Ahmed Birafane

On the linearity and efficiency of outphasing microwave amplifiers

A new approach for the analysis of the efficiency and linearity of Chireix-outphasing combiners is presented. The approach takes into account, in an explicit manner, the effect of impedance mismatch between the amplifiers and the lossless combining structure. It is shown that the impedance mismatch leads to new expressions for the output voltages from both branches of the amplifier. These expressions explain the origin of the lack of linearity reported in the literature for the Chireix architecture and lead to a new expression for the instantaneous efficiency of a Chireix combiner. Various simulations using a commercial simulator are performed and their results are compared to those predicted by the derived equations. A quasi-exact agreement between the simulator and derived equations is found for all simulations conducted, including voltage expressions, linearity analysis, and instantaneous and average efficiency calculations for a code-division multiple-access signal. The impact of combiner parameters on its linearity and efficiency is also studied.

Phase-only predistortion for LINC amplifiers with Chireix-outphasing combiners

Nonlinearities in linear amplification with nonlinear components (LINC) amplifiers using Chireix-outphasing combiners are studied and their sources are identified. A generalized analytical approach for finding phase-only predistortion functions to linearize these amplifiers is presented. Two distinct analytical solutions are found and are applied for phase-only linearization. The first solution consists of a static constant phase predistortion, which is introduced in one of the two RF amplifier branches. This constant phase is determined analytically as function of the value of the reactance of the stub used in the Chireix combiner. The second solution is a variable phase-distortion function, which is derived in explicit form. The two predistortion functions preserve constant envelope operation of the amplifiers. The impact of the two predistortion functions on the combiner efficiency is also investigated. It is found, through simulation with a code-division multiple-access signal and an experimental measurement with a 16 quadrature-amplifier-modulation signal that the two predistortion functions perfectly linearize the amplifier. It also shown analytically that, with perfect linearity, the efficiency of Chireix-outphasing amplifier is reduced and is, at best, equal to that of the LINC amplifier with a resistive combiner.

An analytical approach to LINC power combining efficiency estimation and optimization

A new approach for studying the combiner efficiency in a LINC amplifier based on analytical expressions of the instantaneous combiner efficiency and the signals probability distribution function (PDF) is proposed. The analytical expressions, derived for both a matched combiner and a Chirex outphasing combiner, are applied to compute the combiner efficiency of a LINC amplifier under CDMA and 64QAM signals. A direct approach to determining the optimal shunt reactance used in an outphasing combiner is also proposed and applied to both signals. Validation with a commercial simulator is given.

Analyzing LINC Systems

In this article, an analysis of LINC PA efficiency and linearity is observed. The use of two highly efficient PAs operated with constant envelope signals in the LINC transmitter does not in and of itself guarantee that the overall efficiency of the LINC amplifier will be high. Its shown that the use of matched combiner gives good linearity but the efficiency is decreased when the peak-to-average of signal increases. The same isolation between the two amplifiers is what gives the high linearity at the output. To avoid the efficiency losses in the matched combiner, the lossless Chireix-outphasing combiner has been presented. The nonisolating combiner presents time-varying impedances to the output of RF amplifier of each branch as the phase difference between the branches increases, thus significantly improving the LINC average efficiency. Impedance mismatching between the amplifiers and outphasing combiner have been explicitly taken into account. Closed-form expressions for the voltages at the output of the combiner, the instantaneous impedance seen by each amplifier, as well as the instantaneous efficiencyof the combiner, have been developed and presented. It has been shown that the new voltage expressions are consistent with the nonlinearity behavior of the Chireix combiner/thus demonstrating that the combining structure is a source of nonlinearity. To validate this approach many types of signals are considered as CDMA WCDMA and OFDMA (QPSKA, 16-QAM, 64-QAM) signals. Other sources of distortion such as imbalances between two branches are presented and how to compensate this imbalance is proposed. In light of the results obtained, the use of the Chireixoutphasing combiner with saturated nonlinear amplifiers to reach the objective of both high-efficiency and high-linearity amplification does not seem possible. The potential use of phase predistortion to reach this objective remains to be investigated.

