Schmitt trigger. V I is converted from a sine wave into a square wave. V O switches between +V SAT SAT and is in phase with V I.

Transcription

1 When you have completed this exercise, you will be able to operate a sine wave to square wave converter. You will verify your results with an oscilloscope. Schmitt trigger. V I is converted from a sine wave into a square wave. V O switches between +V SAT SAT and is in phase with V I. FACET by Lab-Volt 251

2 Op Amp Fundamentals TH H. Two trigger points are established by the positive feedback. V TH provides a voltage at the noninverting input of U1. Therefore, the circuit is not zero based. V O is in phase and equals a. half the frequency of V I. b. the frequency of V I. c. twice the frequency of V I. Hysteresis, or positive feedback, creates a window, or band, through which V I must cross before the comparator switches V O. 252 FACET by Lab-Volt

3 O occurs at trigger points A and B. V O cannot switch back until V I crosses a trigger point. Within the window, variations of V I do not cause U1 to switch. The voltage difference between points A and B, labeled +V T T, equals the circuit hysteresis voltage. An increase of positive feedback increases the width of the hysteresis window. A decrease of positive feedback decreases the width of the hysteresis window. a. increase b. decrease FACET by Lab-Volt 253

4 Op Amp Fundamentals Hysteresis reduces the sensitivity of a comparator, increasing the noise immunity of the circuit. Based on V I generates many changes in V O. The number of transitions in V O decreases as hysteresis increases. T T T +V T = +V SAT T SAT The trigger voltages are based on the saturation voltage of U1. Therefore, the trigger points are not symmetrical and vary from op amp to op amp. 254 FACET by Lab-Volt

5 Connect channels 1 and 2 of your oscilloscope to the circuit, and monitor V I and V O. Adjust V I for a 2 V pk-pk, 1000 Hz sine wave. Is V I converted into a square wave output? a. yes b. no FACET by Lab-Volt 255

6 Op Amp Fundamentals Expand the oscilloscope time base setting to 20 voltage. I? a. yes b. no Momentarily monitor the noninverting terminal of U1 on channel 2 of your oscilloscope. Based on your observations of V O and V I, is positive feedback generated at the noninverting terminal of U1? a. yes b. no T on the V I your circuit. Measure V to +V. +V T = V pk Recall Value 1 T on the V I your circuit. Measure V I at a point where U1 just starts to switch from +V SAT SAT. Based on a comparison between +V T of V pk Recall Value 1 T, a. the circuit trigger points are symmetrical. b. hysteresis is effective for both positive and negative alternations of V I. c. hysteresis is effective only for the positive alternation of V I. 256 FACET by Lab-Volt

7 If the level of positive feedback increases, the circuit trigger voltage a. increases. b. decreases. If the level of positive feedback increases, the phase shift between V O and V I a. increases. b. decreases. Place CM switch 13 in the ON position to increase the level of positive feedback. You can toggle the CM off and on to see the difference. a. The phase shift increases because V O is triggered later in the V I cycle. b. Changing the level of positive feedback has no effect on the circuit phase shift. c. The phase shift decreases because V O is triggered later in the V I cycle. An op amp comparator can convert a sine wave input into a square wave output. Positive feedback sets the trigger points of a comparator circuit. An increase of positive feedback increases the phase shift of a comparator circuit. FACET by Lab-Volt 257

8 Op Amp Fundamentals 1. Adjust V I for a 2 V, 1000 Hz sine wave. Place CM switch 14 in the ON position to change the positive feedback in the circuit. How does the CM affect the trigger voltage of the comparator? a. The trigger voltage increases. b. The trigger voltage decreases. c. There is no change in the trigger voltage. d. The trigger voltage of the comparator cannot be determined. 2. Based on your observations, the CM a. increases circuit hysteresis. b. decreases circuit hysteresis. c. has no effect on circuit hysteresis. d. reduces circuit hysteresis to 0 V. 3. Based on your observations, the circuit a. has positive feedback. b. is a Schmitt trigger. c. has a square wave output waveform. d. All of the above. 4. In this circuit, V O and V I are a. exactly in phase. b. slightly out of phase. c. 180 out of phase. d. None of the above. 258 FACET by Lab-Volt

9 5. Which term best describes hysteresis? a. negative feedback b. degenerative feedback c. positive feedback d. slew rate FACET by Lab-Volt 259