Chireix Combiner Research Articles

A 28 GHz Single-Input Linear Chireix (SILC) Power Amplifier in 130 nm SiGe Technology

This article presents an integrated high-efficiency 28 GHz power amplifier (PA) employing a combination of Chireix outphasing and Doherty architectures in order to simultaneously achieve power back-off efficiency and linearity with a single RF input signal and no predistortion. The amplifier consists of a dual-input high-efficiency outphasing PA and a simple input network which serves as a power splitter and feeds the same signal to the inputs of the main and auxiliary PA cells that are biased in class-AB and class-C regions, respectively, similar to the Doherty architecture. The operation of the PA cells together with the Chireix combiner result in back-off efficiency enhancement plus systematic AM-AM and AM-PM variations which are used to correct the distortions caused by transistors, resulting in a linear response. The implemented PA demonstrates 19 dBm saturation power (Psat) with 34.4% peak power-added efficiency (PAE) and 6 dB back-off PAE of >23% at 27.5 GHz. The modulated signal performance using a 100 MHz 64QAM OFDM signal shows an average output power of 11.9 dBm with PAE >20%, EVM < 5%, and ACLR < −33 dBc without using predistortion.

Reconfigurable Chireix Outphasing Power Amplifier Over Multiple Frequency Bands

This brief presents a new design scheme to expand the operating frequency range of outphasing power amplifier (PA) which is useful for software-defined radio applications. The proposed design scheme offers reconfigurability to multiple operating bands by using reconfigurable Chireix combiner along with wideband PA. For efficiency enhancement at output power back-off at multiple operating frequencies, Chireix combiner should provide correct reactance compensation at each operating frequency. This is achieved by utilizing tunable compensating network, which can be reconfigured to multiple operating frequencies. The proposed methodology is experimentally verified in radio frequency/digital hybrid DSP/field programmable gate array-based platform. The measurement results show efficiency enhancement upto 6–7-dB output power back-off over ±25-MHz bandwidth around 1.65, 2.1, and 2.35 GHz.

New Mixed-Mode Design Methodology for High-Efficiency Outphasing Chireix Amplifiers

A new design methodology providing optimal mixed-mode operation for dual-input class-F outphasing Chireix amplifiers is presented. The design starts with single-transistor class-F simulations at the intrinsic $I$ – $V$ reference planes to directly select the optimal peak and backoff resistive loads ${R}_{\mathrm{min}}$ and ${R}_{\mathrm{max}}$ and input RF gate drives yielding the best combination of efficiencies and output powers without needing to perform a load pull simulation or measurement. New analytic equations expressed only in terms of ${R}_{\mathrm{min}}$ and ${R}_{\mathrm{max}}$ are given for designing the Chireix combiner at the current source reference planes. Nonlinear embedding is then used to predict the incident power and multi-harmonic source and load impedances required at the package reference planes to physically implement the power amplifier (PA). An analytic formula solely expressed in terms of ${R}_{\mathrm{min}}$ and ${R}_{\mathrm{max}}$ is reported for the peak and backoff outphasing angles required at the PA input reference planes. A Chireix outphasing PA is designed using two 15-W GaN HEMTs. A Chireix outphasing PA exhibits a peak efficiency of 72.58% with peak power of 43.97 dBm and a 8-dB backoff efficiency of 75.22% at 1.9 GHz. Measurements with 10-MHz LTE signals with 9.6-dB PAPR yield 59.4% average drain efficiency at 1.9 GHz while satisfying the 3GPP linearity requirements.

Ring-type Magnitude Modulation for LINC: a pragmatic approach to the efficiency challenge

This paper considers the use of the linear amplification with nonlinear components (LINC) technique for the power amplification of spectrally compact offset quadrature phase shift keying (OQPSK) signals allowing the use of highly efficient, low cost, and strongly nonlinear high power amplifiers (HPAs). However, the performance of the LINC signal separation and power combining procedures decreases with the rise of the signal’s peak-to-average power ratio (PAPR). A new ring-type magnitude modulation (RMM) method is proposed for OQPSK signals that limits both its maximum and minimum complex envelope excursions avoiding zero crossings, without spreading the transmitted signal’s spectrum. The performance results show that band-limited OQPSK signals whose envelope has low fluctuations produce LINC components with a narrower spectrum, with a considerable impact on the LINC transmitter regardless of the type of combiner chosen: when using a passive/matched combiner, the transmitter’s power efficiency is significantly increased without spreading the combined signal’s spectrum; for the highly efficient non-linear Chireix combiner, there is a reduction of the amount of spectral leakage produced by nonlinearly combining the LINC signal components. Finally, an iterative decoding scheme is also proposed, which employs estimates of the received symbols’ RMM coefficients to compensate the RMM distortion.

