Envelope Tracking Transmitter Research Articles

Linearization of a Highly Nonlinear Envelope Tracking Power Amplifier Targeting Maximum Efficiency

This letter presents a linearization algorithm designed for envelope tracking transmitters with an optimal highly efficient envelope shaping function. Targeting high efficiency at all power regions of the PA results in a severe nonlinear input-output response, and broadening of the PA output spectrum compared with a fixed voltage, or other envelope shaping functions. A two-box modeling approach is used, where a static nonlinearity function focuses on reducing the nonlinearity of the transmitter, and a memory polynomial model is used in cascade to reduce remaining distortions. The results show a definite improvement over traditional one-box models, and is suitable for PAs exhibiting unique nonlinear responses, especially for ET applications.

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 $L = 2^{N}$ voltage levels, with a binary distribution in the range $\Delta V = V_{M} - V_{O}$ (maximum voltage $V_{M}$ , offset voltage $Vo$ ). This solution provides a better voltage resolution $V_{S} = \Delta V/(2^{N}-1)$ with respect to typical multilevel switched-sources topologies $(V_{S} = \Delta V/N)$ . 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.

Concurrent Multi-Band Envelope Modulated Power Amplifier Linearized Using Extended Phase-Aligned DPD

This paper reports an envelope tracking (ET) transmitter architecture for multi-band/widely-spaced carrier signals. Due to the bandwidth limitations of the envelope modulator, the dynamic supply voltage is controlled by combining the low frequency envelopes of the multi-inputs rather than the true composite envelope of the input signals. The ET transmitter is tested for dual-band and tri-band power amplifiers (PAs) operating in concurrent mode using various bandwidth carrier aggregated long term evolution (LTE) signals. The transmitter is linearized using a phase-aligned Volterra digital predistortion that considers the inter-modulation distortion (IMD) products around the carrier frequencies as well as the third-order IMD products in the tri-band case. Experimental results are shown to validate the proposed techniques for linearization and efficiency enhancement of the multi-band ET transmitter and compared to the case of fixed supply biasing of the PA.

A Multimode/Multiband Envelope Tracking Transmitter With Broadband Saturated Amplifier

A multimode/multiband envelope tracking (ET) transmitter consisting of a hybrid switching amplifier (HSA) and a broadband saturated power amplifier (PA) is developed for operation across 1.3 to 2.7 GHz. For the various standard signals with different bandwidth (BW) and peak-to-average power ratio, the HSA efficiently provides supply signals to the PA by changing the reference value of the hysteresis comparator. The saturated amplifier employs the nonlinear output capacitor to shape the voltage waveform, resulting in the half-sinusoidal or rectangular waveform. Since the nonlinear capacitor generates large out-of-phased second harmonic component with small higher order harmonics, the voltage shaping is mainly carried out by the capacitor and slightly supported by the harmonic loading circuit. Thus, with the harmonic load higher than that of output capacitor, the saturated amplifier can operate with high efficiency. This characteristic enables the saturated PA to operate with broadband characteristic and high efficiency because the design is mainly focused on the fundamental matching problem while the harmonics can be easily kept out of a low efficiency region through the subsequent optimization of the matching circuit. The broadband saturated PA is implemented based on load/source-pull methodology. The broadband matching networks for the high efficiency are synthesized using the simplified real frequency technique. For the BW from 1.3 to 2.7 GHz (70% fractional BW), the measured output power, drain efficiency, and power-added efficiency (PAE) performances are between 39.8-42.0 dBm, 55.8-69.7%, and 51.2-65.3%, respectively. The multimode/multiband ET transmitter with the designed broadband saturated PA is demonstrated at 1.8425-GHz long-term evolution (LTE), 2.14-GHz wideband code division multiple access (WCDMA), and 2.6-GHz mobile world wide interoperability for microwave access (m-WiMAX) applications. This transmitter delivers a PAE of 32.16, 37.24, and 28.75% for LTE, WCDMA, and m-WiMAX applications.

