# Active Resistor Circuits

## Abstract

Active resistor circuits represent complex structures, implemented using exclusively MOS transistors that are functionally equivalent with a classical resistor, being able to simulate, between two input pins, a linear current–voltage characteristic. The goal of designing this class of active structures is mainly related to the possibility of an important reduction of the silicon area, especially for large values of the simulated resistances. The areas of utilization of active resistors include, but are not limited to amplitude control in low distortion oscillators, voltage-controlled amplifiers, active RC filters, canceling of an operational amplifier load, or improved performances Deboo integrators. The first generation of MOS active resistors uses MOS transistors working in the linear region, having the main disadvantages of an equivalent resistance inherently nonlinear and of obtaining distortion components that are complex functions on MOS technological parameters. A better design of CMOS active resistors is based on MOS transistors working in saturation. Because of the quadratic characteristic of MOS transistor, linearization techniques have to be developed in order to minimize the nonlinear terms from the current–voltage active resistor characteristic. The presented techniques for designing active resistor structures are based on six different elementary mathematical principles, each of them being illustrated by concrete implementations in CMOS technology of their functional relations.

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