Lab 4 - Lab 4 Op Amps

By: Gracie Miller, Katelyn Rice, and Rachel Rohrer

Summary

The goal of this lab was to examine the limitations of an Op Amp given three different gains. To achieve the varied gains, three different circuits were built. For each circuit, eight input voltages were recorded, the resulting output voltages were recorded, and the expected output voltages were calculated. After collecting all the readings, it was clear that the Op Amp reaches a voltage at which it can not amplify the input signal more and the output voltage plateaus.

Materials

• Resistors
  • 10 kΩ
  • Two 68 kΩ
  • 150 kΩ
  • 8.2 kΩ
  • 330 kΩ
  • Two 1 kΩ
  • 270 kΩ
  • 1.5 MΩ
• 8 pin DIP Op Amp
• 1 kΩ Potentiometer
• 10 kΩ Potentiometer
• JAMECO Breadboard JE27

Assembly Procedures

1. Using the Op Amp, one 10 kΩ resistor, two 68 kΩ resistors, and the 10 kΩ potentiometer, build a circuit as shown
2. Using the Op Amp, one 10 kΩ, one 150 kΩ, one 8.2 kΩ, one 330 kΩ resistor, and the 10 kΩ potentiometer, build a circuit as shown
3. Using the Op Amp, one 270 kΩ, two 1kΩ, one 1.5MΩ, and the 1 kΩ potentiometer, build a circuit as shown

Test Equipment

• Two Fluke 87 True RMS Multimeter
• Global Specialties 1301A Power Supply

Test Procedures

Finding True Values

In order to ensure all calculations were done with correct values, the actual resistance of all the resistors were measured and recorded. The values are shown below:

Comparing Vi and Vo:

For each of the constructed circuits, the voltage provider was sent to 15 V. After each circuit was built, a multimeter was attached to the resistor connecting the potentiometer and the Op-Amp. This multimeter measured the voltage entering the Op-Amp. A second multimeter was attached to a free wire leaving the Op-Amp and measured the voltage output. Both voltages were recorded for 8 separate measurements for each of the 3 circuits. Additionally, the expected output voltage was calculated by multiplying the input voltage by gain of the Op-Amp which is the ratio of the resistors for that particular Op-Amp circuit.

Test Results

Circuit 1, Unity Gain:

The first circuit produces an output voltage of equal magnitude of the input but of the opposite sign. The measured output values were relatively close to the expected values. The results can be seen below:

A graph of the results is shown below:

Circuit 2, Moderate Gain:

The second circuit produced an output voltage greater in magnitude than the input and of the opposite sign. It was found that not all of the measured output values were as closely related to the expected output values because the Op-Amp had reached its saturation point and could not produce as high of an output as would be mathematically expected. The results can be seen below:

A graph of the results is shown below:

Circuit 3, High Gain:

The third circuit behaved similarly to the second circuit but magnified the voltage more so than the second circuit did. Similarly to the second circuit, it was found that not all of the measured output values were as closely related to the expected output values because the Op-Amp had reached its saturation point and could not produce as high of an output as would be mathematically expected. The results can be seen below:

A graph of the results is shown below:

Discussion

Q1: The first circuit performed exactly as expected because the saturation point was never reached; therefore all of the measured output voltages were relatively close to the expected values. This was not true for the second and third circuits because in both cases, the saturation point was reached and the output voltage reached a plateau past this point.

Q2: Op-Amp circuits reach their saturation points when their output voltage nears the voltage of the power supply. Since high gain circuits produce higher output voltages than moderate gain circuits with the same input, the high gain circuits reach saturation faster. Since the gain of the Unity Gain circuit is one, it takes the highest input voltage of the three for the Op-Amp of this circuit to reach saturation.

Q3: It was found that the LM741 were fairly symmetrical. As it can be seen in the first circuit, all of the output voltages were relatively close to the input voltage value in magnitude.

Q4: The integrating and differentiating circuits did perform the mathematical operations as expected. For example, the differentiating circuit produced a cosine wave when a sine voltage wave was generated. Additionally, the integrating circuit produced a triangular voltage output wave when a rectangular input voltage wave was generated.