High-Efficiency Envelope-Tracking Transmitter With Optimized Class-${\hbox{F}}^{-1}$ Amplifier and 2-bit Envelope Amplifier for 3G LTE Base Station

In this paper, a high-efficiency envelope-tracking (ET) transmitter incorporating a novel efficiency-boosting function is proposed and implemented. An inverse Class-F power amplifier is utilized and optimized using the proposed output loading condition, which enhances its efficiency at the high probability region. This matching network can be conveniently implemented by controlling the nonlinear capacitance of the power transistor. For an accurate analysis, the output waveforms are modeled in terms of the nonlinear capacitance, and the efficiency and output power are subsequently analyzed and successfully optimized. For a high-efficiency envelope amplifier (EA), we propose a new EA utilizing a 2-bit switching stage in place of a 1-bit switching stage. This proposed architecture effectively reduces the ripple current, improving the efficiency of the EA. To verify our analysis, we have fully implemented a 3.54-GHz high-efficiency ET transmitter including a digital processing block. The experimental results show that the ET transmitter using a commercial 60-W peak-envelope-power GaN device operates at a drain efficiency of 44% and power-added efficiency of 39.6% with a gain of 10.1 dB at an average output power of 40 dBm for a 10-MHz third-generation long-term evolution signal with 8.5-dB peak-to-average power ratio. Using a digital pre-distortion function, the adjacent channel leakage ratio is less than -47.5 dBc.

A MASH-Controlled Multilevel Power Converter for High-Efficiency RF Transmitters

Envelope tracking (ET) is one of the most promising transmitter architectures proposed to increase the efficiency of modern wireless communication system. An ET transmitter consists of a linear RF power amplifier and a high-efficiency envelope amplifier, which is always a switched-mode power converter. The envelope amplifier provides a varying drain (collector) bias voltage, which tracks the envelope of the RF input signal, for the RF power amplifier. Thus, the RF power amplifier keeps working in high-efficiency region of most of the time. Therefore, the overall efficiency of the transmitter is increased. This paper proposes a multistage noise-shaping-technique-controlled multilevel power converter to perform as the envelope amplifier in an ET transmitter. Experimental results show that the proposed power converter can reproduce any envelope signals with the maximum spectrum component of 300 kHz and give maximum instantaneous power of 20 W. Compared with pulsewidth modulation, the noise-shaping technique can shape the ripple into noise and the noise will be attenuated by the low-pass filter, which results a better performance, while maintaining high efficiency as well.

Analysis and Optimization of Transformer-Based Series Power Combining for Reconfigurable Power Amplifiers

Transformer-based series power combiners are analyzed within the context of reconfigurable, watt-level, wideband digital envelope tracking transmitters. A model is developed for designing and analyzing transformer-based switchable series combiners. This model takes into account losses in the windings and nonideal mutual coupling, as well as the effect of switching and scaling the input amplifiers on the combiner efficiency, input impedance, and power combining ratio. Optimum combining efficiency is derived for a combiner with scaled sources. Based on the developed model, a four-element 29.5-dBm power amplifier with 31% peak efficiency at 1.9 GHz, using a 1.5-V supply is designed and simulated in IBM 130-nm CMOS technology. The efficiency drops to 14% at 12 dB power back-off when elements are turned off. The amplifier maintains an output power >; 25 dBm from 1.8 GHz to 2.5 GHz. The studies presented in this paper can be extended to any series combiner implementation and any frequency range, which can be effective in the design of watt-level power amplifiers in submicrometer CMOS technologies.

Optimized Envelope Tracking Operation of Doherty Power Amplifier for High Efficiency Over an Extended Dynamic Range

This paper presents an optimized envelope tracking (ET) operation of a Doherty amplifier. Compared to the general ET/envelope elimination and restoration transmitter, it has an advantage of the extended dynamic range of 6 dB for the load modulation of a Doherty amplifier. Moreover, by modulating the supply voltage of the carrier amplifier, while that of the peaking amplifier is fixed, the supply modulator provides just half of the current for the same power amplifier output power. It results in a reduced chip size and the crest factor of the supply modulating signal is reduced by 6 dB, enhancing the efficiency of the supply modulator. The designed ET transmitter consisting of the Doherty amplifier and the supply modulator are fabricated in 2- mum HBT and 0.13-mum CMOS processes, respectively. It presents the efficiency improvement over the broad output power region. Especially at the 16-dB backed-off power level, more than 23% of power-added efficiency (PAE) is achieved. For WiBro application, it shows the PAE of 38.6% at an output power of 24.22 dBm with a gain of 24.62 dB. The error vector magnitude is 3.64%.

Envelope tracking power amplifier with static predistortion linearization

AbstractThis paper presents the design method, properties and driving techniques of a linear 0.5 W 1 GHz LDMOS power amplifier used in a supply modulated envelope tracking transmitter with real‐time predistortion. Causes of nonlinearities are identified, and various supply voltage drives are experimented. Measured results show that the power efficiency can be improved while maintaining high linearity. Copyright © 2008 John Wiley & Sons, Ltd.