PURE NONLINEAR TRANSFER CIRCUIT

摘要

1333287 Logarithmic amplifiers BECKMAN INSTRUMENTS Inc 29 Oct 1971 [27 Nov 1970 50324/71 Heading G4G A pure non-linear, e.g. logarithmic, transfer function is obtained by compensating for undesired linearity in the characteristic of a non- linear element, e.g. a diode or transistor. The voltage V1 across such an element may be written where V(i) is the non-linear (current dependent) part of the voltage and iR e is the undesired linear part, R e being, e.g. the bulk resistance of the emitter of a transistor. In the dual-channel logarithmic amplifier shown, the two collectoremitter paths of a dual transistor 19 are connected respectively between the inverting inputs and the outputs of operational amplifiers 5, 25. The amplifiers are also provided with input and output resistors respectively 13, 29 and 15, 31, the terminals of these resistors remote from the amplifier being connected by respective potentiometers 38, 40 and 39, 41 which provide the desired compensation. By considering the currents at node 16, it can be shown that a purely non-linear transfer function can be obtained if R 40 /K 1 = R 13 R 15 /R e where K 1 is the setting of the potentiometer 38, 40 and the subscripts refer to the designations of the resistors on the drawing. A similar result is obtainable for the other channel of the dual amplifier. The outputs of the channels are connected by a double pole switch 36, 37 to respective inverting and non-inverting inputs of an operational amplifier 33. Letting the ratio R 35 /R 31 equal a, it can be shown that where the subscripts refer to the two channels. In the case of a transistor, this becomes where b is equal to aKT/q, K is Boltzmann's constant, T the absolute temperature and q the charge on the electron. The last equation may also be written V out = b.ln(i 1 /i 2 ) or, making R 13 equal R 29 , V out = b.ln(V s /V r ). By using a dual transistor, temperature effects are eliminated. By moving the double-pole switch 36, 37 to its other position, the alternative result may be obtained.