Patent US 4,100,433

A monolithic circuit is disclosed which is suitable for providing a pair of logarithmic differential output current signals in response to an applied linear control voltage. The circuit comprises first and second operational amplifiers connected as voltage followers which act as low impedance driving sources. The linear control voltage is applied to a first one of the operational amplifiers and a reference voltage to the other. These two voltages substantially appear at the respective outputs of the operational amplifiers and are summed across a resistor connected therebetween to establish a difference potential thereacross of which the magnitude is a function of the applied input voltage. The difference potential is utilized to differentially drive the base emitter junctions of a pair of transistors which establish differential logarithmic currents at the collectors thereof.