Patent GB-1372832-A (GB)
Title: Logarithmic circuit
IPC: G01D3/02
Filed: 1973-01-11
Abstract: 1372832 Logarithmic amplifiers PHOTO ELECTRONICS CORP 11 Jan 1973 [14 Jan 1972] 1591/73 Heading G4G In order to compensate for changing conditions, e.g. of temperature, during the operation of a logarithmic amplifier, the amplifier is continuously recalibrated by repeatedly sampling a pair of standard input signals and using the resultant output signals to modify the bias and the gain of the amplifier. In the photometer shown in Fig. 3, the output of a photomultiplier 114 is amplified by a circuit 10A, 22 having an output current proportional to the log of the input voltage. The logarithmic amplifier is coupled by an operational amplifier 24 to the output section of the photometer which consists of three identical circuits for sequential processing of red (R), green (G) and blue (B) signals. The corresponding inputs are generated by a colour wheel (not shown), associated with the photometer 114, which interposes colour filters in the sequence indicated by signals 180, 182, 184, Fig. 4. The colour disc intercepts light from the source being measured and also from a standard source to produce two sequences of R, G, B calibration signals 186, 188. A second rotating blanking disc (not shown) produces zero light input between the colour sampling periods, as indicated by signal sequence 186. The various gating signals shown in Fig. 4 are generated by rotary switches driven in synchronism with the colour disc and the blanking disc. The zero input signal 186 is used to close switch 96A and complete a feedback circuit 97, 98 which applies zero-input bias to the amplifier 22 whereby zero-illumination currents from the photometer are connected. Gaincontrol signals 188 are on during a first set of standard-source R, G, B sampling periods and operate switches 156, 158, 160 to couple sequentially the output of amplifier 24 to capacitors 56 R, G, B which respectively store R, G and B gain calibration signals. Signals 180, 182, 184 operate switches 170, 172, 174 and gate the gain calibration signals sequentially to a differential amplifier 32 during the measured-source R, G and B sampling periods. The output of amplifier 32 is fed to a photo-diode 66 which controls the current in a photoconductor 33 whereby the gain of amplifier 24 is regulated in correspondence with the calibration signals. The above gain-control feedback channel is arranged in parallel with a bias-control channel which is controlled by gating signals 190, 180, 182, 184 in a manner similar to that described above. Thus, during the R, B and G measuredsource sampling periods, the bias of amplifier 24 is regulated by bias-control calibration signals applied through operational amplifier circuit 34, 88, 90, 168. In order to regulate the amplifier 24 over a wide amplitude range, the gaincontrol calibration signals are established at a high input level and the bias-control signals at a low input level. This may be done by using two different intensities of standard-source illumination during the corresponding sampling periods. An output-gate signal 192 occurs between each pair of gain and bias gating signals 188, 190 and operates gates 142, 144, 146 to couple sequentially the output of amplifier 24 to storage capacitors 29 R, B, G during the measured-source sampling periods. The R, G and B outputs are fed by amplifiers 116 to output terminals 14. The various calibration signals may be manually adjusted by means of potentiometers 60, 84, 122. The output circuit of amplifier 24 includes a non-linear circuit 136, 138, 140 which presents a low impedance to high current outputs whereby the time constants of the calibration storage capacitors 56, 80 are substantially reduced whenever there is a sudden increase of input signal upon change from one sampling period to the next.