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So we're studying Schmitt triggers at university and we got these representations. Searched them on google for more info and got other representations. Now I'm confused. Can anyone help me with some info regarding the correctitude of these gates?

The representation we got for a positive hysteresis trigger:

Positive hysteresis trigger

This one was ok, found it the same online.

The representation for inverting trigger I found online:

Inverting Schmitt trigger

Another representation I found online:

Inverting Schmitt trigger with reversed hysteresis symbol

The representation we got on our course:

Inverting Schmitt trigger with reversed hysteresis symbol and dot on output

Now, with my logic, the one we got on class for inverting trigger is wrong. May anyone confirm or help understand it if I got it wrong? Also, how does it affect the signal output, is it the same as the positive hysteresis one with non inverting gate?

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3 Answers 3

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Most Schmitt triggers started as inverting, and the "wrong symbols "show an inverting transfer function could have been copied from examples prior to the standard.

Schmitt Triggers do not have to be inverting. They may be buffered or combined with other inversions to make them non-inverting just as the accepted symbol.

enter image description here

The inversion dot, ° may be on the input or output.

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    \$\begingroup\$ Great, cleared some other things that were in my course. Appreciate ! \$\endgroup\$
    – Alex Prapugicu
    Jun 13, 2019 at 20:44
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According to IEEE Std 91, Standard Graphic Symbols for Logic Functions, the correct way to indicate an input with hysteresis, such as for a Schmitt trigger, is:

enter image description here

So your third and fourth examples, which show the hysteresis symbol mirrored left-to-right, are not correct.

The correct way to indicate an inverted output is with an open circle:

enter image description here

So it looks like your first image is correct for a non-inverting buffer with hysteresis at the input, the second image is almost correct for an inverter with hysteresis, and your third and fourth images are incorrect.

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  • \$\begingroup\$ Okay, this makes sense now. So we apparently got one of them wrongly represented. Thanks for your answer. \$\endgroup\$
    – Alex Prapugicu
    Jun 13, 2019 at 18:42
  • \$\begingroup\$ Glad to help. If an answer on this site helps you, we would appreciate it if you could "accept" the answer. That helps to promote better answers. \$\endgroup\$
    – Elliot Alderson
    Jun 13, 2019 at 18:44
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First, the dot on the right point should be a small hollow circle called a bubble. It indicates that the gate inverts the logical polarity of the signal from positive-true to negative-true. Technically, to invert from negative-true to positive-true the bubble should be at the input, but you rarely see that anymore. Whether of not the gate is inverting or just a non-inverting buffer, that part is independent of whether or not its input stage exhibits hysteresis.

In both the logic and magnetics areas, I've rarely seen it as in your third image, and never have heard that its orientation indicated logical inversion. My vote is for image 1 for non-inverting hysteresis, and image 2 for inverting hysteresis (with a bubble).

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  • \$\begingroup\$ I drew the dot on the last picture in paint because i couldn't find something like that on the internet, i made sure it was 100% as in my course. Thanks for your answer! The course is pretty old and it might have things that are not that updated, regarding the "rarely seen that anymore" part. \$\endgroup\$
    – Alex Prapugicu
    Jun 13, 2019 at 18:46

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