GB1297364A - - Google Patents

Abstract

1297364 Logarithmic amplifiers MITSUBISHI DENKI KK 16 April 1970 [16 April 1969] 18339/70 Heading G4G An amplifier 20 whose output voltage V is proportional to the log of the input current I is compensated for temperature changes by means of temperature dependent resistances 26, 28 and a source 22 of constant voltage U related to a quantity U o which expresses the temperature-dependent properties of a logarithmic transfer element, e.g. a transistor 4, in the amplifier 20. The current I (at terminal 1) can be related to the voltage V (at terminal 2) by where I 1 is the saturation input current, T = absolute temperature, k = Boltzman's constant, q = unit elementary charge. If V ” 0À1 volt, this equation approximates to i.e. V is proportional to log I. The temperature dependence of the saturation current I 1 may be expressed by where C, d and V o are constants inherent in the transistor (or diode) 4. This equation may be written If T approximates an arbitrarily selected T 0 , it can be expressed by These equations lead to where Combining this with equation (2) we finally obtain, in common logs, Source 22 of voltage U and log amplifier 20 are connected by respective temperature-dependent resistances 26, 28 to an inverter amplifier 30 having a feedback resistance 32 of value r. The input of amplifier 30 is connected via resistance 34 of value r 3 to a source of variable reference potential V 3 . The values of resistances 26, 28 are r 1 , r 2 and their temperature dependence can be expressed by the equation where the bracketted figures indicate the temperature at which the resistance is measured. For the output voltage V out at 17 we have, The quantities U and r 2 (T) are selected to satisfy U/r 2 (T) = U 0 /r 1 (T), whereby we have, Eliminating V, U 0 and T between this equation and equation (9) we obtain, V out = A log I+B where A and B depend upon T 0 and circuit parameters only (equations 17 and 18, not given). The range of V out may be controlled by r 3 and the zero by V 3 . The Specification describes a modification (Fig. 5, not shown) in which resistance 32 provides only partial feedback and the inductances of (wire-wound) resistors 26, 28 are compensated by a parallel-connected series RC circuit. In the circuit shown, the + input of amplifier 30 is a virtual earth, and this necessitates use of amplifier 46 to generate U. Fig. 6 shows an alternative which enables elimination of amplifier 46 and resistor 28. A theoretical account of the operation of this circuit is similar to what is given above and is fully set forth in the Specification. It is stated that the resistance of the copper wire wound resistors 26, 28 follows a temperature law r = 0À4176T - 14À18. For these resistors, a series combination of a wire-wound resistor of copper and a resistor having a resistance equal to about 14.2% of the resistance of the former and made of a low temperature-coefficient material, is used. The log transfer circuit 4 may consist of a diode or a PNP transistor instead of the NPN transistor shown.