GB565378A - Improvements in or relating to electric circuits incorporating resistance strain gauges - Google Patents

Abstract

565,378. Thermionic valve voltmeters. AUGHTIE, F. April 22, 1943, No. 6517. [Class 40 (v)] [Also in Group XXXVI] An amplifier having negative feed-back is used in conjunction with an electrical resistance strain gauge so that the effective amplification becomes inversely proportional to the gauge resistance. This results in an amplifier output which is substantially proportional to strain irrespective of stain gauge resistance, so that any one of a series of gauges of the same order of resistance can be used to give a direct indication of strain on the same scale. Fig. 1 shows a two-valve amplifier in which a strain gauge resistance R1 is connected in the gridcircuit of the first valve V1 and is fed from the cathode of the second valve V2 (which operates as a cathode follower) through a resistance R2. Resistances R3, R4 are included in the anode and cathode circuits of the valve V1. A meter M is connected between the cathode of the valve V2 and the junction of two resistances R5, R6 connected across the A.C. supply, or it may be connected directly in the cathode circuit. The instrument M gives the same reading for the same strain irrespective of the gauge resistance. A change of gauge resistance, however, alters the zero setting and this must be corrected by altering the bias of the valve V1 by means of a resistance R7. The resistance R4 may serve as a compensating gauge to correct for changes of temperature of the gauge R1. In the circuit shown in Fig. 2, the main and compensating gauges R8, R9 are connected in the grid circuits of two valves V3, V4 operating in push-pull. The anodes are connected, through an intermediate stage V5, V6, to the output stage V7, V8. As the extra stage causes a reversal of phase the feed-back connections are crossed, so that the valve V7 feeds through a resistance R11 to the resistance R9, and the valve V8 feeds through a resistance R10 to the resistance R8 and valve V3. This provides negative feed-back with respect to differential changes of voltage between the grids of the valves V3, V4. In order to ensure stability, a common cathode resistance R14 is connected to the cathodes of the valves V3, V4 so as to reduce the voltage gain of both valves for voltage changes which are in phase, whilst being without effect on the gain of voltages which are out of phase. A common cathode resistance R18 to the valves V5, V6 similarly controls the operation of the second stage. The screen grids of the valves V3, V4 are supplied from the cathodes of the valves V5, V6 to reduce the amplification of in-phase changes. If the screen-grid current of the valves V3, V4 is less than the cathode current of the valves V5, V6, a shunt resistance R19 is connected as shown. A slider resistance R15 provides for zero adjustment. Resistances R20 R21, R22, R23 are provided for inhibiting parasitic high-frequency .oscillations.