from nonlinear vector network analyzer acquisitions. A fast, single-shot measurement technique is described, in which the two device-under-test (DUT) ports are excited by single-tone sources at carefully selected tone frequencies, acquiring calibrated waveforms at the on-wafer DUT ports with an almost complete coverage of the voltages domain. The gate and drain charge functions identification is executed by the integration of the displacement currents in the frequency domain. A suitable approach for separating the conductive and displacement drain current components is provided. The presence of thermal self-heating and charge trapping phenomena is empirically evaluated, and accounted through an equivalent voltage approach. Experimental validation is provided at 2.5 and 5 GHz for a continuous-wave excitation, and at 2.5 GHz for a two-tone excitation.