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I've have been trying to make a current to voltage converter using a transimpedance amplifier circuit. I have been simulating the circuit in LTspice and everything seems fine, but when I built the circuit in real life it has shown a problem I don't understand.

The circuit is as follows:

enter image description here

The problem is that in the simulation and as expected, the node at the inverting input of U4 is around 100 uV. In reality it is measured at around 900 - 1100 mV depending on the current source. First the current was controlled via a voltage source and a resistor, then I tried with a Howland current source and now the inverting input node is at 2.4 V.

Shouldn't the node be at virtual ground since the non-inverting input is grounded?

I would like for the output to be in the interval 0-3.3 V for currents in the range of -300 - 300 uA.

Note: I'm currently using a TL072 opamp instead of the LM324.

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  • \$\begingroup\$ What about the negative supply voltage V2? Why didn´t you use it? \$\endgroup\$
    – LvW
    Nov 16, 2023 at 10:21
  • \$\begingroup\$ V2 or V- is being used to offset the inverting input and to supply other OP-amps beside this circuit. \$\endgroup\$ Nov 16, 2023 at 10:23
  • \$\begingroup\$ My recommendation: Try a symmetrical supply for the opamps. \$\endgroup\$
    – LvW
    Nov 16, 2023 at 10:26
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    \$\begingroup\$ Please note that the LM324 can sink its output pin to the negative rail. I think the TL072 is not able to do the same. \$\endgroup\$
    – hennep
    Nov 16, 2023 at 10:26
  • \$\begingroup\$ The minimum output voltage of the TL072 is above 1.5V. That would explain the 900-1100mV on the inverting input. It is caused by feedback from the output pin through R5 \$\endgroup\$
    – hennep
    Nov 16, 2023 at 10:43

1 Answer 1

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Shouldn't the node be at virtual ground since the positive input is grounded?

This would happen if the op-amp was provided with a negative supply voltage. Currently it is using ground as the negative rail and this won't permit the op-amp's output to go to a negative voltage (and restore the virtual ground condition).

On the other hand, try reversing the current direction from \$I_1\$ and, you will see it now works reasonably well (with an LM324).

I would like for the output to be in the interval 0-3.3 V for currents in the range of -300 - 300 uA.

That will happen with \$I_1\$ reversed.

Note: I'm currently using a TL072 opamp instead of the LM324

In that case you will need a negative supply rail because the common-mode input range of that op-amp does not include the negative power rail

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