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Part 3: positive feedback (hysteresis of Schmitt trigger circuit) Modify the comparator sub-circuit of Part 2 by applying a positive feedback between the op amp's output and its positive signal input. Note the addition of R1 and R2. This is called a Schmitt trigger (negative Schmitt trigger since the comparator is a negative comparator). R4 R5 Vin V4 1k 1k R3 40+ U2 V3 1k LM741/NS SINECO 100m 1k000) SINE(O 10.01k000) V+ 1 Vi 11 R1 Vin Vout 15 1k LM741/NS V2 Vref R2 15 19k .tran 100m include C:\Program Files\LTC LTspiceXVII\lib\sub\LM741.MOD Record: Use a scope to record and overlay both the Vin and Vout waveforms. Explain: why the positive feedback removes the glitches at the zero-crossing region. Hint: what circuit topology does R1 and R2 effectively form? How does this topology affect the actual reference voltage seen by the op amp comparator? Explore: Sketch Vout vs. Vin. Use the following concepts to explain the hysteresis, which is the difference between the upper trip point (UTP) and the lower trip point (LTP). Feedback fraction B = R1 / (R1 + R2) Upper trip point UTP = B Voh, where Von is the op amp's positive saturation voltage (which is Vcc -1, and is usually about 14V) Lower trip point LTP = -B Vol, where Vol is the op amp's negative saturation voltage (which is -Vcc + 1, and is usually about -14V) VHYSTERESIS = UTP - LTP = 2B Vcc =

VHYSTERESTS = UTP - LTP = 2B Vcc Vout Hysteresis VOH VOL VUTP VLTP Explore: Vary R2 (by using a decade box) to see how it affects the hysteresis. What will happen to Vout is R2 is too small? What will happen to Vout if R2 is too big? | Part 3 data table Overlay of Vin and Yout waveforms Explain why the positive feedback removes the glitches at the zero-crossing region. UTP value LTP value Hysteresis sketch with actual values How does R2 affect Vout if it is too small? Explain why. How does R2 affect Vout if it is too big? Explain why.