Schmitt
Trigger: [Square Circuit]
This circuit converts an irregular shaped waveform to a square wave or pulse. The circuit is known as Schmitt Trigger or squaring circuit. The input voltage Vin triggers (changes the state of) the o/p V0 every time it exceeds certain voltage levels called the upper threshold Vut and lower threshold voltage.
These
threshold voltages are obtained by using the voltage divider R1– R2,
where the voltage across R1 is feedback to the (+) input. The
voltage across R1is variable reference threshold voltage that
depends on the value of the output voltage. When V0 = +Vsat,
the voltage across R1 is called upper threshold voltage Vut.
The
input voltage Vin must be more positive than Vut in order to cause
the output V0 to switch from +Vsat to -Vsat
using voltage divider rule,
Voltage
at (+) input terminal is VUT=Vref + R2 (Vsat-Vref)/(R1+R2)
when V0= +vsat.
When
v0= -vsat. Hysteris width VH= VUT - VLT = 2 R2
(Vsat)/ (R1+R2)
When
V0 = -Vsat, the voltage across R1 is called lower
threshold voltage Vlt .the Vin must be more negative than Vlt
in order to cause V0 to switch from –Vsat to +Vsat. for Vin
> Vlt , V0 is at –Vsat.
Voltage
at (+) terminal is VLT=Vref - R2 (Vsat+Vref)/(R1+R2).
·
If
the threshold voltages Vut and Vlt are made larger than
the input noise voltages, the positive feedback will eliminate the false o/p
transitions.
·
Also
the positive feedback, because of its regenerative action, will make V0
switch faster between +Vsat and –Vsat.
·
Resistance
Rcomp=R1 || R2 is used to minimize the offset
problems.
·
The
comparator with positive feedback is said to exhibit hysteresis, a dead band
condition. (i.e) when the input of the comparator exceeds Vut its
output switches from +Vsat to –Vsat and reverts to its original
state, +Vsat when the input goes below VLT. The
hysteresis voltage is equal to the difference between Vut and Vlt.
Therefore
VH=
Vut – Vlt.
·
If
Vref=0, Vut= -VLT =2 R2(Vsat)/(R1+R2)
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