PRECISION_CURRENT_SOURCE

Published:2009/6/18 23:29:00 Author:May | From:SeekIC

A complete, battery-powered current source suitable for laboratory use is shown. The op amp regulates the voltage acrosg the range resistors at a level determined by the voltage on the arm of the calibrated potentiometer, R3. The voltage on the range resistors is established by the current through Q2 and Q3, which is delivered to the output.The reference diode, D1, determines basic accuracy. Q1 is included to ensure that the LM11 inputs are kept within the common-mode range with diminishing battery voltage. A light-emitting diode, D2, is used to indicate output saturation. However, this indication cannot be relied upon for output-current settings below about 20 nA, unless the value of R6 is increased. The reason is that very low currents can be supplied to the range resistors through R6 without developing enough volt-age drop to turn on the diode.If the LED illuminates with the output open, there is sufficient battery voltage to operate the cir-cuit. But a battery test switch is also provided. It is connected to the base of the op-amp output stage and forces the output toward V+.



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