Digitally programmable accurate current sources for logarithmic control of the amplification or attenuation in a gain cell

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