Circuit and method for accurately mirroring currents in application specific integrated circuits

Abstract

A method and apparatus for mirroring currents in application specific integrated circuits provides higher current mirroring accuracy than previously obtainable with matched active devices by using small groups of resistors with local matching to create a summing node which represents the average voltage across the source resistors of the active output devices and by forming a reference resistor through the combination of resistors from the local resistor groups such that the reference resistor has properties which will largely cause cancellation of location gradients and initial value variation in the resistor groups. An error amplifier compares the voltage at the summing junction with the voltage across the reference resistor and adjusts its output voltage to drive the paralleled gates of each active mirror output device such that the summing junction and reference resistor voltages remain equal. The number of active devices forming an output array is typically an integer squared, and a local resistor group of three matched resistors is provided for each active mirror device. The error amplifier output voltage driving the gates of the active output devices causes the current flowing through each device to mirror the reference input current flowing through the reference resistor. The result is that the current flowing from the output array of active devices is closely equal to the integer squared times the reference input current flowing in the reference resistor.