Jean-Michel Nebus

Joint optimization of the power-added efficiency and the error-vector measurement of 20-GHz pHEMT amplifier through a new dynamic bias-control method

This paper presents a method for the optimization of the power-added efficiency (PAE), as well as the error-vector measurement (EVM) of a 20-GHz power amplifier (PA) applied in this case to the M quadrature and amplitude modulations. A first key point lies in that both input and output biasing voltages of the solid-state power amplifiers (SSPAs) are dynamically controlled according to the RF power level associated with the symbol to be transmitted. The leading idea is that the dynamic biasing control is designed and implemented to keep fixed amplitude (AM/AM) and phase (AM/PM) conversion values, while the RF input power level changes. The power gain of the PAs can then be dynamically tuned to a fixed power gain corresponding to the compression gain behavior for which the PAE is optimum at low-, medium-, and high-input RF power levels. As a main consequence, PAE performances can be drastically improved as compared to classical backoff solutions and optimized while keeping a very good EVM. A Ka-band hybrid amplifier has been realized using an 8/spl times/75 /spl mu/m power pseudomorphic high electron-mobility transistor. The proposed linearization technique is validated by comparisons between measured PAE and EVM on the SSPA when a fixed and controlled bias are used.

Measurements of time-domain voltage/current waveforms at RF and microwave frequencies based on the use of a vector network analyzer for the characterization of nonlinear devices-application to high-efficiency power amplifiers and frequency-multipliers optimization

A new time-domain waveform measurement system based on the combination of an harmonic source and load-pull setup with a modified vector network analyzer (VNA) is presented. It allows the visualization, the measurement, and the optimization of high-frequency currents and voltages at both ports of nonlinear microwave devices. Measurements of GaAs field effect transistor (FETs) and GaInP/GaAs heterojunction bipolar transistor (HBTs) at L-band were performed to demonstrate the great capabilities of the system. On one hand, voltage and current waveforms at both ports of transistors, working as power amplifiers, were optimized for maximum power-added efficiency. On the other hand, time-domain waveforms of transistors operating as frequency multipliers were optimized for maximum conversion gain. Such results prove the capabilities offered by this new nonlinear time-domain measurement system to aid in designing optimized power amplifiers or frequency multipliers. They also provide valuable information for nonlinear transistor model validation.

An active pulsed rf and pulsed dc load-pull system for the characterization of power transistors used in coherent radar and communication systems

This paper presents a new automated and vector corrected active load-pull system allowing the characterization of microwave power transistors under coherent pulsed RF and pulsed DC operating conditions. Measurements of an S band-Class C-8 Watt silicon bipolar amplifier are shown and demonstrate the ability of our system to accurately characterize power variations and carrier phase shift within the pulse. Source and load-pull measurements of an 8/spl times/30 /spl mu/m/sup 2/ GaInP/GaAs HBT (Thomson LCR) are also reported for different pulse widths.

A novel, accurate load-pull setup allowing the characterization of highly mismatched power transistors

The measurement of highly mismatched power transistors has always been a difficult problem. A novel, active load-pull technique providing an attractive solution is proposed in this paper. It consists of using suitable mismatched sources to drive the device under test. By using the proposed measurement setup, an electronic simulation of highly reflective loads very close to the edge of the Smith chart can be achieved (reflection coefficients larger than 0.9). Furthermore, the magnitude and phase of the reflected power waves at the output of the transistor under test are accurately controlled so as not to damage the component. Some examples of load contour mappings are given. They demonstrate the promising capabilities offered by this improved large signal measurement tool. >

A system level model of solid state amplifiers with memory based on a nonlinear feedback loop principle

This paper presents a new behavioral modeling approach for solid state amplifiers (SSAs) used in communication systems. The model topology consists of the combination of two low pass equivalent transfer functions that process the envelope of the signal. The first one is a nonlinear filter which is well suited to handle short term memory effects. The second one is a nonlinear impulse response that takes into account long term memory effects. In the present paper, the proposed behavioral model is extracted from harmonic balance and envelope simulation results of a BiCMOS LNA and a HBT PA. It is then tested for the prediction of the amplifier response to a 16-QAM signal.

