Patent GB-748802-A (GB)
Title: Improvements in or relating to electric multiplying devices
IPC: G06G7/163
Filed: 1953-08-17
Abstract: 748,802. Electric analogue calculating apparatus. SOC D'ELECTRONIQUE ET D'AUTOMATISME. Aug. 17, 1953 [Aug. 28, 1952], No. 22675/53. Class 37. In an electric multiplying device comprising a servo-system responsive to a first input analogue voltage correspondingly adjusting the slider of a potentiometer supplied with a second analogue input voltage to derive on the slider the analogue product voltage, there is provided an electronic multiplier of comparatively low accuracy supplied with the high-frequency components of the inputs, whereby the servosystem multiplier derives a partial product voltage of the high- and low-frequency input components and the electronic multiplier produces another similar partial product voltage, the algebraic sum of the partial product voltages being proportional to the product of the input voltages irrespective of the inaccurate response of the servo-system multiplier to highfrequency components of the input voltages. Fig. 1 shows an electric analogue multiplying device in which input voltage x is applied over resistance 24 to an amplifier 20 energizing the armature of a separately excited servomotor 21 driving the sliders of potentiometers 23, 33, 35. Potentiometer 23 is energized from a positive and negative D.C. source and the slider voltage - x 1  is fed back over resistance 25 in opposition to the amplifier input so that the motor rotates the potentiometer sliders to a position corresponding to the input voltage at which the difference voltage at the amplifier input tends to zero. Since the input voltage x comprises all frequencies and the servo-system has a low cut-off frequency, x is always greater than x 1  so that a residual voltage x-x 1  is obtained by subtraction across resistances 26, 27, comprising frequency components higher than the servo system cut-off, which are applied to a feedback amplifier 28 of gain k (equal to the ratio of the input voltage x to the maximum possible value of the residual voltage x - x 1 ) and a feedback phase inverter 30 to produce voltages of Œ x 2  on inputs 3, 4 of an electronic multiplier III described below. Similarly the other input voltage y is applied over resistance 44 to an amplifier 40 energizing the armature of a separately excited servomotor 41 driving the sliders of potentiometers 43, 46. Potentiometer 43 is energized from a positive and negative D.C. source and the slider voltage - y 1  is fed back over resistance 45 in opposition to the amplifier input so that the motor rotates the potentiometer sliders to a position corresponding to the input voltage at which the difference voltage at the amplifier input tends to zero. A residual voltage y - y 1  is derived by subtraction across resistances 46, 47 and is applied to a feedback amplifier 48 and a feedback phase inverter 50 to produce voltages Œ y 2  on inputs 5, 6 of the electronic multiplier III. Potentiometer 33 is energized from voltages Œ Y 2  to give a slider output x 1 y 2 /k = x 1  (y - y 1 ) and potentiometer 36 is energized from voltages Œ x 2  to give a slider output x 2 y 1  = Y 1  (x - x 1 ). Potentiometer 35 is energized by voltages Œ y 1  derived from the slider of potentiometer 43 over resistances 54, 59 and feedback phase inverter 55, and its slider output x 1 y 1  is additively combined over series resistances at the input of feedback amplifier 37 with the outputs of potentiometers 33, 36 and with output x 2 y 2 /k<SP>2</SP> = (x - x 1 ) (y - y 1 ) of a feedback amplifier 64 having gain 1/k<SP>2</SP>. The combined output voltage of amplifier 37 is proportional to xy, irrespective of the cut-off frequency of the servo-systems. In a modification (Fig. 2, not shown) the servo-system responsive to input voltage x drives a single multiplying potentiometer energized together with one input of the electronic multiplier from the output of a phase-inverting feedback amplifier receiving the input voltage y to produce a slider voltage x 1 y. A residual voltage Œ x 2  = (x - x 1 ) derived as in Fig. 1 energizes the other input of the electronic multiplier, whose output energizes a feedback amplifier of gain 1/k to produce an output x 2 y/k = (x - x 1 )y which is combined with the slider voltage x 1 y to produce an output signal xy. Fig. 3 shows an electronic multiplier of the kind referred to, in which the input voltages x 2 , y 2  are applied respectively over variable resistances 92 introducing an attenuation of ¢ to two pairs of positive and negative signal feeders. Plural networks 70, 72, 74, 76 connect the positive x 2 , y 2  input lines to the output line 90; the respective inputs being connected over equal resistances to a junction point connected to line 90 over a third equal resistance and to the anode of a diode whose cathode receives a positive bias while similar plural networks 71, 73, 75, 77, &c., connect the negative x z , Y 2  input lines to the output lines in similar manner. Plural networks 78, 80, 82, 84 connect respectively the positive x 2 line and negative Y 2  line over equal resistances to a junction point connected to line 90 over a third equal resistance and to the cathode of a diode, whose anode receives a negative bias, while similar plural networks connect the negative x 2  line and the positive Y 2  line to the output line in similar manner. The diodes of networks 70, 71 ; 72, 73 ; 74, 75; 76, 77, &c., receive progressively increasing positive bias voltages, while the diodes of networks 78, 79; 80, 81; 82, 83; 84, 85, &c., receive progressively increasing negative bias voltages. Networks 70-76 transfer voltages proportional to + ¢(x 2  + y a ) to the output line 90, networks 71-77 transfer voltages proportional to - ¢(x 2  + Y 2 ) while networks 78-84 transfer voltages proportional to + ¢(x 2  - Y 2 ) and networks 79-85 transfer voltages proportional to - ¢(x 2  - y 2 ) thus accommodating positive and negative excursions. The effect of the biased diodes is to successively cut off the networks as their input voltages rise to levels equal to those of the progressively increasing bias values, and the combined characteristics of the elementary networks constituting groups 70-77 and 78-85 develop parabolic characteristics, that of each group being a mirror image about the abscissa of that of the opposite group. The output on line 90 is therefore a voltage proportional to