REGENERATIVE COMPARATOR (Schmitt trigger)

  • Schmitt trigger is a comparator circuit with positive feedback ,as gain can be increased greatly.The major application of this comparator is to overcome the noise voltages which reduced the problem with zero crossing detector circuit.it converts the analog signal to digital signal.

the above circuit is an inverting Schmitt Trigger circuit.

  • From the circuit .

                                                   

                              Vref =Reference voltage

                              Vo=Output voltage

  • The input voltage Vi triggers the output  Vo every time it exceeds certain voltage levels .These voltage levels are called upper threshold  voltage and lower threshold voltage.
  • These thrreshold voltages are calculated as follows .

Va =VUT (upper threshold voltage)   for  Vo=+Vsat  then 

                           

                                               

  • As long as Vi is less than the Vut the output remains +Vsat .When ever the input voltage exceeds the  Vut  the output shifts to –Vsat  and remains constant till the input voltage is greater than Vlt. When Vi become lesser than VLt then the output switches from –Vsat to +Vsat .And goes on as mentioned above…..

                 Va =VLT (lower  threshold voltage)   for Vo=–Vsat  then 

  • All the transitions takes place are instant .
  • The input and output  wave form is as follows .

  • The transfer characteristics for Vi are as follows.

  • This circuit exhibits a interesting phenomenon called hysteresis or backlash ,as the        curve looking like the hysteresis curve of magnetic materials . The difference between the two voltages(Vut,VLt) is called hysteresis voltage or hysteresis width.

  • Because  of hysteresis the circuit triggers at a higher voltage for increasing signals than for decreasing signals. if the peak-to-peak noise voltage is within the hysteresis voltage ,there will be no false triggering .Depending on the expected noise voltage ,the hysteresis voltage can be designed by the feedback resistors R1 and R2. 

   

  • For Vref=0.

   

        Therefore a square wave of phase shift  ‘‘ is obtained from the sine wave (input signal ).

  • let

       Vi=Vm sin and Vi=Vut

then 

         sin=Vut/Vm

Applications

consider a clean signal and with noise signal crossing zero axis a number of times then the normal zero crossing detector  will not deliver the same output even though both signals are same but of course the noise is included in other signal .But the trigger circuit reject the interference with zero crossings  and takes the crossings of Vut and VLt only .

a noisy signal (u) fed into a comparator (A) and a schmitt trigger(B).

Note points 

Vi < VLt ,Vo = +Vsat

Vi >Vut , Vo = -Vsat

VLt  < Vut







The images and content are obtained from various references .