GB707656A - Improvements in or relating to differential amplifiers - Google Patents

Abstract

707,656. Valve amplifying circuits. GENERAL ELECTRIC CO., Ltd., and BLOCH, A. July 2, 1952 [July 2, 1951], No. 15660/51. Class 40 (6) A differential amplifier for amplifying a small difference of potential between two points whose potentials may vary with respect to a reference, e.g. earth, potential, and comprising two matched valves having their cathode circuits connected to a common point and their grids connected to respective ones of the two points and means responsive to the difference between the anode currents in the two valves, further comprises means for applying a potential to the common point in the cathode circuits such that the gridcathode potential of each valve is kept at a small value and the valves consequently operate over linear portions of their characteristics. As shown in Fig. 1, the differential amplifier comprises valves 1, 2 to which the potentials at the points 6 are applied. The valves operate as cathode followers to provide an input to the valve 7 whose anode current is measured by the meter 8, the tapping on resistor 3 being so adjusted that this anode current is dependent solely on the difference between the anode currents of valves 1 and 2. The cathode resistors 3, 4 are connected together and to the cathode of a further valve 9 whose grid is also connected to one of the points 6. The common point to which the resistors 3, 4 are connected is consequently maintained at a potential close to that of the grids of the valves 1 and 2 so that these valves operate in a linear fashion independently of the difference of potential between the points 6 and earth. In Fig. 2 (not shown) is illustrated a modification comprising a further valve whose control grid is connected to that of valve 1, the common point of resistors 3, 4 being connected to the mid-point of a potential-divider circuit connected between the cathodes of the further valve and of valve 9 In Fig. 3 the differential amplifier is used as a null detector for an A.C. bridge, the common point of the resistors 3, 4 being connected to a point E between two impedances connected across one diagonal AC of the bridge and so chosen that the potentials at D and E are substantially equal.