Experiment #3 EEL4314L Schmitt Trigger Comparator Page 1
Digital Applications of the Operational Amplifier
This experiment shows the versatile operations of an operational amplifier as a voltage comparator
(Schmitt trigger) circuit and a sample-and-hold circuit.
A voltage comparator is a two-input circuit that compares the voltage of one input to the voltage of
another input. Usually one input is a reference voltage and the other input is a time-varying signal.
When the time-varying input is below or above the reference voltage, then the comparator provides a
low or high logic output accordingly. The ideal operation of a comparator is shown in Figure 1. If VREF
is zero, the comparator can be used as a zero-crossing detector. If VREF is not zero, the comparator can
be referred to as a level detector. The comparator is the basis for most A/D circuits. In these A/D
circuits, the reference voltage is gradually changed by a counter and compared to the analog input
voltage until it equals the analog input voltage.
The Schmitt trigger is a comparator that uses hysteresis. The transfer characteristic of the Schmitt
trigger is shown in Figure 2. The Schmitt trigger basically has two thresholds, one on each side of the
reference voltage. This gives a buffer zone for the rejection of noise and interference of the input
signal. The hysteresis enables the comparator to turn on at one voltage value and possibly turn off at
another voltage value. For example, a decreasing signal will remain at Out+ until V TL is crossed and
then remain at Out- until the signal increases and crosses VTH . Hysteresis in the Schmitt trigger also
ensures that the output is the same frequency as the input. In the case that level detection does not have
buffer zone, noisy input signals may cross the threshold several times while rising and falling. The
Schmitt trigger is especially useful for slowly varying and noisy input signals. The operational
amplifier in the comparator circuit of Figure 3 allows for flexibility in setting the gain, thresholds, and
Experiment #3 EEL4314L Schmitt Trigger Comparator Page 2
Figure 1: Ideal Comparator Transfer Characteristic
Figure 2: Comparator Transfer Characteristic with Hysteresis
Figure 3: Comparator with Hysteresis
Experiment #3 EEL4314L Schmitt Trigger Comparator Page 3
Design and pre-lab: Show all works, including schematics and calculations.
1. Find the relationship between the resistances in the comparator circuit and the thresholds VTL , VTH
and VOUT. . Remember VTL and VTH are the threshold at input voltages. Let the reference voltage equal
to zero (Vref = 0).
2. Modify the above equations for a reference voltage (Vref) not equal to zero
For both questions, Show all works, including schematics and calculations.
1. Construct the comparator circuit of Figure 3. Set the reference voltage to zero and the threshold at
1.5 V above and below zero. Use ±15V supplies for the op-amp.
2. Apply -5 Vdc as the input to initialize the negative output saturation. Then float the input pins.
3. Apply 0 Vdc as the input and step up the input voltage for 0.1 Volts/step from 0 Vdc to + 5 Vdc.
Record the output voltage of each input level. Plot the Output-Input graph (Output: Y-axis, Input:
X-axis). Record the VTH value. The VTH is the input voltage level that changes the output voltage
4. Make sure that the output level is now at high saturation level. Then float the input pins.
5. Apply -0.1 Vdc as the input. Step down the input for 0.1 Volts/step from 0 Vdc to -5Vdc. Record
the output voltage of each input level. Plot the Output-Input graph (Output: Y-axis, Input: X-axis).
Record the VTL value. The VTL is the input voltage level that changes the output voltage level.
6. Now change the input into a 4 Vpp sine wave at 3 KHz. Monitor the input and output waveforms
on the oscilloscope. Measure the slope of the line between the low to high output limits and the
high to low output limits. Capture this waveform from oscilloscope, and use this waveform to find
the slew rate.
7. Show the simulation results and graphs of 1-6.
1. Determine the maximum frequency at which the Schmitt trigger can operate from the slew rate data
collected, assume the minimum time at high or low is 0.5 us? What is the maximum frequency using
the slew rate specification from the data sheet, assume the minimum time at high or low is 0.5 uS
Report Project 3 Digital Application of the Operational Amplifier.
Name:___________________ Date:________ Lab Bench#____________
Remember to include all of your prelab assignment.
Answer the questions at the end of the lab procedure and turn in with the report.
1. Values of components from the design of the comparator with hysteresis.
Include schematics drawing.
2. High threshold
Experiment #3 EEL4314L Schmitt Trigger Comparator Page 4
VTH = ________________VDC
Plot the graph.
3. Low threshold
VTL = ________________VDC
Plot the graph.
4. Calculate from the oscilloscope display.
Slew rate from low to high. = ______________ measured.
Slew rate from high to low = ______________ measured.
Maximum frequency calculated from data = ________________. Assume 0.5 us is minimum time of
High or Low.
Print oscilloscope waveform.