Realization of a triple-tail cell with high input resistance for low-voltage application

Abstract

An operational transconductance amplifier (OTA) is a useful function block for analogue signal processing. A triple-tail cell possesses a wider linear input voltage range than an emitter-coupled pair, in spite of its simple configuration. In practical implementation of the triple-tail cell, the maintenance of the intrinsic linearity necessitates a voltage divider which has low resistance. This lowers the input resistance of the triple-tail cell. This paper describes a method of realizing high input resistance of the triple-tail cell. An active voltage divider composed of two OTAs is connected to the triple-tail cell. The linearity of the triple-tail cell with the divider deteriorates little. The whole circuit can operate at a low supply voltage. Although an emitter-coupled pair is employed as the OTA in the active divider, the linearity of the divider is good. Simulation results of an OTA-C filter that consists of the triple-tail cell and capacitors show that satisfactory characteristics are obtained.