Temperature and process independent exponential voltage-to-current converter circuit

Abstract

A voltage to current conversion circuit is described. The circuit comprises a first differential amplifier for receiving an input voltage and producing an output voltage, and a second amplifier for converting the output voltage of the first amplifier to a current. The transfer function of the voltage to current conversion circuit is proportional to an exponential function that depends on the input voltage. The circuit is temperature and process independent. In a first preferred embodiment, the first amplifier comprises a first transistor for receiving an input voltage at its base terminal, a temperature dependent current source coupled to the emitter of the first transistor, and a positive voltage supply coupled to the collector through a diode coupled transistor, and a second transistor paired with the first transistor and having a base terminal coupled to an input voltage terminal, an emitter coupled to a temperature dependent current source, and a collector coupled to a voltage supply. The output voltage is a differential signal taken from the collector terminals to the second amplifier, which comprises a third transistor coupled to a fixed current source, the base terminal for receiving the voltage output of the first amplifier, and an emitter coupled to a fourth transistor's emitter, the fourth transistor receiving the output voltage of the first stage at its base terminal, and the collector providing an output terminal. A feedback circuit is coupled to the emitters of the transistors of the third and fourth circuits and to the collector of a fifth transistor, the feedback circuit providing negative feedback to limit the current available at the output when the current through the feedback circuit exceeds a predetermined limit.