Patent GB-999483-A (GB)
Title: Improvements relating to instrumentation amplifiers
IPC: H03F3/387
Filed: 1961-12-11
Abstract: 999,483. Potentiometric voltage measurement. CONSOLIDATED ELECTRODYNAMICS CORPORATION. Dec. 11, 1961, [Dec. 9, 1960]. Heading G1U. [Also in Divisions H3 and H4] In an amplifier, the amplitude of a direct current input signal is compared with that of a square wave feedback signal to produce a square wave error signal, from which, after amplification, the feedback signal is derived. Referring to Fig. 1, D. C. signals derived from a thermocouple 40 are fed into a comparator unit 11 together with a feedback signal on line 44. The output signal is fed via square wave amplifier 12 to a demodulator and D. C. amplifier 13 together with a reference square wave from generator 32 to produce a D. C. output signal at 50. This D.C. is modulated with the reference square wave in the translating circuit 14 to produce the feedback signal fed to the comparator 11, and also an A. C. output which is fed to a pen recorder 20. A further output from the reference generator is also fed to the comparator to provide zero suppression and add junction compensation facilities. The comparator circuit 11, Fig. 2a, comprises a transistor 103, operated in an inverted manner, between the collector and emitter of which the D. C. input signal is applied in series with the feedback signal obtained from the secondary of transformer 106, and the zero suppression signal from the secondaries 115, 120 of transformer 116. The transformer 116 is also provided with a cold junction compensation winding 125 across which is connected a potential divider comprising a zero temperature coefficient resistor 126 and a positive temperature coefficient resistor 127, the output from the potential divider being connected in series with the previously mentioned signals. A further transformer 131 having its primary connected to the reference signal provides an output which is connected between base and collector of transistor 103 to switch it on and off. During the "ON" periods the D. C. signal is compared with the sum of the square wave signals connected in series with it to produce a square wave error output at the secondary of transformer 102. Coarse and fine range and zero adjustments are made by adjusting tappings on the secondaries 108, 115 and potentiometers 109, 121. Square wave demodulator and D.C. amplifier 13. After amplification in the amplifier 12, the square wave signal is fed via capacity 150 to the collector of transistor 151, to the base of which is applied the reference square wave to switch the transistor on and off. During the "ON" period capacitor 150 charges to earth, and during the OFF period, it charges capacitor 155. The potential produced across this capacitor is amplified and smoothed to provide a D. C. output (if required) and also the input for a square wave modulator 14. The square wave modulator 14, comprises a series transistor 170, and a shunt transistor 172, both of which are operated in an inverted manner, with the switching reference square wave connected between base and collector. The switching reference square wave is fed via a centre tapped widing on transformer 176 so that the two transistors are switched ON and OFF alternatively. The resulting square wave output is fed to the comparator 11 as the feedback signal applied to transformer 107, and also to the recorder 20 on line 44. The recorder 20, Fig. 1, employes a null balance system in which the square wave signal on line 44 is compared with a square wave representing the actual position of the recorder to produce an error signal which is amplified in amplifiers 21, similar to amplifier 12, Fig. 2a, and applied to the rotor coil of a moving coil pen recorder 23 via pen drive amplifier 22. Mechanically coupled to the recorder is a position indicating device 30 to the stator coils 77, 78 of which is applied a regulated square wave derived from the reference square wave. The output from the rotor 79 depends on its angular position, and is fed back to the input to the square wave amplifier 21. A potential divider 80, 81, 82 is connected across the regulated square wave, and from this a further square wave is developed which is connected in series with that from the rotor 79 to provide zero and range setting of the indicator. The pen drive amplifier 22, (Fig. 2b,) comprises a balanced demodulator in which the square wave signal output from amplifier 21 is applied to the junction of two capacitors 220, 221 connected to the collectors of transistors 222, 223. The reference square wave is applied to the bases in pushpull to switch the transistors on and off alternatively connecting the capacitors 220, 221 to earth. Unidirectional potentials are developed across capacitors 226, 227, diodes 228, 229 producing voltage doubler action. After amplification by transistors 230, 231 these potentials are applied to opposite ends of the rotor coil 63, 64 of the recorder so as to rotate the coil from its midposition independence on the relative magnitudes of these potentials. Regulated square wave. In order to obtain a regulated square wave of good wave shape, the reference square wave is applied to the base of transistor 192 in the comparison circuit, Fig. 2a, to switch it on and off. In the "ON" condition the transistor bottoms typing the output at capacitor 196 to earth, in the "OFF" condition the negative potential is limited by zener diode 194.