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I've got a few LM741 op-amps that I purchased by accident months ago, but now I'm in a situation where I believe they may be useful.

What I want to do is convert a positive voltage to the exact equal output, but as a negative.

So, if my input is +2.2v, I want the output to be -2.2v (actually the ranges are 0-0.237v). I have read quite a bit on how op-amps work, but a lot of the details go quite a bit above my head and quickly go beyond my understanding when I research what I'm trying to achieve.

It looks like I need to wire it as an "inverting amplifier" which I do understand how to do, but I don't know exactly what is required to ensure the output is exactly the same as the input, but reversed polarity.

My question is whether someone could possibly point me in the right direction as to how to achieve what I'm attempting to do (if possible with the chip specified).

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    \$\begingroup\$ Related: electronics.stackexchange.com/questions/148356/… \$\endgroup\$
    – Big6
    Aug 11 '17 at 23:47
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    \$\begingroup\$ To make this work you will need to provide both positive and negative supply rails. You might be able to get away with using one 9V battery to make the +9 rail and another 9V battery to make the -9V rail, but that's not guaranteed to work. LM741 wants more like +15V/-15V supply rails if I remember correctly. It definitely cannot drive an output voltage that is below its V- supply rail, so a negative supply voltage is required. See this canonical question about the limitations of the LM741: electronics.stackexchange.com/questions/304521/… \$\endgroup\$
    – MarkU
    Aug 11 '17 at 23:54
  • \$\begingroup\$ Thanks to both of you thus far. I will follow up on the links. \$\endgroup\$
    – stevieb
    Aug 11 '17 at 23:57
  • \$\begingroup\$ @MarkU I now see why the op-amps I have aren't ideal, thanks for the link. Requiring ~10v is kind of an issue, as I was hoping to run the rails at the same output at 3.3v (Raspberry Pi). I've learned quite a bit from this question both on the operation of the ICs and the limitations of specific chips. \$\endgroup\$
    – stevieb
    Aug 12 '17 at 0:47
  • \$\begingroup\$ Pay attention to how much current you need the op amp tovvsource or sink \$\endgroup\$ Aug 12 '17 at 2:19
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Generally, an inverting op-amp circuit with a gain of 1 will accomplish that. Choose R1 and R2 to be the same and you will get a gain of 1, with an inverted signal. I am not a proponent of the 741 since there are newer op-amps that don't require as much fussing. But the concept is the same regardless of which op-amp you use. Operationally, in the circuit shown, the output will swing to whatever voltage is needed to keep the (-) input at 0v, which turns out to be equal in magnitude, but opposite in sign to your input signal. Your choice for R1/R2 is flexible, but as an example you could use R1=R2=10K.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ This is great, thank you very much. I'll give this a try. I think what I was looking for here was the R1 and R2 values. Not looking for a recommendation here, but specifically wondering what op-amp equivalent that you personally prefer to the ones that I have. \$\endgroup\$
    – stevieb
    Aug 12 '17 at 0:41
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    \$\begingroup\$ I have used and been satisfied with both the LM224 and LM1458. I don't have any attachment to them. When I was choosing devices for my projects they probably had the right number of devices per IC and were readily available. \$\endgroup\$ Aug 12 '17 at 0:54
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    \$\begingroup\$ MCP6001? They're ridiculously cheap \$\endgroup\$
    – Reinderien
    Aug 12 '17 at 1:12
  • \$\begingroup\$ As for the resistor values, 10k is a real good starting point. How precise you need the values depends on how how exact you want your inversion to be. You can get resistors in 1% easily, and 0.1% fairly cheaply, with even better precision for escalating price. But going for much better than 0.1% is probably not worthwhile, since you then start seeing errors produced by the op amp itself. \$\endgroup\$ Aug 12 '17 at 2:26
  • \$\begingroup\$ @Entrepreneur: you are aware that the 1458 is nothing but two 741s in one chip, right? No real difference, there's just two of the grotty things in one package. \$\endgroup\$
    – JRE
    Aug 12 '17 at 9:07

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