Designing A Very High Output Resistance Current Source

Abstract— A new technique is proposed in this paper in order to design a current source in bipolar technology with very high output resistance while minimising output capacitance. Such techniques are useful in Electrical Impedance Tomography applications. First of all, D.C. relationships are established. Then, an expression based on the analysis of a simplified equivalent circuit, is formulated for the output resistance and tested against simulation results for both ideal and practical biasing circuits. The effect of circuit capacitances, and in particular the role of the collector-base capacitance of the output transistor, in determining the output impedance as a function of frequency is considered. Finally, some new circuits with output impedances much greater than that of the basic Baxandall & Swallow (B&S) configuration are proposed and simulated results presented.