GB669064A - Thermionic direct current amplifiers - Google Patents

Abstract

669,064. Valve amplifying circuits. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AERONAUTIQUES. April 29, 1949 [April 30, 1948; April 28, 1949], Nos. 11460/49 and 11461/49. Class 40 (vi). A direct current amplifying arrangement for the double-polarity output signal from an impedance bridge comprises a valve whose anode current energizes the bridge and which is periodically cut-off by rectangular switching pulses for short correction periods, a direct current amplifier and a zero drift correction circuit connected in parallel with the output meter of the arrangement and having gating means operated by pulses coincident with the switching pulses to cause the output of the amplifier during correction periods to charge a condenser connected to an electrode of an amplifier valve so as to reduce the zero drift and to cut off a valve in the charging circuit between correction periods. As shown, Fig. 1, the elements 6, 7, 8, 9 of a strain-gauge bridge are energized by the anode current of a valve 2, which is cut off during correction periods by rectangular pulses applied to its grid. The D.C. output of the bridge is fed to the first stage of a push-pull amplifier. During correction periods, any change in the anode voltage of a valve 37 in the last stage causes a condenser 59 to charge through valve 50. A second valve 54, in shunt to condenser 59, provides a discharge path to prevent valve 50 being cut off. The voltage appearing across condenser 59 is applied to the grid of valve 60, the cathode of which feeds a correcting voltage to the anode of one valve 21 of the first stage. During measurement periods the valves 50, 54 are cut off by the flow of anode current in the sections of double valve 42, which are cut off during correction periods by rectangular pulses applied to the grids. In a modification, Fig. 3 (not shown), correcting voltages are applied to both valves of the first stage, from two opposed condensers, each with an individual charging valve controlled by the anode voltage of one valve of the final stage. In this case, it is unnecessary to provide a discharge path for each condenser. Specifications 620,140, and 633,551 are referred to.