Comparators are electronic devices used in circuits to compare two voltages. When one voltage exceeds the other, the comparator changes its output state. An inverting comparator, specifically, outputs a low signal when the input voltage is higher than the reference voltage. A Schmitt trigger, on the other hand, adds hysteresis to the comparator, changing its output not just on the crossing of the threshold, but also requiring a significant difference before switching back, reducing noise sensitivity.
Objective
This experiment aims to demonstrate the functionality of an inverting comparator using the OP37 OpAmp and to explore the modifications necessary to transition the comparator into an inverting Schmitt trigger. By varying input signals and observing output behavior, we’ll understand how comparators respond to changes and how adding a potentiometer can introduce hysteresis, characteristic of Schmitt triggers.
Materials and Setup
- OP37 OpAmp for the comparator circuit
- Red Pitaya or equivalent signal generator and oscilloscope
- Power Supply: Symmetrical ±3.3V source for the OpAmp
- Potentiometer: For exploring the effects of variable resistance and implementing hysteresis
- Resistors: Load resistor for testing output stability
Circuit Assembly
- Comparator Configuration: Assemble the inverting comparator circuit using the OP37 chip, powered by a ±3.3V source. Set one Red Pitaya output to generate a DC voltage (reference voltage) and the other to generate a sine wave (input signal).
- Measurement Setup: Connect oscilloscope probes to the comparator’s output and the sine wave output to monitor the behavior as the input signal varies in relation to the reference voltage.
Conducting the Experiment
- Observing Comparator Output: With the circuit assembled, observe how the comparator’s output changes as the sine wave input crosses the reference voltage threshold. The output should be negative when the input exceeds the reference and positive otherwise.
- Introducing a Load Resistor: Adding a load resistor to the comparator’s output should not alter its behavior significantly, provided the resistor value does not overload the comparator’s output stage.
- Implementing a Potentiometer: Replace the load resistor with a potentiometer connected in series. Initially, this change does not affect the comparator’s function. However, disconnecting the reference voltage and connecting the potentiometer’s wiper to the noninverting input introduces a variable reference level, transitioning the behavior towards that of an inverting Schmitt trigger.
Results and Analysis
- Schmitt Trigger Behavior: Adjusting the potentiometer reveals the inverting Schmitt trigger's characteristic hysteresis. The output becomes positive only after the input signal goes slightly negative and becomes negative as the input goes above zero. This hysteresis effect stabilizes the output against rapid fluctuations, a desired property in noisy environments.
