Scott Schafer

Codesign of PA, Supply, and Signal Processing for Linear Supply-Modulated RF Transmitters

This paper presents a method for achieving high-efficiency linear transmitters by codesign of the RF power amplifier (PA), dynamic supply, and signal processing. For varying amplitude signals, the average efficiency of the PA is improved by adding a supply modulator with requirements derived from nonstandard PA modeling. The efficient PA and supply modulator both introduce signal distortion. A targeted linearization procedure is demonstrated with reduced complexity compared to standard digital predistortion. Experimental results on a 2.14-GHz 81% efficient 40-W peak power GaN PA illustrate the codesign method by achieving 52.5% composite power-added efficiency with high linearity for a W-CDMA signal with a 23-MHz supply modulator bandwidth.

Sex differences in extinction recall in posttraumatic stress disorder: A pilot fMRI study

Recent research has found that individuals with posttraumatic stress disorder (PTSD) exhibit an impaired memory of fear extinction compounded by deficient functional activation of key nodes of the fear network including the amygdala, hippocampus, ventromedial prefrontal cortex (vmPFC) and dorsal anterior cingulate cortex (dACC). Research has shown these regions are sexually dimorphic and activate differentially in healthy men and women during fear learning tasks. To explore biological markers of sex differences following exposure to psychological trauma, we used a fear learning and extinction paradigm together with functional magnetic resonance imaging (fMRI) and skin conductance response (SCR) to assess 31 individuals with PTSD (18 women; 13 men) and 25 matched trauma-exposed healthy control subjects (13 women; 12 men). Whereas no sex differences appeared within the trauma-exposed healthy control group, both psychophysiological and neural activation patterns within the PTSD group indicated deficient recall of extinction memory among men and not among women. Men with PTSD exhibited increased activation in the left rostral dACC during extinction recall compared with women with PTSD. These findings highlight the importance of tracking sex differences in fear extinction when characterizing the underlying neurobiological mechanisms of PTSD psychopathology.

X-band MMIC GaN power amplifiers designed for high-efficiency supply-modulated transmitters

The design and measured performance of X-band power amplifier MMICs that utilize 0.15μm GaN on SiC process technology are presented. Under continuous wave operating conditions these single and 2-stage MMICs demonstrate peak power added efficiencies (PAE) from 45% to 69%, output powers from 2.5-13W, and up to 20dB of large signal gain. Designed for drain modulated applications, the power amplifiers maintain good performance at reduced drain bias voltage. The output power of the two stage MMIC can be varied from 2W to 13W when the drain bias is varied between 7.5V and 20V while maintaining a PAE above 54%.

High-Efficiency X-Band MMIC GaN Power Amplifiers Operating as Rectifiers

This paper presents a performance evaluation of GaN X-Band power amplifiers operating as self-synchronous rectifiers. Two single-stage MMIC power amplifiers are characterized under continuous wave conditions at 10.1GHz. One PA is designed with a single 10 × 100μm HEMT in a 0.15μm GaN process, while the other contains two 10 × 100μm power-combined devices. The MMICs exhibit 67% and 56% power added efficiency at VDD = 20V in deep class-AB bias, respectively. In rectifier mode, biased in class-C, the same MMICs show a RF-to-DC efficiency of 64%. The output powers of the two MMIC PAs are around 3.2W. In rectifier mode, the gate DC bias and the load-pull determined RF gate impedance are set for optimal efficiency. The DC load does not affect the efficiency substantially, and can be chosen for a desired voltage or current. The paper demonstrates that high-power efficient GaN rectifiers can be achieved by designing high-efficiency PAs at least up to X-band.

Characterization and reduction of cardiac- and respiratory-induced noise as a function of the sampling rate (TR) in fMRI

It has recently been shown that both high-frequency and low-frequency cardiac and respiratory noise sources exist throughout the entire brain and can cause significant signal changes in fMRI data. It is also known that the brainstem, basal forebrain and spinal cord areas are problematic for fMRI because of the magnitude of cardiac-induced pulsations at these locations. In this study, the physiological noise contributions in the lower brain areas (covering the brainstem and adjacent regions) are investigated and a novel method is presented for computing both low-frequency and high-frequency physiological regressors accurately for each subject. In particular, using a novel optimization algorithm that penalizes curvature (i.e. the second derivative) of the physiological hemodynamic response functions, the cardiac- and respiratory-related response functions are computed. The physiological noise variance is determined for each voxel and the frequency-aliasing property of the high-frequency cardiac waveform as a function of the repetition time (TR) is investigated. It is shown that for the brainstem and other brain areas associated with large pulsations of the cardiac rate, the temporal SNR associated with the low-frequency range of the BOLD response has maxima at subject-specific TRs. At these values, the high-frequency aliased cardiac rate can be eliminated by digital filtering without affecting the BOLD-related signal.

