Taylor W. Barton

A 2.4-GHz, 27-dBm Asymmetric Multilevel Outphasing Power Amplifier in 65-nm CMOS

We present a 2.4-GHz asymmetric multilevel outphasing (AMO) power amplifier (PA) with class-E branch amplifiers and discrete supply modulators integrated in a 65-nm CMOS process. AMO PAs achieve improved modulation bandwidth and efficiency over envelope tracking (ET) PAs by replacing the continuous supply modulator with a discrete supply modulator implemented with a fast digital switching network. Outphasing modulation is used to provide the required fine output envelope control. The AMO PA delivers 27.7-dBm peak output power with 45% system efficiency at 2.4 GHz. For a 20-MHz WLAN OFDM signal with 7.5-dB PAPR, the AMO PA achieves a drain efficiency of 31.9% and a system efficiency of 27.6% with an EVM of 2.7% rms.

Asymmetric multilevel outphasing architecture for multi-standard transmitters

We describe a new outphasing transmitter architecture in which the supply voltage for each PA can switch among multiple levels. It is based on a new asymmetric multilevel outphasing (AMO) modulation technique which increases overall efficiency over a much wider output power range than the standard LINC system while maintaining high linearity. For demonstration, the overall transmitter is simulated in a 65nm CMOS process with HSUPA and WLAN signals. The simulation results show an efficiency improvement from 17.7% to 40.7% for HSUPA at 25.3dBm output power and from 11.3% to 35.5% for WLAN 802.11g at 22.8dBm while still meeting system linearity requirements.

Transmission line resistance compression networks for microwave rectifiers

This work presents a development of multi-way transmission-line resistance compression networks (TLRCNs) and their application to rf-to-dc conversion. We derive analytical expressions for the behavior of TLRCNs, and describe two design methodologies applicable to both single- and multi-stage implementations. A 2.45-GHz 4-way TLRCN network is implemented and applied to create a resistance-compressed rectifier system that has narrow-range resistive input characteristics over a 10-dB power range. It is demonstrated to improve the impedance match to mostly-resistive but variable input impedance class-E rectifiers over a 10-dB power range. The resulting TLRCN plus rectifier system has >50% rf-to-dc conversion efficiency over a >10-dB input power range at 2.45 GHz (peak efficiency 70%), and SWR <;1.1 over a 7.7-dB range.

A highly efficient 1.95-GHz, 18-W asymmetric multilevel outphasing transmitter for wideband applications

A 1.95-GHz asymmetric multilevel outphasing (AMO) transmitter with class-E GaN power amplifiers (PAs) and discrete supply modulators is presented. AMO transmitters achieve improved efficiency over envelope tracking (ET) transmitters by replacing the continuous supply modulator with a discrete supply modulator implemented with a fast digital switching network. Outphasing modulation is used to provide the required fine output envelope control. A 4-level supply modulator is implemented that allows for fast and efficient discrete envelope modulation with up to 28-V supply voltages using low-voltage gate drivers and time-alignment logic. With two class-E GaN PAs that achieve 62.5% power-added efficiency (PAE) at 40- dBm peak output power, the AMO transmitter delivers 42.6- dBm peak output power at 1.95-GHz. For a 16-QAM signal at 36-dBm output power, the transmitter achieves 44.2/42.8/41.4% average system efficiency and 2.0/2.1/3.1% EVM for 10/20/40-MHz channel bandwidth, respectively.

Transmission Line Resistance Compression Networks and Applications to Wireless Power Transfer

Microwave-to-dc rectification is valuable in many applications, including RF energy recovery, dc-dc conversion, and wireless power transfer. In such applications, it is desired for the microwave rectifier system to provide a constant RF input impedance. Consequently, variation in rectifier input impedance over varying incident power levels can hurt system performance. To address this challenge, we introduce multiway transmission line resistance compression networks (TLRCNs) for maintaining near-constant input impedance in RF-to-dc rectifier systems. A development of TLRCNs is presented, along with their application to RF-to-dc conversion and wireless power transfer. We derive analytical expressions for the behavior of TLRCNs, and describe two design methodologies applicable to both single and multistage implementations. A 2.45-GHz four-way TLRCN network is implemented and applied to create a 4-W resistance compressed rectifier system that has narrow-range resistive input characteristics over a 10-dB power range. It is demonstrated to improve the impedance match to mostly resistive but variable input impedance class-E rectifiers over a 10-dB power range. The resulting TLRCN plus rectifier system has >50% RF-to-dc conversion efficiency over a >10-dB input power range at 2.45 GHz (peak efficiency 70%), and standing wave ratio <;1.1 over a 7.7-dB range, despite a nonnegligible reactive component in the rectifier loads.

