AN Improved Solid-State Logarithmic Amplifier

A logarithmic amplifier is described which is composed entirely of solid state components. Logarithmic characteristics can be improved by the compensation of low saturation current Si-diodes with an appropriate Ge-diode. By the use of reverse-biased Si-diode dielectric modulator as the input stage of the amplifier, an input resistance of 5 × 1010 ohms and a signal threshold of 10-11 amp. are obtained at room temperature. A range of 7 decades is covered, that is, from 5 × 10-11 to 5 × 10-4 amp., with the accuracy less than ±2% of a decade and the temperature drift less than ±0.005 dec./°C over a temperature range from 0°C to 50°C.