Patent GB-766571-A (GB)
Title: Improvements relating to electrical multiplying circuits
Inventor: ALLEN GORDON STANLEY JAMES
IPC: G06G7/24
Filed: 1954-04-20
Abstract: 766,571. Electric analogue calculating systems. CROSFIELD Ltd., J. F. April 7, 1955 [April 20, 1954], No. 11410/54. Class 37. An electric circuit for multiplying or dividing by the summation of signals representing the logarithms of the inputs comprises rectifiers of logarithmic current/voltage characteristics in series with respective high resistances to which signal voltages representing the input quantities are applied, thereby to develop proportionate rectifier currents and resultant rectifier voltages respectively proportional to the logarithms of the inputs, which are connected through high resistances to a single summation resistance ; an input signal voltage variable over a range in excess of that for which the current/voltage characteristic of a single rectifier is logarithmic being applied to plural rectifiers operative over successive working ranges separated by arbitrary values, and signals representing the logarithmic values of such input signal for each range being combined in the summation resistance with signals representing the logarithmic values of the remaining inputs, and such combined voltage being amplified and fed to a rectifier of similar logarithmic characteristics, connected in series with a low value load resistance to develop a voltage proportional to the anti-logarithm of the sum of the logarithms of the input quantities. In Fig. 1 a square waveform of 20 kc/s. amplitude modulated proportionately to variable x is coupled over cathode follower V1 to oppositely connected diodes D1, D2 respectively in series with resistor R2 and resistor R3 in series with germanium rectifier CR1 of law V=K log I, so poled that on negative excursions the current pulses therein are proportional to X and the voltage pulses across R3 are proportional to log x down to a limit set by the rectifier logarithmic range. A similar waveform, amplitude-modulated proportionately to variable y, is coupled over cathode follower V2 to oppositely connected diodes D3, D4 respectively in series with resistor R4, and with resistor R5 in series with germanium rectifier CR3 of similar logarithmic law, so poled that on positive excursions the current pulses therein are proportional to y and the voltage pulses across R5 are proportional to log y. The cathode output of V1 is also amplified by cascaded pentodes V3, V4, V5, the cathode of the last of which feeds oppositely connected diodes D5, D6 respectively in series with resistor R6, and with resistor R7 in series with logarithmic germanium rectifier CR2, so poled that on negative excursions the current pulses therein are proportional to x and the voltage pulses across R7 are proportional to log x over a range of values down to a limit set by the allowable logarithmic range of the rectifier divided by the gain of cascaded pentodes V3, V4, V5. The several rectifier voltage waveforms are summed over series resistors R11,R12,R13 and common summation resistor R1, while a phase-splitting pentode amplifier V6 driven by a constant amplitude square waveform synchronous with the input signal waveforms is cathode-coupled through diode D7 to the junction of diode D2 and resistor R3, and is anode coupled through diode D9 to resistor R1. Polarities are such that on negative excursions synchronous with negative excursions of the input signal, and when the input signal amplitude at the cathode of D2 is less than that of the constant signal amplitude of the cathode ofD7, the latter passes constant amplitude pulses to resistor R3 and rectifier CR1 so that D2 is cut off; while when the signal amplitude at the cathode of D2 exceeds the constant signal amplitude at the cathode of D7, the latter is cut off and D2 is conductive, so that over an input amplitude range whose lower limit corresponding with the rectifier logarithmic characteristic is set by the amplitude of the constant voltage waveform, a pulsed waveform whose amplitude represents log x appears across resistor R1, while below such limit a constant amplitude waveform appears across CR1, and then effect across R1 is cancelled by the equal antiphased waveform over diode D9, which is conductive on positive excursions. The amplitude of the waveform applied to rectifier CR2 over diode D6, however, exceeds the limit, so that on negative excursions diode D10 conducts to energize pentode amplifier V7 which feeds back a degenerative signal to the input of V4, whereby the signal across R7, CR2 is held to a constant value wherever the input to amplifier V1 exceeds the rectifier lower limit of logarithmic operation. Thus (Fig. 2, not shown) rectifier CR1 responds to the upper and rectifier CR2 to the lower ranges of values of the signal representing x, separated by the lower limit of logarithmic response of the rectifiers; and the resultant signal is additively combined across resistor R1 with that from rectifier CR3 representing log y, and the combined signal is amplified (Fig. 3) by cascaded pentodes 11, 13, 14, 15 feeding cathode follower triode 16 coupled to a load comprising resistance 18 in series with rectifier 17 similar to rectifiers CR1, CR2, CR3, and low resistance 22. Feedback is provided from the cathode load to the input of pentode 13 whereby, when the gain of pentode 11 is adjusted to compensate the attenuation of network R1, R11, R12, R13, the signal voltage across the rectifier is held at a value proportional to log xy, so that rectifier signal current is proportional to the antilog thereof and the amplified voltage signal at the anode of pentode 23 is proportional to (xy). The gradients of the characteristics of the individual rectifiers may be varied by adjustment of resistances R11, R12, R13, while the characteristics are variable as a whole by adjustment of resistances R3, R5, R7. Either logarithmic signal may be amplified with respect to the other before summation so that the corresponding input value is raised in the result to the power represented by the gain factor. Variation of the gain of pentode 11 with respect to the loss introduced by network R1, R11, R12, R13 raises the product value to a power represented by the gain factor. The apparatus is applicable to a colour transmission signal connection system of the kind described in Specification 738,118, [Group XL (b)].