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Besides eliminating noise, one of the advantage of Schmitt trigger is given as speed.

And what I saw that the reason of this is the positive feedback used by Schmitt trigger.

Does that mean Schmitt trigger responses input faster so it makes the input pulse's rising edge sharper comparing to a comparator? And how is positive feedback making things faster? How does it cause the output's rise time to be shorter comparing to a comparator?

(Question is about comparing a comparator and s Schimtt trigger's speed and assuming input pulse has no noise. How Schmitt trigger resulting faster rising edges...)

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  • \$\begingroup\$ Read references given here. \$\endgroup\$
    – periblepsis
    Sep 11 at 0:40
  • \$\begingroup\$ I will try but at least is my assumption correct? Does Schmitt trigger make the input's rising edge sharper comparing to a comparator? Or sharpening rising edge is irrelevant to being Schmitt trigger or comparator? \$\endgroup\$
    – ty_1917
    Sep 11 at 0:48
  • \$\begingroup\$ Or am I wrong: The response time of both a Schmitt trigger and a comparator largely depends on their design and the specific components used. Neither inherently responds faster than the other??? \$\endgroup\$
    – ty_1917
    Sep 11 at 0:59
  • \$\begingroup\$ A schmitt trigger has several purposes. But one of them is to sharpen one or both output edges with respect to input edges. The word 'comparator' is abused and may just be an opamp that compares two voltages but without positive feedback. But if it does use positive feedback, then it's a Schmitt trigger in all likelihood. \$\endgroup\$
    – periblepsis
    Sep 11 at 1:25
  • \$\begingroup\$ It somewhat depends on technology. But in all cases if the input switch is driven by a larger voltage differential charge carriers will be transferred more rapidly than otherwise. At turn on the Schmitt trigger output voltage is partially transferred to the input, increasing delta-Vin at any given moment. At turn off the delta V is decreased so cutoff is reached faster ((but, drive current is lower sooner so ... :-) )) \$\endgroup\$
    – Russell McMahon ♦
    Sep 11 at 5:28

1 Answer 1

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A Schmitt trigger also is a comparator. The reference input is internal, but not in the same way as an comparator.

The rise and fall times are determined by the speed of the internal transistors in both the Schmitt trigger and the comparator.

Hysteresis occurs when the reference is driven lower while the input is driven higher (and vice versa). This accomplishes two benefits:

  1. "Chattering" on the output is eliminated for noisy inputs up to a point. So this better defines the switching threshold.
  2. It allows the internal transistors to operate at their fastest switching speed.

Hysteresis is the main difference between an op-amp operated in open-loop as a comparator and a properly designed comparator with hysteresis. An op-amp can be operated as a comparator by using positive feedback, but care must be taken to avoid unwanted oscillations or instability.

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