What is the difference between an hex inverting buffer (e.g. TC74HC4049AP) and hex inverting Schmitt trigger (e.g. 74HCT14N)?

How can hysteresis be added to a CMOS hex inverting buffer to make it act like a Schmitt trigger, if they differ?

  • \$\begingroup\$ There might or might not be a difference; tell us the actual names. Is this a homework question, or what is the actual problem you're trying to solve? \$\endgroup\$
    – CL.
    Commented May 12, 2023 at 10:22
  • \$\begingroup\$ For example between TC74HC4049AP and 74HCT14N ? It's my curiosity \$\endgroup\$
    – Ken90
    Commented May 12, 2023 at 10:26
  • \$\begingroup\$ cannot see anywhere online the difference between an Hex inverter buffer and a hex inverter Schmitt trigger, I read about hysteresis of the last one? \$\endgroup\$
    – Ken90
    Commented May 12, 2023 at 10:29
  • \$\begingroup\$ If you want to add information, please Edit your question instead of posting it as a comment. \$\endgroup\$
    – Velvet
    Commented May 12, 2023 at 10:30
  • \$\begingroup\$ ok will post there \$\endgroup\$
    – Ken90
    Commented May 12, 2023 at 10:36

1 Answer 1


The dfference is that the standard inverting buffers don't have hysteresis and the Schmitt trigger inverting buffer have hysteresis.

Hysteresis just means you have different levels for low going threshold and high going threshold, to have a margin for noise. Without hysteresis there is only one threshold to determine if a signal is high or low, so noisy signal right at the threshold level will just amplify the noise to output and noisy slowly changing signals have multiple noisy output transitions. Hysteresis can remove all that.

The hysteresis is added similar to how you would add feedback to an analog comparator to change the threshold of change.

As an internal block diagram it just has feedback from output back to input which will change the level where the input threshold is.

Instead of resistors the feedback can be done with weak PMOS and NMOS transistors in a CMOS chip.

In logic gate level, there is no difference except the standard inverter symbol has added the symbol to mean it's a Schmitt trigger inverter.

  • \$\begingroup\$ I like this better ten the other answers so far.. More should be said though to explain hysteresis better. Regardless it is just good enough for +1 \$\endgroup\$ Commented May 12, 2023 at 11:06
  • \$\begingroup\$ ietresearch.onlinelibrary.wiley.com/doi/10.1049/el.2012.3191 \$\endgroup\$
    – Ken90
    Commented May 12, 2023 at 12:40
  • \$\begingroup\$ thanks I also found this article relative to CMOS feedback useful after what you explained \$\endgroup\$
    – Ken90
    Commented May 12, 2023 at 12:40

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