COMPARATORS AND WAVEFORM GENERATORS
The purpose of this lab is to become acquainted with the use of comparators
and the use of comparators with hysteresis (Schmitt triggers) for waveform
generation. We will use the LM319 dual-comparator integrated circuit. The
LM319 contains two specially-designed, fast comparators in one dual-in-line
package. The output of each comparator is high or low depending on whether
the difference between the two inputs is greater than or less than zero.
(An op-amp without feedback also behaves as a comparator, but is not suitable
for high-speed operation because of slew rate limitations. How long would
it take the output of a 741 Amplifier to switch between +10 and -10 volts?)
- Construct the following simple comparator circuit and investigate how
the output and input waveforms are related as the reference level is varied.
The comparator can be given hysteresis by providing some positive feedback,
as below (section 12.4). (This circuit is called a Schmitt trigger, and
is widely used in digital circuits to prevent multiple output transitions
as a noisy input signal passes through the reference voltage.)
- Use a pot to generate a reference voltage that can be varied between
0 V and 5 V.
- Connect a 10 Vpp triangle wave to the input as shown.
Sketch or plot a typical set of waveforms. Is this an inverting or
- With the reference set to 2 V find at what input levels the output switches.
- Repeat this for several other reference levels.
The Schmitt trigger can be used to construct a relaxation oscillator
as shown below.
If these oscillators do not work, break the loop and input the expected signal.
See if the signal progresses around the loop as expected.
- Choose resistor values in the feedback to give a hysteresis levels
of about 0 and 1.5 volts.
- Construct the circuit
- With the triangle wave input measure the switching levels.
- Sketch the hysteresis curve (vout
- Explain why the switching levels are what they are.
This circuit produces a triangle wave and unipolar and
bipolar square waves.
- Construct and test the circuit and explain how it operates.
- Explain why the offset plus inverting amplifier is needed after the
- Modify the oscillator circuit to have a fixed frequency of 1 kHz, and
compare the observed and theoretical results.
- Finally, explain why the waveforms do not have a 50% duty cycle, and
how would you change the circuit to give a 50% duty cycle.
Voltage Controlled Oscillator. The Schmitt trigger can also be used with a current source to produce
a sawtooth or triangle wave.
© Copyright 2001 New Mexico Institute of Mining and Technology
- Design and build the following sawtooth-wave generator. The input is a
positive DC voltage. The output pulse are very narrow.
- Design the Schmitt trigger so the input threshold levels will be 0
V and 5 V.
- Choose components so the frequency of oscillation will be 5 kHz for
an input voltage of 10 V.
- Use a pot to make the input variable from 0 to 15 V.
- Measure and plot the frequency of oscillation as a function of input voltage.
- Change the 1 k resistor (there are two so find the correct one)
so the sawtooth output becomes a triangular wave.