Figure 1 shows a nice “textbook” current source. If the resistors are chosen so that R3/R2 = R4/R1, then it can be shown that Iload = −Vin/R2.
This sounds great, but there’s a hitch: the resistor ratios must be matched exactly; otherwise it isn’t a perfect current source. Even so, its performance is limited by the
op-amp’s common-mode rejection ratio (CMRR). For large output currents the resistors must be small, and the compliance is limited. Also, at high frequencies (where the loop gain is low) the output impedance can drop from the desired value of infinity to as little as a few hundred ohms (the op-amp’s open-loop output impedance). These drawbacks limit the applicability of this clever circuit.
You can convert this circuit into a non-inverting current source by grounding R1 (where Vin is shown) and applying the control input voltage Vin instead to R2.
Figure 2 is a nice improvement on the Howland circuit, because the output current is sourced through a sense resistor Rs whose value you can choose independently of the matched resistor array (with resistor pairs R1 and R2).
The best way to understand this circuit is to think of IC1 as a difference amplifier whose output sense and reference connections sample the drop across Rs (i.e., the current); the latter is buffered by follower IC2 so there is no current error. For this configuration you can exploit the internal precision matched resistors in an integrated difference amplifier: use something like an INA106 for R1, R2, and IC1,
wired “backwards” (for G=0.1) to reduce the drop across the sense resistor.
|Engineer.Labs | Hardware Design Engineer (Freelancer)||
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