A wide-range logarithmic electrometer with improved accuracy and temperature stability

A seven-decade temperature-compensated logarithmic electrometer is presented. The amplifier is designed for a dynamic range of 1 pA to 10 μA. Temperature compensation is accurately achieved by using an array of four matched monolithic bipolar transistors and straightforward postprocessing techniques. Use of this method results in <1% error over the temperature range of -18 to 71°C for the upper five decades of input current. In addition, errors resulting from variable emission coefficients of the logarithmic elements are eliminated. Problems associated with the temperature compensation of logarithmic amplifiers are identified and discussed. Implementation of the new temperature-compensation technique is fully described and compared with conventional methods. Selection of devices and associated temperature characterization are presented. A complete stability analysis of the amplifier is performed and verified with circuit simulations and laboratory testing. Experimental results and data analysis illustrating the performance of the design over temperature are presented.