A Low-Frequency Amplifier with Fractional-Ohm Input Impedance†

Publisher Website
Google Scholar
Abstract
A method of amplification of d.c. and low frequency (several hundred cycles/sec) potentials arising in sources of very low internal impedance is described. The essential element is a two-button carbon microphone which is excited by a sound wave of constant amplitude and which in series with the potential source takes the place of the interrupter in the well-known arrangements designed to amplify chopped ‘d.c’. potentials. It is found that the a.c. output of the microphone under these conditions is strictly linear within a range of applied potentials from a fraction of one volt down to potentials in the neighbourhood of the thermal - circuit noise. The input circuit is matched to the grid of the first amplifying valve by means of a tuned transformer wound on a ferrite core. Input impedances as low as 1/10 of one ohm have been obtained. The method is being used to investigate transients of some tens of milliseconds duration in thermocouples of some tens of milliohms internal resistance.

This publication has 0 references indexed in Scilit: