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I am designing a non-inverting op-amp circuit that maps an input signal (V3) 0-3.3V to an output signal (Vo) 0.2-2.4V. I opted to offset the output up by 0.2V and then used a second op-amp circuit as a negative impedance converter (NIC) to attenuate the 3.3V input to 2.4V at the output. The schematic can be seen below:

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Simulating the circuit using the spice model for the LM258 gives roughly excellent results (as it always does) that I would be happy with:

enter image description here

In reality, when I prototype the circuit, the output hits 2.4V just fine at the high end, but cannot go below ~0.7V at the low end. When I disconnect the NIC and short circuit the inputs of X1 it works just fine at the low end (but of course the output goes beyond 2.4V at the high end). Can someone explain to me why the NIC is preventing the output from going as low as I want it to and how to overcome that?

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    \$\begingroup\$ What are your supply voltages? (in the real circuit and in the simulation) \$\endgroup\$ – Spehro Pefhany Feb 24 at 14:46
  • \$\begingroup\$ The op-amp supply rails are +12V and 0V. There is an additional 3V3 rail used to generate the offset connected to pull-up resistor R5. Same in both simulation and reality. \$\endgroup\$ – illancha Feb 24 at 14:51
  • \$\begingroup\$ You can do this with three resistors and no opamp, so why bother? \$\endgroup\$ – Brian Drummond Feb 24 at 15:31
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The LM324/LM358 can only sink a few tens of uA down to (near) the negative rail. Try increasing your resistors and maybe add a pull-down resistor. From the datasheet:

enter image description here

Note that the 50uA is nominal, only 12uA is guaranteed, and that only at 25°C.

I would suspect your SPICE model is not very accurate if the simulation does not reflect actual behavior better than that.

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    \$\begingroup\$ Your suggestion to increase the resistances worked well, thanks! I increased R2 and R3 by ten times each and now I can stably hit both outputs 0.2V and 2.4V at the extremes. However, a significant amount of instability is introduced somewhere in the middle. I will continue to tinker with it tomorrow and see where I get. Much appreciated! \$\endgroup\$ – illancha Feb 24 at 15:24
  • \$\begingroup\$ I assume you're just doing this for academic purposes, because this isn't the easiest way to accomplish the stated task. \$\endgroup\$ – Spehro Pefhany Feb 24 at 15:28
  • \$\begingroup\$ How would you do it, bearing in mind that I am limited by 12V and 3V3 rails? \$\endgroup\$ – illancha Feb 24 at 16:22
  • \$\begingroup\$ As @Brian said, you can do it with three resistors, plus a voltage follower if you need low output impedance. A bit trickier if you need to have high input impedance (which your circuit does not have). \$\endgroup\$ – Spehro Pefhany Feb 24 at 17:08

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