Envelope tracking (ET) is by now a well-established technique that improves the efficiency of microwave power amplifiers (PAs) compared to what can be obtained with conventional class-AB or class-B operation for amplifying signals with a time-varying envelope, such as most of those used in present wireless communication systems. ET is poised to be deployed extensively in coming generations of amplifiers for cellular handsets because it can reduce power dissipation for signals using the long-term evolution (LTE) standard required for fourthgeneration (4G) wireless systems, which feature high peak-to-average power ratios (PAPRs). The ET technique continues to be actively developed for higher carrier frequencies and broader bandwidths. This article reviews the concepts and history of ET, discusses several applications currently on the drawing board, presents challenges for future development, and highlights some directions for improving the technique.

ET Comes of Age: Envelope Tracking for Higher-Efficiency Power Amplifiers

Integration is a key step in utilizing advances in GaN technologies and enabling efficient switched-mode power conversion at very high frequencies (VHF). This paper addresses design and implementation of monolithic GaN half-bridge power stages with integrated gate drivers optimized for pulsewidth-modulated (PWM) dc–dc converters operating at 100 MHz switching frequency. Three gate-driver circuit topologies are considered for integration with half-bridge power stages in a 0.15- $\mu$ m depletion-mode GaN-on-SiC process: an active pull-up driver, a bootstrapped driver, and a novel modified active pull-up driver. An analytical loss model is developed and used to optimize the monolithic GaN chips, which are then used to construct 20 V, 5 W, 100 MHz synchronous buck converter prototypes. With the bootstrapped and the modified pull-up gate-driver circuits, power stage efficiencies above 91% and total efficiencies close to 88% are demonstrated. The modified active pull-up driver, which offers 80% reduction in the driver area, is found to be the best-performing approach in the depletion-mode GaN process. These results demonstrate feasibility of high-efficiency VHF PWM dc–dc converters based on high levels of integration in GaN processes.

Very High Frequency PWM Buck Converters Using Monolithic GaN Half-Bridge Power Stages With Integrated Gate Drivers

This paper introduces an envelope tracking (ET) power supply for 4G cell phone base stations using EPC eGaN® FETs. An analytical loss model is developed for design optimization and verified by a single phase synchronous buck converter using the zero-voltage switching (ZVS) technique. The model is then extended to four phases and is used to design a 60 W ET power supply with 20 MHz large signal bandwidth (BW). At 25 MHz per-phase switching frequency, measured static power stage efficiency peaks at 96.5% with 68 W output power delivered from 30 V. Experimental results demonstrate accurate tracking of 20 MHz 7 dB Peak-to-Average Power Ratio (PAPR) LTE envelope with 92.3% total efficiency, delivering 67Waverage power from 30 V.

/pdf/envelope-tracking-gan-power-supply-for-4g-cell-phone-base-4nnop7u81x.pdf

Envelope tracking GaN power supply for 4G cell phone base stations

This paper describes a synchronous buck converter based on a GaN-on-SiC integrated circuit, which includes a halfbridge power stage, as well as a modified active pull-up gate driver stage. The integrated modified active pull-up driver takes advantage of depletion-mode device characteristics to achieve fast switching with low power consumption. Design principles and results are presented for a synchronous buck converter prototype operating at 100 MHz switching frequency, delivering up to 7 W from 20 V input voltage. Measured power-stage efficiency peaks above 91%, and remains above 85% over a wide range of operating conditions. Experimental results show that the converter has the ability to accurately track a 20 MHz bandwidth LTE envelope signal with 83.7% efficiency.

100 MHz, 20 V, 90% efficient synchronous buck converter with integrated gate driver

All-digital outphasing transmitter architecture using multidimensional power coding (MDPC) is proposed for noncontiguous concurrent multiband transmission with a high power efficiency. MDPC transforms multiband digital baseband signals into multibit low-resolution digital signals that drive switching-mode PAs. A prototype digital outphasing transmitter consists of two 1-GHz bandwidth GaN Class-D PAs and a Chireix power combiner. The two GaN PAs are driven by bipolar radio frequency (RF) pulse-width modulation (PWM) signals, which are transformed from a concurrent dual-band LTE signal by MDPC. The dual-band LTE signal with 15-MHz aggregate channel bandwidth at 240 and 500 MHz frequency band is transmitted with $-$ 30 and $-$ 37 dBc out-of-band emissions, respectively. Digital outphasing achieves more than two times higher coding efficiency than conventional concurrent dual-band digital transmitters with the same PAs in Class-S operation. Measured power coding efficiencies of 35.4% and 47.1% are observed with outphasing bipolar and 3-level RF PWM signals respectively, which are encoded from the dual-band LTE signal.

