Patent GB-1273750-A (GB)
Title: Electrical function generator
Inventor: EDELSON ROGER HARVEY
IPC: G06G7/28
Filed: 1969-07-30
Abstract: 1,273,750. Function generators. HUGHES AIRCRAFT CO. 30 July, 1969 [21 Aug., 1968], No. 38256/69. Heading G4G. A junction generator adapted to accept either a D.C. or an A.C. analogue input, comprises an operational amplifier having a linear characteristic determined by its input impedances, and means for switching such input impedances in response to input voltage in such a manner that different linear characteristics are presented to different ranges of input voltage, such linear characteristics approximating over the whole range of input voltage to the curve of the required function. The desired function may be (as shown)f(x)=x<SP>m</SP> and is approximated by e.g. four straight lines 11, 13, 15, 17 of general equation Y=a i X+b i  (i= 1, 2, 3, 4) where the a i  represent the slope of the line and the b i  the intercepts on the Y axis. The locations of the "break points" A, B, C, where i changes value, are determined in accordance with the known "least squares" method (explained in the Specification). The optional amplifier 93 operates over the whole range of input voltages and has linear characteristics determined by input impedances in box 21, these being switched in accordance with input level by the switches in box 19. The inputs consist of a varying analogue voltage x (representing e.g. range) and a constant reference voltage (representing e.g. maximum range). Assuming D.C. operation, and a near-minimum value of input, the analogue variable is fed from input 35 through resistance 81 to the inverting terminal 2 of amplifier 93 and the reference voltage is fed from input 37 through an inverter 34 (producing 180 degrees phase shift) and a resistance 91 to the same terminal 2 of the amplifier. The resistance 81 operates on the variable x and determines a 1 , the slope of line 11, whilst resistance 91 operates on the constant reference voltage and determines the intercept b 1  of line 11. The amplifier operates according to characteristic 11 whilst the variable x is less than A. Instead of D.C. voltages, phase coherent A.C. variable and reference voltages may be fed respectively from terminals 27, 29 selected by operation of switch 25. The two voltages are also fed for comparison to pairs of resistances 49, 51; 53, 55; 57, 59, the pairs having respective common connections to the inverting terminals 2 of operational amplifiers 41, 43, 45. Where the inputs are A.C., the voltages compared are r.m.s. values derived in rectifiers 31, 33 brought into operation upon changeover of switch 25. The ratios of the values of the resistances in each pair 49, 51 etc. are in ascending order and are chosen such that as the analogue variable x increases through values A, B, C, the amplifiers 41, 43, 45 are switched on in turn. Thus, as the value A is exceeded, amplifier 41 is switched on, its output goes negative and turns on transistor 63, whereby a positive selector voltage V SW1  is produced at collector terminal 65. This voltage is applied through resistance 99 to the gate of FET 83 which thereupon becomes conductive and connects the resistance 105 to the analogue variable input 35 (or 27). Resistances 81 and 105 are now in parallel and the slope value a 1  changes to a 2 . At the same time, the selector voltage V SW1  is fed through resistance 101 to the gate of FET 103 whereby (in a manner analogous to that described above) resistance 107 is placed in parallel with resistance 91 and the intercept value b 1  changed to b 2 . The amplifier 93 now operates according to a characteristic Y = a 2 X + b 2  along segment 13. In a similar manner production of selector voltages V SW2  and V SW3  respectively result in the addition to the circuit of parallel resistances 113, 123; 151, 125 which produce the linear characteristics 15 and 17. In a modified circuit, a different single resistance is used for each value of a i  and b i  instead of a parallel combination. In another modification, the resistances 51, 55, 59 and the resistances 47 are removed and negative reference voltages applied to the inputs 3 of the amplifiers 41, 43, 45.