MOSFET enhances low-current measurements using moving-coil meter

摘要

A previous Design Idea describes an interesting and useful method for using a moving-coil analog meter to measure currents in the less-than-lA range (Reference 1). The design offers considerable flexibility in the choice of meter-movement sensitivity and measurement range and simplifies selection of shunt resistors. Although the design uses a bipolar meter-driver transistor, under some circumstances, a MOSFET transistor represents a better choice. The original circuit comprises a voltage-controller current sink that measures the bipolar transistor's emitter current, but the transistor's collector current drives the analog meter. A bipolar transistor's emitter and collector currents, I_E and I_C, respectively, are not identical because base current, I_R, adds to the emitter current.