When you have completed this exercise, you will be able to operate a zener-clamped op amp comparator circuit using dc and ac voltages. You will verify your results with an oscilloscope. U1 is zero based because its noninverting terminal is connected to circuit common. Therefore, the circuit reference voltage is 0 V. If V I O SAT. 242 FACET by Lab-Volt
Voltage Comparators When V O Z feedback. The feedback clamps V O to the value of V Z and prevents negative saturation. In this circuit, V O equals a. SAT. b. 0.7 Vdc. c. 0 V. d. +V SAT. O switches between Z and +V F. V Z is the zener diode breakdown voltage, and V is the forward voltage drop of CR1. FACET by Lab-Volt 243
In this circuit, the cathode of CR1 is connected to the U1 output terminal. As a result, V O switches between a. +V Z F. b. Z and +V F. c. +V SAT SAT. If V O equals +5.1 Vdc, V I a. is negative. b. is positive. c. can be either negative or positive. A zener diode clamped circuit controls the levels of a sine wave or square wave input signal. Z and +V F. If the CR1 connections are reversed, clamping occurs at +V Z F. 244 FACET by Lab-Volt
Voltage Comparators I and V Z a. U1 is saturated. b. CR1 cannot conduct. c. V O d. All of the above. Locate the OPEN-LOOP COMPARATOR circuit block, and connect the circuit shown. Use your oscilloscope to monitor V I and V O. Observe the oscilloscope pattern as you slowly vary V I its total range of voltages. FACET by Lab-Volt 245
Based on your oscilloscope waveforms, the closed-loop comparator switches V O when V I equals a. V Z and V F of CR1. b. Observe the oscilloscope pattern as you slowly vary V I total range of voltages. Based on your observations, V O a. follows the level of V I. b. switches between +V SAT SAT. c. z and +V F. d. switches between +V I I. Place CM switch 6 in the ON position to reverse the direction of zener diode CR1. Vary V I through its total range. Based on your oscilloscope pattern, a. the direction of CR1 does not affect the comparator operation. b. V O is clamped at +V SAT SAT. c. V O is clamped at +V z F. d. U1 is now open-looped because the CR1 connections are reversed. CM 6 is still activated to reverse the direction of zener diode CR1. Modify your circuit as shown, and maintain the oscilloscope connections. pk-pk, 1000 Hz square wave. 246 FACET by Lab-Volt
Voltage Comparators Based on V I and V O, the zero based comparator a. is saturated by a square wave input waveform. b. shifts the voltage levels of a square wave input waveform. c. switches between +V Z F. d. Place CM switch 6 in the OFF position. Based on your oscilloscope waveforms, the anode of CR1 is a. connected to the input of the comparator circuit. b. not connected into the comparator circuit. c. connected to the output of the comparator circuit. Modify your circuit as shown. Move channel 1 of your oscilloscope to the junction of R1, R2, and R3, and adjust V REF for 0.0 V. V. FACET by Lab-Volt 247
V O switches between +V F Z. Based on a comparison between V O and V IN, U1 switches when V IN a. equals +V SAT SAT. b. equals +V Z F. c. Z and +V F. d. is slightly above or below V REF Adjust V REF to its maximum negative level. Based on your circuit waveforms, the comparator trips at about a. +2.7 Vdc. b. 0 V. c. d. +5.1 Vdc. Adjust V REF to its maximum positive level. Based on your circuit waveforms, the comparator trips at about a. +2.4 Vdc. b. 0 V. c. d. +5.1 Vdc. Based on your observations, the comparator trip point is a. not related to the circuit reference voltage. b. essentially controlled by the circuit reference voltage. 248 FACET by Lab-Volt
Voltage Comparators V O switches between +V F Z. If CR1 is reversed, V O F and +V Z. The circuit trip point essentially equals the value of V REF. If V Z is changed, the peak values of V O vary. Although the peak values of V O Z F 1. The trip point of this circuit approximately equals a. V Z. b. V F. c. 0 V. d. V SAT of U1. 2. If zener diode CR1 is reversed, the circuit trip point equals a. V Z. b. V F. c. 0 V. d. V SAT of U1. FACET by Lab-Volt 249
3. If V I equals +10 Vdc, V O equals a. b. +5.1 Vdc. c. d. +0.7 Vdc. 4. If V I is less than 2 Vdc, V O equals a. b. +3 Vdc. c. +2.7 Vdc. d. 5. If V I is greater than 4 Vdc, V O equals a. b. +3 Vdc. c. +2.7 Vdc. d. 250 FACET by Lab-Volt