Patent GB-1173343-A (GB)
Title: Improvements in and relating to Threshold Logic Circuit
Inventor: HENRY MICHEL
IPC: H03K19/14
Filed: 1967-06-07
Abstract: 1,173,343. Phototransistor logic circuits. COMMISSARIAT A L'ENERGIE ATOMIQUE. 7 June, 1967 [8 June, 1966], No. 26354/67. Heading H3T. [Also in Division G4] In a threshold logic circuit having n inputs x 1 ...x 1 ...x n  assuming binary values Œ1 and a reference input x 0  = + 1; with positive or negative real analog weighting coefficients a 0 , a 1 ...   a 1... a n  wherein - a 0  represents the threshold, the output S of the threshold circuit is given by and it is shown that by modifying the coefficients ai a large number of Borlean functions may be produced by the same threshold circuit,   increasing for n input variables as n/2<SP>2</SP> Input terminals 1, 2 and reference terminal 3 (Fig. 1) are connected through symmetrical photo-sensitive transistors 10, 20, 30 to the first input 40 of differential amplifier 4 while connected also through similar transistors 11, 21, 31 to the second input of the amplifier, whose output is fed back to input 41 over resistor 6 equal to the earth return resistor 7 of input 40, and whose output 5 is connected to a trigger 8 producing an output logic signal at 9. The transistors receive luminous fluxes 12, 13, 22, 23, 32, 33 respectively from light source S; and currents where # denotes luminous flux and x the binary inputs so that and the output U of the amplifier 4 is shown to be proportional to a i  x i  and operates trigger 8 to deliver a voltage of the same sign as this sum. Since x 3  = x 0  = + 1 and a 3  = a 0  the threshold is -a 3  and the weighting coefficients a 1  a 2  and threshold coefficient -a 3  are modifiable by adjustment of the luminous flux differences #12-#13; #22-#23; #32-#33 which are obtained by variations of transmission coefficients of different opaque and translucent zones E 10 , E 11  . . . E 20 , E 31  having differing transparencies of a screen E, interposed between a lattice containing the phototransistors and a light source S; the screens being interchangeable to enable the threshold circuit to be adapted to predetermined logic functions. Fig. 2 shows a learning machine for setting up a specific screen in conjunction with the threshold circuit (Fig. 1) which comprises a unit 50 into which binary-variables are introduced at 51, 52 to produce at 53 an output representing the desired response, which is applied to comparator 60 also receiving the output 9 of the threshold circuit, the error signal being received by input 54 of unit 50 which has outputs 71, 72, 73 operatively connected to respective control circuits of which 81 is shown; consisting of terminals 110, 100 receiving positive and negative voltages applied to resistor 91, and parallel arms comprising field effect transistors 14, 15 in series with electroluminescent gallium arsenide diodes 16, 17. The base of transistor 14 is earthed, and that of 15 connected to output 71 having a memory capacitor 101. The Borlean function to be obtained from the threshold circuit is inserted into the learning machine, and (in respect of a given number of working points corresponding to the respective possible combinations of binary values applied to inputs 51, 52), a corresponding signal is developed at 53 and compared at 60 with the signal from the output 9 of the threshold circuit to produce an error signal at 54 controlling unit 50 to modify the output voltages at 71, 72, 73 and thus vary the illumination of diodes 16, 17 &c. transmitted along luminous flux tubes to phototransistors 12, 13 &c. to vary coefficients a 1  &c. according to a convergent algarithm, so as to cancel the error signal. The requisite screen is then produced by photographing the distribution of the luminous flux as received by the photo-sensitive transistors, and such screen thus enables the threshold circuit to produce the requisite function. Asymmetrical phototransistors may be used in the circuit of Fig. 1 (Fig. 3, not shown), and photo-resistors may alternatively be used.