The Application of Some Semiconductors as Logarithmic Elements

Practical and theoretical considerations involved in the application of semiconductors as passive logarithmic elements are discussed. Simple voltage-divider circuits incorporating semiconductors exhibiting special characteristics are used to obtain output voltages proportional to the logarithms of input voltages, with emphasis on the dc case. Data are presented on the logarithmic response, and temperature dependence of a number of specific types of rectifiers. Reasonably good temperature compensation of such circuits can be achieved with negative temperature coefficient resistors. Circuits having a logarithmic response have been demonstrated for input signals from dc up to 100 kilocycles in frequency.