1,721,058 research outputs found
Topologie e metodologie di progetto di blocchi di elaborazione analogica di precisione in tecnologia a canale corto
No full textTopologies and design methodologies for high precision analog processing blocks in short-channel technologies. With the explosive growth of battery-powered portable devices, power reduction in integrated circuits has become a major problem. In many of these portable systems the signal is processed in the digital domain by limiting the role of the analog part to interface circuits between analogous physical quantities and digital processing.
Having analog circuits operating at the same voltage as the digital ones means that I can integrate on the same chip front-end and digital processing functions without the need for additional interface circuits, reducing the overall cost of the system. Another reason that pushes to low supply voltages is given by technological considerations, for sub micrometric channel lengths the thickness of gate oxide becomes so slim that it has been forced to reduce the supply voltage to avoid effects like breakdown of the oxide of gate. With the reduction of the supply voltage there is a consequent reduction in the dynamic of the input signal. To maintain the same dynamic range with a lower power supply voltage, the thermal noise in the circuit must also be proportionally reduced. Therefore capacitors used in the circuit must be increased to lower the KT/C noise. Therefore, for operational amplifiers that have the task of driving larger loads and for high resolution applications, doing it becomes even more difficult. There is, however, a compromise between noise and energy consumption. Because of this strong compromise, under certain conditions, energy consumption will actually increase in proportion to the decrease in power supply [1].
Another aspect that poses a significant problem to the reduction of consumption is the fact that battery technology is currently progressing at a much slower pace than that of electronic circuits. Nowadays, many electronic systems work with the power supplied by batteries alone, in some cases this problem is the most critical feature of the device, just think of networks of wireless sensors or implantable devices in the human body.
There are also many switched-capacitor applications that require fast signal transitions and certain performance in given times. If I use a class A amplifier it would always be on even in the times when it is not necessary, which leads to considerable consumption. In this context, a possible solution can be represented by class AB transconductance amplifiers (OTA), which have the advantage of consuming a small current in quiescent condition and providing a large peak current when a large signal is applied. This peculiarity can be exploited to achieve fast settling times with low average power consumption. In many applications, such as active filters, Sample and Hold Amplifiers (SHA), pipeline ADCs, the use of fully differential amplifiers is required to exploit the advantages offered by differential signals, such as doubled dynamic range compared with that provided by a given voltage generator, low sensitivity to common mode disturbances and even order harmonics suppression. Various ways of achieving class AB OTAs are proposed in the literature. In all fully differential type implementations, there is a need for auxiliary circuits for controlling common mode output voltage (CMFB). These additional circuits introduce into the project additional constraints and static power consumption compared to the basic topologies of class AB amplifiers. This line of research has driven me to focus on two main topics, closely related to the aforementioned issues:
1- Low-voltage and Very-low-voltage design of Class AB OTAs blocks for S/H;
2- Study of a behavioral modeling of Class AB OTAs;
This work is divided into five chapters.
Chapter 1 is an introduction to the state of the art of Class-AB OTAs. From the assessment of the state of the art there will emerge various ways to realize class-AB "OTAs" but only those that comply with certain constraints will be taken into consideration. The topologies chosen and on which the studies will be conducted will be those that will show the best performance in terms of power consumption, and that are implemented in according to the symmetrical current mirror OTAs.
Chapter 2 is an introduction to figures of merit (FOMs) that will be used to characterize OTAs from a performance point of view. Also in chapter 2 the study and comparison of four topologies emerged from the state of the art evaluation will be conducted. These topologies will be compared from the point of view of consumption and performance of both small and large signal using the FOMs. From the comparison the one will be selected that has the best performances from the point of view of power consumption, bandwidth and large signal behavior. This topology that is preferred over the other choices will be shown to have a performance limit linked to the low value of the CMRR.
Chapter 3 regards the improvement of the performance of the topology chosen to make it the most performing of the state of the art. Three possible methods will be proposed to increase the CMRR of the structure with little impact on consumption but without altering the low signal performance. The first two will be based on open loop techniques while the latter on a closed loop technique.
Chapter 4 regards the design of other topologies with the aim of improving their performance.
Chapter 5 regards behavioral modeling of a class AB OTA. Given that there are no guidelines in the literature on how to design class AB OTAs, a model will be proposed in this chapter, in its alpha phase, with the intent to understand how some parameters are linked to the settling time in similar way as to how is done for class A amplifier
Cholesterol and triglycerides of the liver after administration of a chromatographic fraction of thymus: variations in tissue and blood.
No full textIn rats fed on a fat diet and injected with a chromatographic thymus fraction, intrahepatic distribution of cholesterol and triglycerides have been studied. The results show that the hematic decrease of these compounds, always observed in previous works, is accompanied by a smaller infiltration in hepatocytes and the lipidic drops are more finely dispersed. These results lead us to suppose that liver is a target organ of the thymus fraction
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Lateral bending of tapered piezo-semiconductive nanostructures for ultra-sensitive mechanical force to voltage conversion
No full textQuasi-1D piezoelectric nanostructures may offer unprecedented sensitivity for transducing minuscule input mechanical forces into high output voltages due to both scaling laws and increased piezoelectric coefficients. However, until now both theoretical and experimental studies have suggested that, for a given mechanical force, lateral bending of piezoelectric nanowires results in lower output electric potentials than vertical compression. Here we demonstrate that this result only applies to nanostructures with constant cross-section. Moreover, though it is commonly believed that the output electric potential of a strained piezo-semiconductive device can only be reduced by the presence of free charges, we show that the output piezopotential of laterally bent tapered nanostructures, with typical doping levels and very small input forces, can be even increased up to two times by free charges.
Our results confirm that lateral bending is likely not optimal for piezoelectric energy harvesting, but also suggest that lateral bending of tapered nanostructures with typical doping levels can be ideal for transducing tiny input mechanical forces into high and accessible piezopotentials. Our results provide guidelines for designing high performance piezo-nano-devices for energy harvesting, mechanical sensing, piezotronics, piezo-phototronics, piezo-controlled chemical reactions, and more
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