Offset-Adjusting Current Source Circuit

Posted on Jul 15, 2012

Though this setup can act as a cost-effective current source with an output accurate to 1%, the voltage offset will turn on the current source even when Vqq equals Vin. 2. Modifying the configuration of Figure 1 can rectify the problem of the current source being turned on by the voltage offset. The addition of R7 allows an adjustment that guarantees turn-off for any op-amp offset specification. By carefully choosing components, you can create a cost effective circuit for a current source with an output that`s accurate to 1% (Fig. A). 70UT (the current flowing from the collector of Ql) is ^cc^`in (^e voltage at the wiper of R3) divided by the value of R2. In some instances, it`s important to be able to turn off the current source (within the limits of Icm for Ql).

Offset-Adjusting Current Source Circuit
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Unfortunately, in about half ofthese cases, the offset voltage (l70S) of the op amp will turn the current source on even when Vcc = Vm. That`s because the offset voltage (when the noninverting input needs to be at a higher potential than the inverting input to get an output of 0 V from the op amp) is impressed across R2. This offset voltage forces Ql to turn on enough to yield a collector current of V()S divided by R2. Diagram offers a fix for this predicament. The addition of R7 presents the emitter of Q2 with a Thevenin equivalent voltage and resistance represented by: The difference between Vcc and VTH is Vcc + R7). If Vcc (#5/fl5 + R7) is set equal to the maximum Vos spec for the op amp in question, the circuit is then guaranteed to turn off. This circuit has an output current of VTH - VSN divided by RTI}. The compromise of Fig. does present another error term in the circuit. The term (Vm - ISJN) will have to be 2 Vos to guarantee a current output for whole population of the op amp chosen. This error can be made arbitrarily small (but not zero) by increasing the voltage of D2 and Vcc while raising the value of D2 and Vcc while also raising the value of the equivalent resistance Rth.

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