I'm currently using the MP111 hybrid op-amp set up as a voltage controlled current source with a gain of 1. But what I'm finding is that at low input voltages the gain is much higher than it should be - up to 8 A/V. As we increase the input voltage the gain decreases sharply and then becomes 1.

Why is the gain so off at low input voltages? Is there a way to fix this?

I'm calculating the gain by dividing the current seen passing through the shunt resistor (Rs) by the input voltage.

MP111 datasheet: https://www.apexanalog.com/resources/products/mp111u.pdf

MP111 configuration as current source (figure 8): https://www.apexanalog.com/resources/appnotes/an07u.pdf

Rs = 0.01 Ohms, Rload = 0.05 Ohms, Rin = 101 kOhm, Rf = 1 kOhm.

The positive and negative rails are at +15V, -15V respectively. Boost functionality is not being used therefore Vboost pins are connected directly to the power rails.

Image of the gain vs. input voltage

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    \$\begingroup\$ Probably due to input offset voltage. \$\endgroup\$ – Marko Buršič Oct 24 '18 at 6:36
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    \$\begingroup\$ Marko's suggestion about offset may be a part. But that opamp's specs and your curve don't convince me that's all there is to it. Why not a discrete design, by the way? (That opamp is boutique and expensive.) \$\endgroup\$ – jonk Oct 24 '18 at 8:08
  • \$\begingroup\$ What are +/-Vb pins connected to - show your entire circuit please. \$\endgroup\$ – Andy aka Oct 24 '18 at 12:02
  • \$\begingroup\$ Could you show your layout? \$\endgroup\$ – John Birckhead Oct 24 '18 at 12:40
  • \$\begingroup\$ I've added a schematic of the circuit. In terms of layout - It's just on a protoboard with discrete components on top of the MP111 \$\endgroup\$ – VanGo Oct 25 '18 at 2:42

Thermal feedback, of unknown magnitude and polarity, will either buck or boost your desired control-loop signal.

To test, abruptly change the Rload by 50% (simply hold a similar Resistor in parallel), and observe the behavior.

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  • \$\begingroup\$ What should/shouln't happen when I do this ? \$\endgroup\$ – VanGo Oct 25 '18 at 2:00
  • \$\begingroup\$ You should get a 50% change in some voltage. But you've already stated "the gain is 8:1, not 1:1"; I expect something changing by 8:1, or at least far more than 1:1. \$\endgroup\$ – analogsystemsrf Oct 25 '18 at 5:53

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