Multi-Way Lossless Outphasing System Based on an All-Transmission-Line Combiner

A lossless power-combining network comprising cascaded transmission-line segments in a tree structure is introduced for a multi-way outphasing architecture. This architecture addresses the suboptimal loading conditions in Chireix outphasing transmitters while offering a compact and microwave-friendly implementation compared to previous techniques. In the proposed system, four saturated power amplifiers (PAs) interact through an all-TL power-combining network to produce nearly ideal resistive load modulation of the branch PAs over a 10:1 range of output powers. This work focuses on the operation of the combining network, deriving analytical expressions for input-port admittance characteristics and an outphasing control strategy to modulate output power while minimizing reactive loading of the saturated branch amplifiers. A methodology for combiner design is given, along with a combiner design example for compact layout. An experimental four-way outphasing amplifier system operating at 2.14 GHz demonstrates the technique with greater than 60% drain efficiency for an output power range of 6.2 dB. The system demonstrates a W-CDMA modulated signal with a 9.15-dB peak-to-average power ratio with 54.5% average modulated efficiency at 41.1-dBm average output power.

Four-Way Microstrip-Based Power Combining for Microwave Outphasing Power Amplifiers

A lossless multi-way outphasing and power combining system for microwave power amplification is presented. The architecture addresses one of the primary drawbacks of Chireix outphasing; namely, the sub-optimal loading conditions for the branch power amplifiers. In the proposed system, four saturated power amplifiers interact through a lossless power combining network to produce nearly resistive load modulation over a 10:1 range of output powers. This work focuses on two microstrip-based power combiner implementations: a hybrid microstrip/discrete implementation using a combination of microstrip transmission line sections with discrete shunt elements, and an all-microstrip implementation incorporating open-circuited radial stubs. We demonstrate and compare these techniques in a 2.14 GHz power amplifier system. With the all-microstrip implementation, the system demonstrates a peak CW drain efficiency of 70% and drain efficiency of over 60% over a 6.5-dB outphasing output power range with a peak power of over 100 W. We demonstrate W-CDMA modulation with 55.6% average modulated efficiency at 14.1 W average output power for a 9.15-dB peak to average power ratio (PAPR) signal. The performance of this all-microstrip system is compared to that of the proposed hybrid microstrip/discrete version and a previously reported implementation in discrete lumped-element form.

Theory and Implementation of RF-Input Outphasing Power Amplification

Conventional outphasing power amplifier systems require both a radio frequency (RF) carrier input and a separate baseband input to synthesize a modulated RF output. This work presents an RF-input/RF-output outphasing power amplifier that directly amplifies a modulated RF input, eliminating the need for multiple costly IQ modulators and baseband signal component separation as in previous outphasing systems. An RF signal decomposition network directly synthesizes the phase- and amplitude-modulated signals used to drive the branch power amplifiers (PAs). With this approach, a modulated RF signal including zero-crossings can be applied to the single RF input port of the outphasing RF amplifier system. The proposed technique is demonstrated at 2.14 GHz in a four-way lossless outphasing amplifier with transmission-line power combiner. The RF decomposition network is implemented using a transmission-line resistance compression network with nonlinear loads designed to provide the necessary amplitude and phase decomposition. The resulting proof-of-concept outphasing power amplifier has a peak CW output power of 93 W, peak drain efficiency of 70%, and performance on par with a previously-demonstrated outphasing and power combining system requiring four IQ modulators and a digital signal component separator.

Asymmetric multilevel outphasing transmitter using class-E PAs with discrete pulse width modulation

We present a high-efficiency transmitter architecture based on asymmetric multilevel outphasing (AMO), but with a new method of generating discrete amplitude levels from the constituent amplifiers. AMO and multilevel LINC (ML-LINC) transmitters improve their efficiency over LINC by switching the supplies of the power amplifiers (PAs) among a discrete set of voltages. This allows them to minimize the occurrence of large outphasing angles. However, it is also possible to generate a discrete set of amplitudes by varying the duty cycle of the waveform that drives the PAs. The chief advantage of this discrete pulse width modulation (DPWM) is hardware simplicity, as it eliminates the need for a fast, low-loss switching network and a selection of power supply voltages. We demonstrate this concept with a 48-MHz, 20-W peak output power AMO transmitter using a four-level DPWM. At peak output power, the measured power-added efficiency is 77.7%. For a 16-QAM signal with a 6.5-dB peak-to-average power ratio, the AMO prototype improves the average efficiency from 17.1% to 36.5% compared to the standard LINC system.

Transmission-line-based multi-way lossless power combining and outphasing system

This paper presents a non-isolating multi-way outphasing and power combining system that achieves nearly resistive loading of branch amplifiers over the entire output power range through a combiner network comprising only transmission line sections. We derive a design methodology and describe an outphasing control law that selects control angles to minimize the peak susceptive loading of the branch PAs over a specified output power range. The approach is demonstrated in a 2.14-GHz, four-way outphasing amplifier system that achieves >60% drain efficiency over a 6.2-dB output power range.