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

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

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:

• The power supplies were already set to +-5Volts Jul 15, 2023 at 17:10
• 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. Jul 15, 2023 at 17:32