A differential-input,differential-output current mode operational amplifier

A current operational amplifier with differential input and differential output is described. The amplifier is based on the parallel connection of a CCII+ current conveyor and a CCII? current conveyor followed by a differential output transconductance gain stage. The performance of the amplifier is analysed and experimental results obtained from an implementation using standard operational amplifiers and current mirrors realized using transistor arrays are presented and compared to the theoretical analysis. It is concluded that the static small signal open loop gain and the frequency response matches the performance of conventional voltage operational amplifiers. The input offset and bias errors and the common mode rejection are shown to be strongly dependent on the matching accuracy of the current mirrors used in the conveyors. The proposed configuration can easily be integrated into a monolithic amplifier in either CMOS or bipolar technology.     

Short-Time Creep Transition for Hardened Cement Paste

Short-time creep recovery tests in two-point bending were performed on hardened cement paste beams water-cured at #100° and 25°C. The experimental variables were pore structure, equilibrium moisture content, and temperature. The recovery curves were analyzed using linear viscoelastic concepts, which revealed a transition in the time range 0.2 to 10s which is both linear with stress and reproducible. A lognormal distribution of retardation times characterizes the transition. It is tentatively proposed that internal redistribution of capillary water is responsible for the transition. Experimental observations which support this view are: (1) The size of the transition increases with increasing amounts of capillary water for saturated pastes; (2) the transition strength decreases greatly when the capillary water is removed; (3) the apparent activation energy of the transition (3.8 kcal/mol) agrees well with that for viscous flow of water.     

From mixed to multiple land use

This introduction paper to the special issue on multiple land use of Journal of Housing and the Built Environment provides an overview of the differences between the concepts of mixed land use and multiple land use. Multiple land use is a successor of mixed land use concept but is not yet very well well-defined. It covers more dimensions and types of land use than mixed land use, and emphasises the institutional dimension of decision-making. These different dimensions are used to analyse a large multiple land use project in Amsterdam in the Netherlands. This project called the Zuid As (South Axis) is introduced in this paper.