Experimental characterization of LINC outphasing combiners' efficiency and linearity

In this paper, an experimental characterization of the efficiency and linearity of resistive and outphasing combiners for linear amplification using non linear components (LINC) amplifiers is presented. The combining efficiency is measured for three combiners, i.e., one Wilkinson combiner and two Chireix combiners, with different electric stub lengths. The measured results are compared to theoretical predictions for the three cases. The nonlinear effects of Chireix combiners are highlighted and phase adjustment of one of the systems branches is proposed to linearize it. We show that linearization is feasible in this manner only at the expense of reduced efficiency.

Sources of linearity degradation in LINC transmitters for hybrid and outphasing combiners

The LINC (linear amplification using non linear components) is a potential candidate as a transmitter system for providing both high linearity and high efficiency. In this paper, we review all the origins of linearity degradation in LINC transmitters for both matched and outphasing combiners and group them into three main sources. We first show that the linearity degradation in the LINC transmitter with an outphasing combiner is inherent to the combiner structure. We further show the impact of phase and amplitude imbalance in the two LINC branches on the transmitter linearity. Finally the filtering effect on the linearity in the LINC transmitter is studied. Extensive simulations using the advanced design system (ADS) software with CDMA signals (7 dB of peak-to-average) have been carried out and results are presented to validate the approach used in each case studied.

Analytical Design Methodology of Outphasing Amplification Systems Using a New Simplified Chireix Combiner Model

An analytical design methodology of outphasing amplification systems is proposed using new simplified analytical expressions for the instantaneous efficiency and the input/output voltages of Chireix outphasing combiners. These expressions are derived first for ideal voltage sources with nonzero internal impedance and later for ideal Class-B amplifiers where dc current variation is incorporated. They take into account the impedance mismatch between the amplifiers and the lossless combining structure and account explicitly for the all of the combiners electrical parameters. We show that the maximum achievable instantaneous combining efficiency can be controlled in position and in value through the judicious choice of the combiners stub length and transmission-line characteristic impedance. We further show that, when this choice is combined with the amplified signals distribution, the average combining efficiency can be precisely controlled and easily maximized. Various validations of the proposed expressions are performed by comparison with experimental and simulation results for various combiners as well as for a complete linear amplification with nonlinear components amplifier. Excellent agreement between simulations and measurements is obtained in all cases considered.

Impact of PA Mismatching in Chireix-Outphasing System without Stubs

In this paper; new expressions for the Chireix combiner without stubs are derived to study the impact of the combiner mismatching to the efficiency and the linearity of the system. We show analytically that the Chireix outphasing combiner without the shunt reactance is perfectly linear but the instantaneous efficiency degrades when impedance mismatching between the combiner and the PA occurs. We varied the length and the impedance of the lambda/4 transmission line of the Chireix combiner consequently we can see by using ADS software that the system stays linear and the best efficiency exist at length of lambda/4 and impedance of 1.4Z0. In the other hand, we note that the efficiency is reduced and is at the best equal to that of the hybrid combiner. This work is validated through simulation with ADS software using CDMA and multi-tones and one tone 2 GHz envelope modulated signals.

A Simplified Model for Chireix Outphasing Combiner Efficiency

In this paper, a simplified model for the instantaneous efficiency of the Chireix combiner is proposed. This model is given in the form of a very simple and intuitive expression that can replace the more complex expressions recently proposed in the literature. The model takes into account the impedance mismatch between the amplifiers and the lossless combining structure and is shown to be valid for all combiners considered. Validation of the simplified model is performed by comparison of the models predictions to experimental and simulation results for five different Chireix combiners. Excellent agreement between the proposed model, simulations and measurements is obtained in all cases considered

On the large-signal modeling of AlGaN/GaN HEMTs for RF switching-mode power amplifiers design

A large-signal model for GaN HEMT transistor suitable for designing switching-mode power amplifiers (SMPAs) is presented along with its parameters extraction procedure. This model is relatively easy to construct and implement in CAD software since it requires only DC and S-parameter measurements. The modeling procedure was applied to a 4-W packaged GaN-on-Si HEMT and the developed model is validated by comparing its small- and large-signal simulation to measured data. The model has been employed for designing a switching-mode inverse class-F power amplifier. Very good agreement between the amplifier simulation and measurement shows the validity of the model.