A Vacuum-Tube Voltmeter with Logarithmic Response

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Abstract
A new vacuum‐tube voltmeter which has a response proportional to the logarithm of the input voltage is described. The operation of the device is based upon the distortion of the plate‐current wave form occurring in a tube of the ``variable‐mu'' type, and upon the use of a diode rectifier to select only that portion of the wave which has been ``compressed'' by the curvature of the tube characteristic. The elementary circuit may be cascaded for extension of the range of input voltage over which the response is logarithmic. The speed of response of the device to changes in signal intensity is practically unlimited, and the frequency fidelity is only slightly inferior to that of a comparable impedance‐coupled amplifier designed for the same frequency band. Commercially available tubes are used throughout.

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