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I am building a square wave generator using LM741 just like in this Falstad simulation.

enter image description here

My problem is that the output has a small offset and I don't understand where it comes from.

When measuring input signal at the - pin, I see the charging and discharging of the capacitor which also has an offset error.

Why is that? Is it discharging faster than it is charging? From what I know it can't be. When I measured the same circuit but I used LTspice and the internal circuitry of LM741 I get the same offset (as it should have been, theoretically speaking.)

How can I get rid of it or in the real case make it smaller?

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    \$\begingroup\$ A schematic is better than words. There's a CircuitLab button on the editor toolbar if you don't have your own. Use Save and Insert to place an inline, editable schematic in your post. \$\endgroup\$
    – Transistor
    Commented Oct 26, 2021 at 15:08
  • \$\begingroup\$ the root of your problem is probably that you're using a 741, a terrible, terrible outdated op amp that should never be used in any modern designs. \$\endgroup\$
    – Hearth
    Commented Oct 26, 2021 at 15:09

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The 741 model has an unequal high vs. low swing, which you can see in your simulation. Since you’re making a square wave that swings to the max and min output, this unequal swing will add a bias to your signal. This bias is also present on the fed-back inputs.

Input bias currents would also add an offset. But it’s mostly about the unequal swing here.

If it’s absolutely essential that the swing be symmetrical about ground, you could add a FET pair to the output and get rail-to-rail swing. Watch out for the gate-source voltage though, since you have a nearly 30V swing. Use voltage dividers if you have to.

Example (simulate it here):

enter image description here

Or, use a better op-amp that has rail-to-rail swing.

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  • \$\begingroup\$ The 741 opamp design is 53 years old and it has many problems. A simulation does not show all its problems. Get rid of it and use a modern opamp instead. \$\endgroup\$
    – Audioguru
    Commented Oct 26, 2021 at 21:12
  • \$\begingroup\$ Ok thank you for your answer! I have one more question though, what exactly causes this swing effect? \$\endgroup\$
    – PowerTb321
    Commented Oct 28, 2021 at 11:37
  • \$\begingroup\$ The fact that you’re driving the output to the max and min means that the output transistors are driven into saturation. These have an unequal collector-emitter saturation voltage. \$\endgroup\$
    – hacktastical
    Commented Oct 28, 2021 at 16:21

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