High-efficiency X-band MMIC GaN power amplifiers with supply modulation

This paper presents measurement results on supply-modulated X-band 0.15μm gate width GaN HEMT MMIC power amplifiers for OFDM signals. Two PAs at 10GHz with 4 and 10W output powers show peak CW efficiencies of 69% and 55%, with gains of 8.5 and 20.4 dB respectively. Supply modulation trajectories are designed by static characterization of each MMIC PA, and the drain modulation is performed though both a linear broadband modulator and a high-efficiency 5-MHz switching modulator. The PAs are tested in four modes: (1) with a constant 20-V supply and the 18-MHz OFDM signal input through the drive; (2) pure envelope tracking; (3) signal-split supply modulation; and (4) envelope elimination and restoration (EER) with a 5-MHz switching modulator. Average PAE and composite power added efficiencies are compared, reaching 65.4% and 35%, respectively, with the 4-W PA and a linear modulator.

X-band wireless power transfer with two-stage high-efficiency GaN PA/ rectifier

A 2-stage X-band GaN MMIC operating as a power amplifier and rectifier is measured in a wireless power transfer link. The PA operates at 9.9 GHz in class-AB and achieves 10W of output power and >20 dB of gain. As a rectifier, the MMIC achieves over 52% RF-DC conversion efficiency at a power level of >8W. In a wireless powering link at a distance of 5 cm, the system achieves 10% DC-DC efficiency. The applications are bi-directional high power directional wireless power transfer (WPT).

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Partial Amelioration of Medial Visceromotor Network Dysfunction in Major Depression by Sertraline.

Objectives Major depression is associated with reduced cardiac vagal control, most commonly indexed by heart rate variability. To examine the dynamics of this abnormality, we examined within-participant covariation over time between brain activity, cardiac vagal control, and depressive symptoms in depressed patients treated with sertraline and in healthy volunteers. Methods Patients with depression and nondepressed control participants were enrolled in a 12-week protocol. After Week 0 assessment, patients began treatment with sertraline. Neural activity and vagal control were measured for all participants at Weeks 0, 2, 6, and 12 using functional magnetic resonance imaging and synchronized electrocardiographic recordings. At each of the four assessments, a moving window analysis was used to estimate vagal control as assessed by respiratory sinus arrhythmia (RSA) from the electrocardiographic data, which was then regressed onto functional magnetic resonance imaging activity. Results At baseline, patients showed reduced blood oxygen level–dependent RSA covariation compared with controls within multiple a priori brain regions associated with vagal control, collectively described as the medial visceromotor network (MVN). Sertraline treatment led to a significant increase in brain-RSA covariation for patients compared with controls, despite a lack of improvement in mean RSA. Conclusions These data suggest a partial normalization of MVN dysfunction in depression during sertraline treatment. Specifically, results indicate a partial recovery of MVN function. However, this recovery was insufficient to cause a significant change in RSA levels. These results may help to explain both improvements with and limitations of sertraline treatment of depression.

Multi-Frequency Measurements for Supply Modulated Transmitters

Transmitters for high peak-to-average power ratio communication are increasingly using supply modulation to improve efficiency. In addition to a dc component, the dynamic supply may contain ac components up to 500 MHz. The low-frequency (LF) dynamic impedance of the supply terminal of a power amplifier (PA) is often unknown and available nonlinear transistor models are unable to predict dynamic LF effects required for design of wideband efficient supply modulators (SMs). This paper describes a technique to calibrate and measure multi-port multi-frequency parameters of a transistor and PA under supply modulation conditions. The measurement setup is used to characterize the complex drain impedance of GaN transistors and PAs in large-signal operation at X-band with 1-500-MHz LF excitation on the drain terminal, over a range of input power levels. In addition, the LF drain impedance of a 10-GHz monolithic microwave integrated circuit PA with 4-W output power and 60% peak power-added efficiency is measured when the PA is connected to a simple switched resonant SM. The main motivation for this work is to obtain knowledge of the dynamic supply-port impedance that can enable improved PA and SM co-design.