Browse Prior Art Database

Test Circuit

IP.com Disclosure Number: IPCOM000098607D
Original Publication Date: 1959-Jun-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Ross, HN: AUTHOR

Abstract

The circuit provides indication of the magnitude of an electrical potential resulting from an extremely minute current flow. The device is used to measure the contact potential between the control grid and cathode of a vacuum tube T, which is inherently an existing potential caused by differences in the chemical composition of the electrodes.

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Test Circuit

The circuit provides indication of the magnitude of an electrical potential resulting from an extremely minute current flow. The device is used to measure the contact potential between the control grid and cathode of a vacuum tube T, which is inherently an existing potential caused by differences in the chemical composition of the electrodes.

The output E(O) of a high gain, chopper stabilized differential amplifier, which comprises a summing stage A(1) and a chopper stage A(2), is directed to a voltmeter V and a voltage dividing network consisting of resistors R(1) and R(2). The voltmeter V is a conventional zero-center digital volt meter with, for example, a maximum reading of 100 volts full scale. The The values of R(1) and R(2) are selected so that a fractional part of the output voltage E(O) of the differential amplifier, such as 1/100, appears at point X. The tube T, under test, is inserted in the circuit so that its grid to cathode path and resistor R(3) are in parallel with resistor R(2).

Contact potential, in conventional testing methods, has been defined, for comparative purposes, as the potential required to establish 0. 1 microampere of current flow between the grid and the cathode of a tube under test.

If the values of the resistors in the illustrated embodiment are selected so that R(1) is 9900 ohms, R(2) is 100 ohms and R(3) is 1000 ohms, and if the differential amplifier has a high gain, such as 5000, for example, so that feedback erro...