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I think this might be a bit of a stupid question, but I've got a bit of mental block on it.

I'm trying to power a CA3140 op-amp. I have a 12V source, and an AMD660 inverter to provide the negative voltage.

schematic

simulate this circuit – Schematic created using CircuitLab

However, when I connect the negative supply pin of the op amp to the pin to the output pin of the ADM660 inverter, the voltage reading on the ADM660 output goes to a few hundred (positive) mV instead of -6V(ish) it should be. Obviously the output of the op-amp does not descend into the negative region.

I've tried placing various resistors in series with the output of the ADM660 to reduce the current as it was fairly high, but this just results in a voltage drop across the resistor, and still leaves a positive voltage on the op-amp pin. What should I be doing? I feel like I'm missing something easy here, sorry if it's really obvious, I'm fairly new to electronics.

Thanks.

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    \$\begingroup\$ What you're doing wrong is trying to feed your ADM660 via a voltage divider. \$\endgroup\$
    – brhans
    Apr 28, 2016 at 17:45
  • \$\begingroup\$ Thanks for the reply. What's wrong with that sorry? \$\endgroup\$
    – ram
    Apr 28, 2016 at 17:54
  • \$\begingroup\$ ADM660 does not generate electric power from nowhere, it takes it from its VIN pin. You limit the input current of this pin to low values by R3 (R4) in this divider. You have to add resistors' values on schematics. \$\endgroup\$
    – Master
    Apr 28, 2016 at 18:00
  • \$\begingroup\$ And: you have to consider the negative supply pin of your OP AMP as a constant current sink pin. You can not make this current smaller by any external components. You can only change the voltage (as it happens in your design). \$\endgroup\$
    – Master
    Apr 28, 2016 at 18:01
  • \$\begingroup\$ Voltage Dividers Are NOT Power Supplies. Never. Not Ever. Just Don't Do It. \$\endgroup\$
    – brhans
    Apr 28, 2016 at 18:05

2 Answers 2

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Here's a quick fix that may do the job. Add an NPN transistor to your circuit using the existing potential divider to provide the base voltage. Its emitter will be about 0.6V lower but it will be able to boost the current available to your ADM chip.

enter image description here

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This is a nice trick, and you can use the ADM660's doubling feature if you need -12 for the opamp.

You could also use the Zener as a shunt regulator if that would be more appropriate.

enter image description here

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