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 nega tive voltage until the circuit is

driven into negative saturation. 

Schmitt Trigger transfer

Low state

High state

0

0

Vo

Vt- Vt+Vi

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

Hysteresis

Hold and measure

Hold and measure

VThreshold

Vinput

Voutput

The End

……. Thank You …….