GB932835A - Function generator - Google Patents

Abstract

932,835. Logarithmic function generators. EASTMAN KODAK CO. March 27, 1962 [March 27, 1961], No. 11694/62. Class 37. A function generator (Fig. 1) comprises a phase reversing D.C. amplifier 10, a resistor 14 having one end connected to the input of the amplifier and its other end adapted to be connected to a source of operating voltage, a capacitor 11 connected in shunt with the amplifier, a tapped resistive chain 30 in series with a first feedback diode 31 also connected in shunt with the amplifier and a second feedback diode 33 connected between the tap on the resistive chain and a source of a first predetermined reference potential. The arrangement of the feedback diodes and reference potential is such that upon connection of the operating voltage, the output voltage will change from its initial value at first linearly with respect to time, then non-linearly and finally, linearly, but at a different rate than during the first period of linear change. The function generator is used to provide a voltage output which varies logarithmically with time, the specified equation being V = -25 log t, where V is the time after switching on the generator. Under non-operating conditions a positive voltage is applied to amplifier 10 from terminal 71 so that the voltage at the output of the amplifier is permanently negative, clamped to say -10V by Zener diode 24 and diode 22. By operating switch 70, a negative input is applied to the amplifier from terminal 72, and this is held at a fixed level by diode 18. This causes the output from the integrating amplifier to increase linearly (Section 611, Fig. 2) until it reaches zero volts, when diode 31 conducts so that the resistive path 30 is also brought into shunt with the amplifier and the continued output voltage rise is no longer linear (section 63<SP>1</SP>). When the voltage at tapping point 32 is such that diode 33 conducts, feedback through the resistive path ceases and the output voltage rise becomes linear again, but at a lower rate than at first (Section 65<SP>1</SP>). By suitable choice of parameters the variation can be made approximately logarithmic. The output from the integrating amplifier stage at 13 is applied to a further amplifier 41 which has a resistive input network 40. The output from this amplifier at 43 follows the output from amplifier 10 to point 66 (Fig. 3), corresponding to 66<SP>1</SP> (Fig. 2), at which point diode 51 in the input network starts to conduct, thus reducing the overall gain of the amplifier, so that the output voltage decreases more slowly (Section 67). As the diodes 52, 53 also start to conduct so further changes in the rate of decrease are brought about. The overall system can be arranged to closely generate a logarithmic output over time changes of 100 : 1, and the voltage output may be used for example in the accurate control of exposure times in photographic colour printing.