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Schmitt trigger
Introduction

A Schmitt trigger is an electronic circuit that is used to detect
whether a voltage has crossed over a given reference level.

A Schmitt trigger circuit is a fast-operating voltage-level
detector.

When the input voltage arrives at the upper or lower trigger levels,
the output changes rapidly.
Schmitt trigger
Symbols of Schmitt trigger
Positive and Negative Feedback

The input voltage vin is applied to the inverting input terminal
and the feedback voltage goes to the non-inverting terminal.

This means the circuit uses positive voltage feedback instead of
negative feedback, that is, in this circuit feedback voltage aids the
input voltage rather than opposing it.
Feed back
Voltage Divider Feedback

The voltage divider feedback a negative voltage to the non-
inverting input, which results in a larger negative voltage.

This feedback more negative voltage until the circuit is driven into
negative saturation.
Schmitt Trigger transfer
Vo
High state
Low state
Vi
0
0
Vt-
Vt+
Calculation of Resistor

To design a Schmitt trigger, potential divider current I2 is once
again selected to be very much larger than the op-amp input bias
current.
Then the resistor R2 is calculated from equation
R2 = UTP/I2
and R1 is determined from
R1 = (VOUT – UTP) / I2
Threshold level

Whenever the input voltage goes over the High Threshold Level,
the output of the comparator is switched HIGH or LOW.

The output will remain in this state, as long as the input voltage is
above the second threshold level, the Low Threshold Level.

When the input voltage goes below this level, the output of the
Schmitt Trigger will switch.
Threshold level
Transistor Schmitt trigger

A transistor implementation of a Schmitt Trigger is rather
important due to the single voltage supply that requires to operate
the basic Schmitt Trigger circuit with two NPN transistors.
Transistor Schmitt trigger
Hysteresis

Hysteresis is desirable in a Schmitt trigger because it prevents
noise form causing false triggering.

The difference between the two threshold voltages is known as
hysteresis
Hysteresis
Hold and measure
VThreshold
Vinput
Voutput
Hysteresis
Hold and measure
The End
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