Digitally-Programmable Accurate Current Sources for Implementation in Controllable Logarithmic Amplifiers and Attenuators

A novel method is presented for realizing current sources driving a current equal to a programmable fraction of a reference current. The multiplication factor is determined by the digitally programmable power of a selected radix that is set by the ratio between the area of two emitters. The number of identical current sources can be expanded virtually unlimited without loss of accuracy or an increase in complexity. Several applications are possible; however, this paper is focussed on applying this technique to digitally-programmable logarithmic amplifiers or attenuators. A radix equal to 1.122 allows a gain cell to be programmed in dB steps.