A Tuned Low-Frequency Amplifier for use with Thermocouples

A system is described for the detection of very small signals of heat radiation. The output of a suitable thermocouple, receiving radiation `chopped' at 5 c/s, is stepped up by a transformer and applied to an amplifier tuned to the `chopping' frequency. A narrow band-width (0.63 c/s) and special design of the first stage of amplification are employed to keep the amplifier noise level low. The equivalent noise input resistance of the system referred to the transformer primary terminals is only 1 or 2 Ω, so that the Johnson noise inherent in the thermocouple resistance normally limits the minimum signal which may be detected. The final amplified signals are rectified by a mechanical switch; this has certain advantages over more conventional electronic circuits.