# Why is the opamp's inverting input not at virtual ground?

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:

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.

• What about the negative supply voltage V2? Why didn´t you use it?
– LvW
Nov 16, 2023 at 10:21
• V2 or V- is being used to offset the inverting input and to supply other OP-amps beside this circuit. Nov 16, 2023 at 10:23
• My recommendation: Try a symmetrical supply for the opamps.
– LvW
Nov 16, 2023 at 10:26
• 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. Nov 16, 2023 at 10:26
• 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 Nov 16, 2023 at 10:43

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