In the given problem, I know that the answer should be (d), because it should act like a Schmitt trigger.

Schmitt trigger

But when I try to model this circuit in Flastad, I suddenly get that the answer (b) is right.


Perhaps I miss something again. The op amp should act like an output of either 5 of -5 volt, depending on the non-inverting voltage value and whether it is lower or higher than zero reference inverting voltage input.

  • 1
    \$\begingroup\$ Well, this is "normal" some simulation programs will have a problem with positive feedback. And you can nothink do about it. \$\endgroup\$
    – G36
    Jul 15, 2023 at 15:11
  • \$\begingroup\$ Changed the amplitude to 10 Volts, now it acts like a Schmitt trigger \$\endgroup\$
    – KrtekUser
    Jul 15, 2023 at 15:34
  • \$\begingroup\$ In theory, Vin slightly larger than 5V should be enough. \$\endgroup\$
    – G36
    Jul 15, 2023 at 15:46
  • \$\begingroup\$ What is the concept of this theory? \$\endgroup\$
    – KrtekUser
    Jul 15, 2023 at 16:14
  • \$\begingroup\$ The concept of this theory is that Falstad's simulator is giving you the wrong answer. \$\endgroup\$ Jul 15, 2023 at 23:10

1 Answer 1


You've used an op-amp model that has hidden power supplies. The default in this online simulator appears to be +/-15V for the limits on the output. So you'd expect the ST to switch at +/-15V thresholds.

Real op-amp outputs will saturate at some voltage less than the power supplies, depending to some degree on the loading (your 1KΩ resistors) are a relatively heavy load for some op-amps).

Anyway, change the output limit numbers (or use a model with power supply connections and appropriate power supply voltages) and it should behave more as you expect:

enter image description here

  • \$\begingroup\$ The power supplies were already set to +-5Volts \$\endgroup\$
    – KrtekUser
    Jul 15, 2023 at 17:10
  • 1
    \$\begingroup\$ For the ST to switch the non-inverting input has to reach 0V, so the thresholds will be +/-5V with an output of +/-5V. Your signal has to be a bit larger than that. Try 5.1V peak. If it's being driven by a similar op-amp you'd want to make the feedback resistor higher than the input resistor or it will never switch. \$\endgroup\$ Jul 15, 2023 at 17:32

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