Schmitt Trigger: [Square Circuit]

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 V_ut 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 = +V_sat, the voltage across R1 is called upper threshold voltage V_ut.

The input voltage V_in must be more positive than Vut in order to cause the output V₀ to switch from +V_sat to -V_sat using voltage divider rule,

Voltage at (+) input terminal is V_UT=V_ref + R₂ (V_sat-V_ref)/(R₁+R₂) when V₀= +v_sat.

When v0= -v_sat. Hysteris width VH= V_UT - V_LT = 2 R₂ (V_sat)/ (R₁+R₂)

When V0 = -V_sat, the voltage across R₁ is called lower threshold voltage V_lt .the Vin must be more negative than V_lt in order to cause V0 to switch from –V_sat to +V_sat. for V_in > V_lt , V0 is at –V_sat.

Voltage at (+) terminal is V_LT=V_ref - R₂ (V_sat+V_ref)/(R₁+R₂).

·           If the threshold voltages V_ut and V_lt 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 +V_sat and –V_sat.

·           Resistance R_comp=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 V_ut its output switches from +V_sat to –Vsat and reverts to its original state, +V_sat when the input goes below V_LT. The hysteresis voltage is equal to the difference between V_ut and V_lt. Therefore

V_H= V_ut – V_lt.

·           If V_ref=0, V_ut= -V_LT =2 R₂(V_sat)/(R₁+R₂)