High-Side Current-Sense Circuit in One BJT Array

by Dennis L Feucht

High-side current-sense circuits are often implemented with an op-amp or other circuitry that requires a separate supply to power it. The positive supply must exceed or at least equal the high-side voltage and the amplifier often must either be floated or designed to operate from a relatively high voltage for a high common-mode range. This impediment can be eliminated by using circuits that are powered entirely from the sensed current loop.

Zetex (now Diodes, Inc) has a series of such circuits, and one of them (ZDS1009) is a four-BJT (2 pnp, 2 npn) circuit that has a common-mode voltage range equal to the breakdown voltage of the transistors and requires no supply. (It is an odd circuit in that it requires a finite r_o as a current path for leakage current (I_SC) to start the circuit.)

My June 2009 TechNote introduced a number of high-side schemes using 5 to 8 BJTs.

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