/pdf/concurrent-multiband-digital-outphasing-transmitter-1mr8ch0qaq.pdf

Concurrent Multiband Digital Outphasing Transmitter Architecture Using Multidimensional Power Coding

In this paper, we report a DC/DC converter based on GaN HEMT's with a switching frequency of 200 MHz that can be used to generate envelope-modulated power supply voltages for use in envelope tracking power amplifiers. The converter consists of switching circuits using 0.25-um GaN HEMTs, inductor, and low pass filter, and can provide output voltages above 28V. An integrated bootstrap driver of the switching circuits is employed in order to reduce DC power consumption of the driver stage. Generation of envelope power supply voltages for 20 MHz LTE signals was demonstrated using 200 MHz switching rates with efficiency of 73%(including dissipation in final and driver stages). The chip size is 1075×990 um2.

High efficiency GaN switching converter IC with bootstrap driver for envelope tracking applications

A high-frequency amplifier module includes a driver-stage amplifier 3 that amplifies an RF signal input thereto from an RF input terminal 1 , and a final-stage amplifier 5 that amplifies the signal amplified by the driver-stage amplifier 3 and outputs the signal after the amplification to an RF output terminal 7 . The driver-stage amplifier 3 is fabricated on a silicon substrate 11 , while the final-stage amplifier 5 is fabricated on a gallium arsenide substrate. This configuration downsizes the cost while maintaining a high-frequency characteristic comparable to that in the case where all components of an entire module are fabricated on a gallium arsenide substrate 71.

High-frequency amplifier module and high-frequency amplifier module unit

A broadband push-pull power amplifier with power added efficiency (PAE) greater than 47% from 500 MHz to 25 GHz is reported The power amplifier comprises two GaN integrated circuits whose outputs are combined with a broadband balun to achieve power up to 20W Each IC contains two stacked GaN HEMTs and is designed to employ a load impedance of 25Ω without any reactive matching in order to achieve broadband operation To provide a better input impedance match to 25Ω and to reduce gain variation over frequency, an RC feedback network is used Measurements, under CW excitation, demonstrated greater than 63% power added efficiency at select frequencies and greater than 47% over the entire bandwidth

Broadband high PAE GaN push-pull power amplifier for 500MHz to 2.5 GHz operation

We report a digitally-driven switching-mode amplifier using GaN devices in Voltage-Mode Class D operation, for use at 465 MHz (the recently designated frequency for LTE Band 31). The amplifier is fabricated as an integrated circuit including output devices and drivers. The output circuit employs two stacked GaN FETs; a bootstrap-drive configuration is used in order to increase the driver-stage efficiency for the uppermost GaN FET. The GaN VMCD amplifier achieves peak power-added efficiency of 66.6%, and an output power of 3.3W. At 6dB power backoff, power-added efficiency of 36.3% is achieved. These power-added efficiency values are the highest for reported GaN VMCD amplifiers at a commercial wireless frequency.

Over 65% PAE GaN voltage-mode class d power amplifier for 465 MHz operation using bootstrap drive

A front-end amplifier has an impedance detector that detects an impedance seen looking into an antenna side from a power amplifier from a radio-frequency signal output from the power amplifier and a radio-frequency signal reflected from the antenna, in which a control circuit decides on whether the impedance detected by the impedance detector belongs to a specific region or not, and controls, if the impedance belongs to the specific region, at least one of the bias condition of the power amplifier and the impedance of a variable-matching circuit.

Kenji Mukai

Papers

High efficiency GaN switching converter IC with bootstrap driver for envelope tracking applications

High-frequency amplifier module and high-frequency amplifier module unit

Broadband high PAE GaN push-pull power amplifier for 500MHz to 2.5 GHz operation

Over 65% PAE GaN voltage-mode class d power amplifier for 465 MHz operation using bootstrap drive

Front-end amplifier