Measurement based modeling of power amplifiers for reliable design of modern communication systems

The characterization and the modeling of nonlinear memory effects are, nowadays, an integral part of the design process of modern communication systems. Notably, the nonlinear long term memory effects occurring in solid state devices impact considerably system performances. Recently, a new method to characterize and integrate low frequency memory effects in nonlinear behavioral models of SSPAs has been presented. This paper presents a detailed mathematical study and a measurement based extraction principle of the proposed behavioral model. Calibrated time-domain envelope measurements are used for the model extraction and verification procedures. The extraction technique is illustrated by the modeling of a L-Band HFET amplifier.

Optimum design of very high-efficiency microwave power amplifiers based on time-domain harmonic load-pull measurements

Due to the large expansion of wireless communications, the need for high-efficiency power amplifiers has emerged. In mobile communication systems, power amplifiers are the most critical elements for the power-dissipation budget. Thus, the operating conditions of active devices have to be optimized using accurate and complementary computer-aided design (CAD) and experimental tools. This paper reports two design methods of high-efficiency power amplifiers. The first one is CAD oriented and based on the substitute generator technique using the nonlinear model of transistors. The second one is based on a specific measurement system of time-domain waveforms using a modified vector network analyzer, coupled with harmonic active load-pull techniques (three active loops). This new setup enables the measurement and optimization of time-domain waveforms at both ports of transistors driven by constant-wave test signals. These two design methodologies are applied to the optimization of an S-band 1-W class-F GaInP/GaAs heterojunction-bipolar-transistor power amplifier.

Characterization and modeling of bias dependent breakdown and self-heating in GaInP/GaAs power HBT to improve high power amplifier design

It is usual to say that power GaInP/GaAs heterojunction bipolar transistors (HBTs) have many advantages for power amplification at microwave frequencies, because of their high gain and high power density. Furthermore, the possibility of controling the base biasing conditions (voltage, current, self-bias control) compared to a field-effect transistor offers additive degrees of freedom to make a tradeoff between linearity and power-added efficiency. Nevertheless existing devices are limited because of the relatively low breakdown voltage whereas high collector voltage swings are required to achieve high power. This drawback makes them not appropriate for use in the next generation of mobile communication base station or radar systems. Silicon technologies such as LDMOS and III-V devices (MESFET and HFET) present competitive performances in term of high power level but for medium power added efficiency. Important improvements have been made in recent years which make possible large breakdown voltages for GaInP/GaAs HBTs. Breakdown value close to 67 V has been achieved. The aim of this work is to significantly improve the modeling of the breakdown voltage on this type of transistor. Furthermore, the in depth characterization and modeling of self-heating effects have been greatly improved in order to improve thermal management solutions which enable us to enhanced design solutions of HBT high power amplifiers.

A Smart Load-Pull Method to Safely Reach Optimal Matching Impedances of Power Transistors

This paper presents a new method to find optimal load impedances of power transistors with a VNA based Load-Pull measurement setup. Most of load pull setups find the optimal load impedance of a device under test (DUT) for a given available input power. If the optimal impedance must satisfy a trade off between several parameters, such as gain compression or power added efficiency, the measurement procedure may become very time consuming. Our method automatically generates a behavioral model of the DUT. Crossing-informations from this model and measurements lead us to the good impedance optimum with a limited number of iterations.

Time-domain envelope measurements for characterization and behavioral modeling of nonlinear devices with memory

This paper presents a calibrated four-channel measurement system for the characterization of nonlinear RF devices such as power amplifiers. The main goal of this study is to perform the characterization of the bandpass response of a nonlinear device-under-test (DUT) driven by modulated carriers. The proposed setup enables the generation of L- or S-band (1-4 GHz) carriers with a modulation bandwidth up to 100 MHz. The carrier harmonics generated by the nonlinear DUT are ignored and considered to be sufficiently filtered. This characterization setup enables calibrated time-domain measurements of the complex envelopes of both incoming and outgoing RF waves at the input and output of the DUT. This means that the fundamental and harmonic frequencies of the envelope are measured and processed. A large set of modulation formats can be generated by using a computer-controlled arbitrary waveform generator. Complex envelopes are measured by using a four-channel sampling scope. The proposed calibrated setup can be used to study or to validate linearization techniques of power amplifiers. This characterization tool is also well suited for the extraction and validation of behavioral bilateral models of nonlinear RF analog equipment exhibiting memory effects