 Welcome to the session on OPAM as smith trigger. Learning outcomes are at the end of session students will be able to explain the working of inverting smith trigger as well as they can sketch the output waveforms for given input signal for the smith trigger circuit. Contents are like this before moving towards OPAM as a smith trigger recall the concept of open loop comparators where the output voltage will be containing errors nothing but noisy output signal due to the false transitions at the output. So, it will produce oscillating output with a noisy inputs or slow varying input. To overcome this disadvantage positive feedback can be used and we can obtain the output without any error smith trigger is nothing but a inverting comparator circuit as we are applying the input signal to the inverting terminal of operational amplifier as well as it convert any regular or irregular shaped input waveform into a square waves output voltage. Therefore, it is also called as a squaring circuit. Look at the circuit which is a inverting comparator nothing but smith trigger where register ROM is used to minimize the offset voltage at the output. ROM can be the parallel combination of R1 and R2 which is connected in series with the input signal source to the inverting terminal of OPAM. OPAM is biased with the given specifications and register R2 is used as a feedback register. R1 and R2 forms the voltage divider circuit and the voltage across register R1 will be treated or will be acting as a reference voltage for the comparator circuit nothing but inverting comparator circuit. So, V1 is nothing but voltage at the non-inverting terminal of OPAM which can be calculated as R1 divided by R1 plus R2 into V0. As we are using positive feedback over here we can obtain the output voltage varying between plus Vsat to minus Vsat and minus Vsat to plus Vsat only. So, as you know the output voltage of operational amplifier is the difference between the two voltages that is V1 minus V2 as the reference voltage at the non-inverting terminal is V R1 by R1 plus R2 into V0 as input signal is varying from 0 to plus positive direction and the negative direction. We can obtain the two reference voltages that is VUT, VU2 can be R1 by R1 plus R2 into plus Vsat. As I told you we are configuring the OPAM with the positive feedback so output voltage will be plus Vsat or minus Vsat. So, here V0 is replaced by plus Vsat and lower threshold point is nothing but VAT where output voltage is minus Vsat. So, look at the waveforms of the Smith trigger circuit initially due to the offset voltage at non-inverting input output voltage will be plus Vsat. So, this is the initial condition where output voltage is plus Vsat and therefore V1 that is voltage at non-inverting terminal of OPAM will be upper threshold voltage that is VUT calculated as R1 by R1 plus R2 into plus Vsat. Now, when input signal is exceeding this voltage that is VN is greater than VUT so output voltage will be now plus Vsat to minus Vsat. So, this is the first condition or first threshold condition. Now, whenever VN is greater than VUT or as long as VN is greater than VUT output will be in the minus Vsat value or output gives the minus Vsat level. So, this is the VUT of the signal. Now, when VIN is less than VLT now see here condition second condition is VIN less than VLT. So, this is lower threshold point and when signal is less than this then output voltage V0 will switch from minus Vsat level to plus Vsat level. So, as long as VIN is less than VLT that is this much portion output voltage will be at plus Vsat as there are only two threshold conditions there will be no any transition in between these two points that is in between VUT to VLT. Now, assume when input signal is one volt pick to pick so we can have the signal from 0 to 0.5 in the positive direction 0 to 0.5 in the negative direction and supply voltages are plus or minus 15 voltage 15 volt nothing, but VCC is plus 15 and VW is minus 15. Assuming R1 has 100 ohm R2 has 56 kilo ohms at that time VUT will be R1 by R1 plus R2 into plus 15 and VLT will be R1 by R1 plus R2 into minus 15. So, output waveform will look like this see here if you put these values you will get the threshold point at approximately 26 milli volt. So, as long as input is less than VUT it will be in the plus Vsat level, but when it crosses the VUT value then it will switch from plus Vsat to minus Vsat it will remain in the same state, but when the output voltage is less than this VLT value then it switches from minus Vsat to plus Vsat. So, see carefully over here this is the input waveform which varies from 0 to 0.5 in the positive direction, 0 to 0.5 in the negative direction these are the two threshold lines and now for these two threshold lines this is the output voltage. So, output voltage will vary between 0 to Vsat. So, as we are taking the plus 15 volt over here, but there is condition for the output voltage swing that is maximum output voltage swing as well as there will be drop across the IC. So, you will get the signal output signal varies between 0 to 14 volt and 0 to minus 14 volt. So, here hysterics voltage plot can be plotted like this. So, as you know comparator with positive feedback produces the dead band condition and very important thing this dead band is depends on the feedback factor. Feedback factor is R1 divided by R1 plus R2. On x axis there is V y axis V0. So, as V in goes on increasing as V in is goes on increasing in the positive direction and it will be at the plus Vsat level initially, but once it crosses the VUT condition it will drastically changes state from plus Vsat to minus Vsat. Again it will be in the minus Vsat, but when it crosses the lower threshold voltage level then it will give you the transition from minus Vsat to plus Vsat. So, you can see here hysterics voltage can be calculated as the difference between two points that is VUT minus VLT. If you put the values of VUT and VLT this is the formula for threshold width. When we go for the inverting comparator then always you will obtain the hysteresis voltage loop in the clockwise direction. So, here this is when negative to positive and here from positive to negative in this way you will get the hysteresis voltage plot. In the Schmitt trigger circuit assume Vcc has plus 5 volt and Vdebury has minus 5 volt, Vn varies between this 10 volt pick to pick, R1 and R2 values are same. Now, find the threshold voltage values and draw the output waveforms for the given condition. So, pause the video and draw the output waveforms. Yes, this is the output for given condition these are the two threshold values keep in mind as Vsat is there and R1 and R2 values are same you will get the threshold voltage at the half of this. So, here threshold is 2.5 and minus 2.5 respectively for positive and negative values and output voltage will now swing from 0 plus 15 to minus 15 respectively. As we are basing the opamp between plus 5 to minus 5 you will get the output voltage transitions from 5 to minus 5 and minus 5 to plus 5. Very important thing the amplitude of square wave is independent of the pick to pick value of input waveform applications of Schmitt triggers. It is used to convert a very slowly varying input voltage into output having abruptly varying waveform occurring precisely at predetermined values of input voltage. It is used when the comparator it is using all applications where comparator circuit is used where input voltage has to be converted to square wave signal. Very important thing for Schmitt trigger is that input signal must have enough excursion to carry the input voltage beyond the limits of the hysteresis range. Therefore, you must see that what is the amplitude of input voltage and what is the threshold points or VU2 and VLT. Then and then you can obtain the hysteresis curve nothing but you will obtain the transitions in the output voltage. References are like this. Thank you.