An ultrahigh-speed GaAs MESFET operational amplifier

A wide-bandwidth GaAs MESFET operational amplifier is reported, with a 65-dB DC gain and a 20-GHz gain-bandwidth product at 500 MHz. The circuit uses a variety of local feedback techniques to enhance the overall gain. The use of an undoped GaAs buffer, grown at a relatively low temperature (≈300°C), eliminates backgating and light sensitivity. The circuit was fabricated in an 80-GHz f_T MESFET process, with 0.2-μm electron-beam defined gates. The high levels of 1/f noise, MESFET frequency-dependent output conductance, and large offset voltage standard deviation limit the application of the circuit to moderate precision applications     

Displacement of residual nonwetting fluid from porous media

Percolation theory of transport in random composites is used to explain the correlation between the residual saturation of nonwetting phase in porous media after displacement by a wetting phase and the capillary number, this number being a measure of the ratio of Darcy-law viscous force in the wetting liquid to interfacial tension force in curved menisci between the two phases. Statistical concepts of percolation theory give estimates of the length distribution of blobs created when the nonwetting phase loses continuity because of displacement by the wetting phase. These estimates agree with the few experimental data. Simple blob mobilization theory and experiments establish that the capillary number required to mobilize a blob is inversely proportional to its length in the direction of the Darcy-law pressure gradient; this and the predictions of percolation theory account for the observed capillary number correlation.     

The FT-30 seawater reverse osmosis membrane--element test results

A thin-film composite reverse osmosis (RO) membrane has been developed that provides excellent performance when used in single-pass seawater systems. This membrane, designated FT-30, gives superior performance in such areas as flux, salt rejection, chemical stability, and microbiological resistance. In addition, it is resistant to some oxidizing disinfectant agents, although it is not fully resistant to chlorine; elements fabricated from the membrane have shown damage after 2,000 to 2,500 hours exposure to seawater at 0.5 milligrams per liter (mg/l) active chlorine. It appears that the membrane can operate continuously at active chlorine concentrations below 0.2 mg/l.Several long-term studies were carried out at the OWRT Wrightsville Beach Test Facilities. In actual trials in seawater systems, salt rejections as high as 99.5 percent and fluxes of 23 gfd have been obtained when tested at 800 psi and 25°C. Some of the elements have been in continuous operation for over 4, 000 hours with no deterioration in salt rejection and with only the normal flux decline usually associated with compaction and scaling. Other elements have been installed in small seawater systems with normal intermittant use for almost two years with no serious loss in performance.     

A Modified Ebers-Moll Transistor Model for RF-Interference Analysis

This paper develops analytical techniques for the study of nonlinear RF and microwave effects in semiconductor devices. Rectification in p-n junctions is discussed, and a novel large-signal transistor model is developed, based upon modifications to standard Ebers-Moll formulations for bipolar transistors. Use of the models in worst-case analysis is discussed, with ranges of parameters given based on a simplified analysis of rectification in ideal diodes.     

Guanidinium-based potentiometric SO2 gas sensor

An SO2 gas sensor was developed by using a hydrogen sulfite-selective electrode positioned behind a gas-permeable membrane (GPM). The hydrogen sulfite-selective electrode was prepared by incorporating a multicyclic guanidinium ionophore in a plasticized poly(vinyl chloride) membrane. This gas sensor presents important advantages over the conventional Severinghaus-type SO2 gas sensor that contains a pH electrode immersed in an internal solution behind the GPM. The Severinghaus gas sensor suffers interferences from weak acids that can cross the GPM as gases and change the pH of the internal solution. In contrast, in the proposed sensor, the excellent selectivity of the HSO3- electrode and the ability of the GPM to discriminate gaseous from nongaseous species combine to generate the most selective potentiometric SO2 gas sensor reported to date.     

Unfolding of Plasmodium falciparum triosephosphate isomerase in urea and guanidinium chloride: evidence for a novel disulfide exchange reaction in a covalently cross-linked mutant

The conformational stability of Plasmodium falciparum triosephosphate isomerase (TIMWT) enzyme has been investigated in urea and guanidinium chloride (GdmCl) solutions using circular dichroism, fluorescence, and size-exclusion chromatography. The dimeric enzyme is remarkably stable in urea solutions. It retains considerable secondary, tertiary, and quaternary structure even in 8 M urea. In contrast, the unfolding transition is complete by 2.4 M GdmCl. Although the secondary as well as the tertiary interactions melt before the perturbation of the quaternary structure, these studies imply that the dissociation of the dimer into monomers ultimately leads to the collapse of the structure, suggesting that the interfacial interactions play a major role in determining multimeric protein stability. The Cm(urea)/Cm(GdmCl) ratio (where Cm is the concentration of the denaturant required at the transition midpoint) is unusually high for triosephosphate isomerase as compared to other monomeric and dimeric proteins. A disulfide cross-linked mutant protein (Y74C) engineered to form two disulfide cross-links across the interface (13-74') and (13'-74) is dramatically destablized in urea. The unfolding transition is complete by 6 M urea and involves a novel mechanism of dimer dissociation through intramolecular thiol-disulfide exchange.