Differential amplifier with exponential gain characteristic

Abstract

A differential amplifier provides a predetermined gain characteristic over a large range differential input voltage signals and operating temperatures. The differential amplifier receives a first differential input voltage signal at a pair of amplifier input terminals and outputs a differential output current signal at a pair of amplifier output terminals. First and second emitter-coupled transistors are connected to receive at their bases an amplifier core differential voltage signal, have their emitters coupled to receive first and second constant currents and coupled together by a load element. The first and second emitter follower transistors have their bases coupled to the amplifier input terminals to receive the first differential input voltage signal from the pair of amplifier input terminals. The emitters of the first and second emitter follower transistors are coupled to provide the amplifier core differential voltage signal to the bases of the first and second emitter-coupled transistors, respectively. A VBE difference replicator receives the current in the collectors of the first and second emitter-coupled transistors and provides, responsive to the emitter-coupled transistors' collector currents, a replicated voltage that substantially replicates a voltage difference between the VBE of the first emitter-coupled transistor and the VBE of the second emitter-coupled transistor. An adaptive current source receives current from the emitters of the first and second emitter followers. The adaptive current source also receives the replicated voltage from the VBE difference replicator. The adaptive current source means draws a differential correcting current signal through the first and second emitter followers responsive to the replicated voltage. As a result, a difference between the VBE of the first emitter follower and the VBE of the second emitter follower compensates the amplifier core differential voltage to correct for a difference between the VBE of the first emitter-coupled transistor and the VBE of the second emitter-coupled transistor.