GB1020960A - Electronic measuring apparatus with minimized quadrature components - Google Patents

Abstract

1,020,960. Bridge measuring circuits. SHEFFIELD CORPORATION. June 11, 1964 [June 14, 1963], No. 24299/64. Heading G1U. In a measuring circuit in which a null indication is observed through an amplifier and in which it is sufficient to observe the inphase component only, the quadrature component of the amplifier output is observed, shifted 180 degrees in phase and fed back to the amplifier input. In the circuit of figure 3 capacitors 32, 34 are compared with the aid of a ratio transformer 38 in a bridge circuit. The bridge output, however, will have a component in quadrature with the supply (due, for example, to losses in the transformer) as well as the inphase component which it is desired to reduce to zero. Consequently at low values of inphase signal, when high amplification is required, the amplifier 40 will be saturated by the quadrature current. The amplifier output is fed to a phase sensitive null detector 41 and, after shifting 90 degrees, to a quadrature detector 44 which feeds a D.C. signal at X, X<SP>1</SP> to the auto-nulling circuit 46. The auto-nulling circuit (figure 4, not shown) receives the main bridge supply at Z, Z<SP>1</SP> which drives transistors 60, 62 in emitter follower connection to feed the variable gain amplifier stage comprising transistors 64 and 66. The gain depends on the emitter voltages at transistors 64 and 66. The D.C. signal at X, X<SP>1</SP> is connected to the bases of transistors 68 and 70 which are in emitter follower configuration and drive transistors 72 and 74. Transistors 76 and 78, which are driven by transistors 72 and 74 control the emitter current of transistors 64 and 66 and hence the amplitude of the output signal which passes through the emitter follower formed by transistors 80, 82 and the summing circuit 84. The output is then shifted 90 degrees at 86 and fed to the amplifier input with the bridge error signal through the summing network 88.