A Low-Power CMOS Class-E Chireix RF Outphasing Power Amplifier for WLAN Applications

This paper describes the design and implementation of a low-power CMOS class-E Chireix RF outphasing power amplifier (PA) for WLAN applications. The proposed circuit is based on cascode class-E topology with two-stage structure and Chireix power combiner with a floating load. This class-E topology adopts self-biased technique and current-reused technique that avoids using thick-oxide transistors and can enhance the driving ability and power gain. Additionally, the Chireix combiner can reduce the loss of power and improve the efficiency by avoiding the use of isolation resistance. With a 2.5 V power supply, the realized prototype achieved 21.4 dBm of maximum output power and 29.9 % of average power-added efficiency (PAE) at 2.4 GHz. When operated for minimum input levels at 2.5 V, it reached 38.8 dB power gain. For a WLAN OFDM signal with 20 MHz, the error vector magnitude of 3 % is achieved with peak drain efficiency of 62 %. Moreover, the outphasing power amplifier is fully integrated with TSMC 0.18-µm CMOS technology and the chip area including testing pads is 1.07 × 1.17 mm2.

Linearization and Imbalance Correction Techniques for Broadband Outphasing Power Amplifiers

This paper presents adaptive phase-only memory digital pre-distortion (DPD) and gain/phase/delay imbalance correction techniques for outphasing power amplifiers (PAs). Least squares adaptation is used for the phase-only DPD and imbalance correction techniques. The performance of these techniques is verified by co-simulations and measurements of an outphasing PA implemented with two identical class-F PAs and a Chireix combiner. The Chireix combiner with compensating stubs added to achieve high efficiency introduces nonlinearity. The proposed adaptive phase-only memory DPD corrects for the nonlinearity due to the Chireix combiner. The proposed gain/phase/delay imbalance correction technique achieves total lower/upper adjacent channel power ratio (ACPR) improvements of 43.8/37.6 dB for 5-MHz and 46.2/38.3 dB for 10-MHz long-term-evolution signals in measurements. The fractional-sample delay imbalance correction reduces distortion at low input amplitudes and demonstrates on the order of a 10-dB ACPR improvement compared to full-sample delay imbalance correction.

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 combiner's 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 combiner's stub length and transmission-line characteristic impedance. We further show that, when this choice is combined with the amplified signal's 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.

A CMOS Outphasing Power Amplifier With Integrated Single-Ended Chireix Combiner

This brief proposes an on-chip outphasing power amplifier that uses a single-ended Chireix combiner for a linear amplification with a nonlinear component amplifier. The proposed combiner structure consists of a lumped inductor and a lumped capacitor that can achieve the simple single-ended configuration of a Chireix combiner. It is also suitable for on-chip implementation with minimum efficiency deterioration. An inductance-capacitance balun using the lumped model of λ/4 and 3λ/4 transmission lines was effectively merged into a simple Chireix combiner for two outphased input signals. The relation between the output resistance and the outphasing angle of the input signals was derived to determine the maximum efficiency. A voltage-mode class-D power amplifier was used with the combiner to illustrate the combiner's effectiveness. The prototype fabricated in a 0.13-μm complementary metal-oxide-semiconductor process shows a maximum 52% power-added efficiency (continuous wave) and a -47-dBc adjacent channel power ratio performance at a 10-MHz offset with a 1.92-GHz wideband code-division multiple-access signal.

A new impedance match method to improve efficiency of LINC with Chireix combiner

AbstractIn this article, we propose a new impedance match method in serial Chireix combiner to improve the efficiency of Linear amplification with Nonlinear Components (LINC) amplifier. Based on the impedance characteristics of the serial Chireix combiner, two transmission lines are added between power amplifiers (PAs) and Chireix combiner in LINC. For experimental verification, the PAs and combiner are implemented and tested. From the measured results, this method is proved available for improving efficiency of LINC, and convenient for design as well as tuning work. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1418–1421, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25205

CMOS Outphasing Class-D Amplifier With Chireix Combiner

This letter presents a CMOS outphasing class-D power amplifier (PA) with a Chireix combiner. Two voltage-mode class-D amplifiers used in the outphasing system were designed and implemented with a 0.18-mum CMOS process. By applying the Chireix combiner technique, drain efficiency of the outphasing PA for CDMA signals was improved from 38.6% to 48% while output power was increased from 14.5 to 15.4 dBm with an adjacent channel power ratio of -45 dBc.

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.

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.