Electronic – Troubleshoot a bipolar amplifier circuit

An op-amp amplifies the difference between its inputs, not the difference between an input and whatever happens to have the ground symbol attached to it in your schematic. Look at the dual-supply version of the inverting amplifier:

It amplifies voltage differences from ground because the non-inverting input (+) is attached to ground. It could just as easily be any other voltage, and this is why single-supply op-amp circuits can work.

In your circuit, the non-inverting input isn't attached to ground. It's attached to 6V. An op-amp with negative feedback will strive to make its inputs equal through the feedback path. Your input is -2.5V, and it wants the inputs to be equal, so the voltage drop over R4 should be \$6V - (-2.5V) = 8.5V\$. The voltage drop over R3 should be equal, since they are the same value, and the op-amp bias current is assumed to be none. Thus, the output would have to be \$ 6V + 8.5V = 14.5V\$. Of course, this is impossible for an op-amp with a 12V supply, so it does the best it can manage, 11.01V.

You can decrease the voltage at the non-inverting output by making R1 bigger and R2 smaller to move the virtual ground to a lower voltage, but you won't be able to move it all the way to 0V relative to ground because an op-amp generally can't operate all the way to the rails. Read more about common-mode input range to learn more. You could use a rail-to-rail op-amp to get closer. Or, you could continue to use a negative supply. Or, you could add a voltage to your input with a zener diode in series with it. Or, you may be able to AC couple the inputs. Which option is best depends on what the desired behavior when the input isn't -2.5V.

Here are some ideas for converting -2.5V to a positive voltage, without an op-amp:

Your circuit is incorrect. What you are trying to do is a gain+offset (negative offset) so you want to gain it up and offset the virtual ground point. You don't need that 10k/10k voltage divider as you've drawn it to provide a 2.5V reference.

Since your full scale output from the DAC is 5V, and you want a full scale output from the opamp of 20V (-10 to +10) then you need to have a gain of 4 in the opamp. That gives you 0-20V. So you want (DAC x Gain) - Offset.

In a non-inverting op-amp, the + input has a gain set by the (feedback resistor/input resistor)+1. However, the negative input has a gain of simply the (feedback resistor/input resistor). So a non-inverting input with a gain of 4 will have a a gain of 3 from the negative input. You can use the negative input to offset the fullscale 20V to -10 to +10. So you want 10/3 = 3.33 volts at the negative input.

^{simulate this circuit – Schematic created using CircuitLab}

At the point I marked 3.333V, for a standard non-inverting op-amp configuration, that point would be GND. But you want to add the negative input's gain to the output too, so you need 3.333V there.

The LM324 will work fine,but it's not a precision opamp and has some significant input offsets itself, and you might lose precision of the DAC input, but it can be trimmed out if you need to. If you don't want to trim anything then you can use a precision opamp with good DC characteristics.