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?)

1. Construct the following simple comparator circuit and investigate how the output and input waveforms are related as the reference level is varied.
• 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 non-inverting comparator?
• With the reference set to 2 V find at what input levels the output switches.
• Repeat this for several other reference levels.
2. 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.)
• 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 vs. vin).
• Explain why the switching levels are what they are.

3. 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.
• 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 Schmitt trigger.
• 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.
• Try it

4. Voltage Controlled Oscillator. The Schmitt trigger can also be used with a current source to produce a sawtooth or triangle wave.
• 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.