I have a project where I want to take the positive and negative outputs of a passive differentiator and produce only positive pulses. E.g. when the switch feeding the differentiator is closed I get a positive pulse which I want to merely pass along; when the switch is opened I get a negative pulse which I want to invert and also pass along as a positive pulse. I got this to work by feeding the output into an active rectifier circuit. But the interesting thing is that while experimenting with Ltspice I found that I could achieve the same result with the voltage follower configuration shown below, and sure enough it worked on the breadboard, too.

I understand that when you exceed the ratings for a particular device you can get bad or unexpected results. (In this case my negative going pulses approach minus nine volts whereas the LM358 VICR is -.3V to 32 volts.) Still, if anyone can explain exactly why this works, I would be glad to know.

Also, is the op amp being damaged? Is abusing an op amp in this way a bad practice?


simulate this circuit – Schematic created using CircuitLab

enter image description here

  • \$\begingroup\$ You got lucky! The data sheet states as you did results are not predictable when going out of specifications. I would not expect the same results from different op-amps. The input protection diodes should be shown in your circuit that will help you to understand it. Note you get a second pulse when the pulse goes positive. The opamp should be OK, you are below the max input current. \$\endgroup\$
    – Gil
    Commented May 12, 2021 at 3:12
  • \$\begingroup\$ @Gil. Ah. I just found this in the LM358 application design guidelines: "If either input or both inputs are lower than –0.3 V with respect to the negative supply, excessive input current can flow and the output may display phase reversal, also called inversion." \$\endgroup\$
    – RichardD
    Commented May 12, 2021 at 14:25

1 Answer 1


The current into the input during the negative pulses is small, so no damage occurs. But leaving the input common mode voltage range can cause phase reversal leading to a high output. This often happens for old opamps.


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