Current Source Adjusts For Offset

By carefully choosing components, you can create a cost-effective circuit for a current source with an output that's accurate to 1% (Fig. 1). I_out (the current flowing from the collector of Q₁) is V_CC –V_in (the 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 I_CEO for Q₁). Unfortunately, in about half of these cases, the offset voltage (V_OS) of the op amp will turn the current source on even when V_CC = V_in. 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 O V from the op amp) is impressed across R₂. This offset voltage forces Q₁ to turn on enough to yield a collector current of V_OS divided by R₂.

Figure 2 offers a fix for this predicament. The addition of R₇ presents the emitter of Q₂ with a Thevenin equivalent voltage and resistance represented by:

V_TH = V_CC \[1 - (R₅/R₆ + R₇)\]

R_TH = (R₅ × R₇)/(R₅ + R₇)

The difference between V_CC and V_TH is V_CC (R₅/R₅ + R₇). If V_CC (R₅/R₅ + R₇) is set equal to the maximum V_OS spec for the op amp in question, the circuit is then guaranteed to turn off. This circuit has an output current of V_TH - V_in divided by R_TH.

The compromise of Figure 2 does present another error term in the circuit. The term (V_TH - V_in) will have to be 2 × V_OS to guarantee a current output for the whole population of the op amp chosen. This error can be made arbitrarily small (but not zero) by increasing the voltage of D₂ and V_CC while also raising the value of equivalent resistance R_TH .