I'm using a single supply op amp. Specifically the LM2904NG. I am trying to invert 0.67 V to -0.67 V. My output is in fact positive. I simulated the first single supply op amp I could find (LT1006) in LTspice in order to see if I could figure it out. I started with just trying to invert 3.3 V before adding in more circuitry but it does not work. It outputs 136 mV in fact.

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My expectation is that the output will be enter image description here where Rf=R4 and Rin=R1. This is not the case. Am I missing something basic about single supply op amps? Can these not provide a voltage below their negative rail? I suppose this seems reasonable.

  • \$\begingroup\$ Where is it supposed to get this negative voltage from? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 22 '15 at 1:37
  • \$\begingroup\$ Basic op-amp circuits can not provide an output voltage below their negative rail. \$\endgroup\$ – Oka Sep 22 '15 at 1:42
  • \$\begingroup\$ I'm not sure. I've had limited experience with single supply op amps. My experience with dual supply op amps leads me to believe an input of 3.3 V will lead to an output of -3.3 V from my schematic. I supposed my question boils down to, can a single supply op amp output a voltage below the negative rail voltage? \$\endgroup\$ – AesBubo Sep 22 '15 at 1:42
  • \$\begingroup\$ @AesBubo no, it cannot. What makes single supply opamps special is, that they allow your inputs to go to 0V and often allow the output to go very close to 0V as well. They are not capable of generating negative voltages though. \$\endgroup\$ – Nils Pipenbrinck Sep 22 '15 at 1:44
  • \$\begingroup\$ Understood, is there a way to generate a positive and negative voltage from a single supply? \$\endgroup\$ – AesBubo Sep 22 '15 at 1:46

A negative supply is required to get this to work. One way of getting a negative supply from a positive is to use a DC-DC converter module.

Another way is to use a switched capacitor part such as an ICL7660 or TPS60402.

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