Project-12 The Operational Amplifier

Objective

The objective of this project is to demonstrate how AC voltage gain changes when you use feedback resistors of different values in an opamp circuit.

General Instructions

After the circuit is set up, you measure Vout for each value of RF, and find AV, using the ratio Vout/Vin. You also determine a calculated A V using the ratio RF/Rin in each case to determine how close the calculated AV is to the measured AV.

Parts List

You need the following equipment and supplies:

• One 0.1 μF capacitor.
• Two 10 kΩ, 0.25-watt resistors.
• One 51 kΩ, 0.25-watt resistor.
• One 100 kΩ, 0.25-watt resistor.
• One 150 kΩ, 0.25-watt resistor.
• One 220 kΩ, 0.25-watt resistor.
• One 270 kΩ, 0.25-watt resistor.
• One 330 kΩ, 0.25-watt resistor.
• One 380 kΩ, 0.25-watt resistor.
• Two terminal blocks.
• Two 6-volt battery packs (4 AA batteries each).
• One function generator.
• One oscilloscope.
• One breadboard.
• One OPA134 operational amplifier. This op-amp comes in a few different packages; get the 8-pin dual in-line (DIP) version. 

Figure8.36 shows the pinout diagram for the OPA134. When you insert the op-amp into the breadboard, try not to bend any of the leads. The leads on dual in-line packages are fragile and will break off if you bend them more than once or twice.

Figure 8.36

Step-by-Step Instructions

Set up the circuit shown in Figure 8.37 using the 51 k resistor for RF. Figure 8.38 shows the battery connections. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help, look at the photos of the completed circuit in the “Expected Results” section. One unusual aspect of this circuit you may want to look for in the photos is how the 2V bus of one 6-volt battery pack should be connected to the +V bus of the other 6-volt battery pack.

Figure 8.37

Figure 8.38

Carefully check your circuit against the diagram.
After you check your circuit, follow these steps, and record your measurements in the blank table following the steps.

1. Connect the oscilloscope probe for channel 2 to a jumper wire connected to Vin. Connect the ground clip to a jumper wire attached to the ground bus.
2. Connect the oscilloscope probe for channel 1 to a jumper wire connected to Vout, and then connect the ground clip to a jumper wire attached to the ground bus.
3. Set the function generator to generate a 10 kHz sine wave with approximately 0.2 Vpp.
4. Measure and record Vout and Vin.
5. Change the feedback resistor to the value shown in the next row of the table (labeled 100 k in this instance). Each time you change the resistor, it's advisable to disconnect the batteries to avoid shorting wires.
6. Measure and record Vout and Vin.
7. Repeat steps 5 and 6 until you have recorded Vout and Vin for the last row of the table.
8. Determine the calculated AV and the measured AV, and record these values in each row of the table.

Expected Results

Figure 8.39 shows the breadboarded circuit for this project.

Figure 8.39

Figure 8.40 shows a function generator and oscilloscope attached to the circuit.

Figure 8.40

The input signal is represented by the upper sine wave shown in Figure 8.41, and the output signal is represented by the lower sine wave.
Count the number of divisions for the peak-to-peak output sine wave, and multiply that number by the corresponding VOLTS/DIV setting to determine Vout and Vin.

Figure 8.41
As you measure Vin and Vout, you may need to adjust the TIME/DIV control, the VOLTS/DIV control, and vertical POSITION controls on the oscilloscope. The controls shown in Figure 8.42 are adjusted to measure Vout when RF = 380 k .

Figure 8.42

Your values should be close to those shown in the following table.

The measured values of AV are quite close to the calculated values of AV, well within variations that could be caused by the ± 5 percent tolerance specified for resistor values.