The Schmitt Trigger

The Schmitt trigger is a comparator application which switches the output negative when the input passes upward through a positive reference voltage. It then uses positive feedback of a negative voltage to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point. That is, it provides feedback which is not reversed in phase, but in this case the signal that is being fed back is a negative signal and keeps the output driven to the negative supply voltage until the input drops below the lower design threshold.

Comparator Applications
Schmitt Trigger Action
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Op-amp concepts

Reference
Simpson
Sec 10.19
 
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Schmitt Trigger Action

The Schmitt trigger action uses a comparator to produce stable level-crossing switches in contrast to the action of a straight reference comparison.

Comparator Applications
For circuit parameters:
= V
=
=
=
the trigger voltage will be
= V
and it will hold until the signal drops to
= V
Show expressions
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Op-amp concepts
 
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The Schmitt Trigger

Schmitt trigger action is a double threshold comparator process. The current equation at A gives:

where

This dependence upon the output voltage gives the dual threshold. The two output states give the thresholds shown at right.

Calculation
Index

Electronics concepts

Op-amp concepts

Reference
Simpson
Sec 10.19
 
HyperPhysics*****Electricity and magnetismR Nave
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