Project-10: The Band Pass Filter

Objective

The objective of this project is to determine how Vout changes as the frequency of the input signal changes for a bandpass filter.

General Instructions

When the circuit is set up, you measure Vout for each frequency. You also generate a graph to show the relationship between the output voltage and the input frequency.

Parts List 

You need the following equipment and supplies.

• One 100 Ω, 0.25-watt resistor.
• One 1000 pF capacitor. (1000 pF is also sometimes stated by suppliers as 0.001 μF.)
• One 100 μH inductor.
• One function generator.
• One oscilloscope.
• One breadboard.
• Step-by-Step Instructions
• Set up the circuit shown in Figure 7.22. 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 building the circuit, look at the photos of the completed circuit in the “Expected Results” section.

Figure 7.22

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, and connect the ground clip to a jumper wire
2. attached to the ground bus.
3. Connect the oscilloscope probe for channel 1 to a jumper wire connected to Vout, and connect the ground clip to a jumper wire
4. attached to the ground bus.
5. Set the function generator to generate a 5 Vpp, 100 kHz sine wave.
6. Measure and record Vout.
7. Adjust the function generator to the frequency shown in the next row of the table (labeled 150 kHz in this instance). Each time you
8. change the frequency, check Vin, and adjust the amplitude knob on the function generator to maintain Vin at 5 Vpp if needed. (If you
9. leave the amplitude knob in one position, the voltage of the signal provided by the function generator will change as the net reactance of
10. the circuit changes.)
11. Measure and record Vout.
12. Repeat steps 5 and 6 until you have recorded Vout for the last row of the table.
13. In the blank graph shown in Figure 7.23, plot Vout versus fin with the voltage on the vertical axis and the frequency on the X axis. The curve should have the same shape as the curve shown in Figure 7.20.

Figure 7.23

Expected Results

Figure 7.24 shows the breadboarded circuit for this project.

Figure 7.24

Figure 7.25 shows a function generator and oscilloscope attached to the circuit.

Figure 7.25

The input signal is represented by the upper sine wave shown in Figure 7.26, and the output signal is represented by the lower sine wave.
Read the number of divisions for the peak-to-peak output sine wave, and multiply it by the corresponding VOLTS/DIV setting to determine Vout.

Figure 7.26

As you set fin to a new value on the function generator, you may also need to adjust the TIME/DIV control, the VOLTS/DIV control, and vertical POSITION controls on the oscilloscope. The controls shown in Figure 7.27 are adjusted to measure Vout when fin = 500 kHz.

Figure 7.27

Your values should be close to those shown in the following table, and the curve should be similar to that shown Figure 7.28.

Figure 7.28.

Because Q = 3.2 (well below 10), the curve for this circuit is not perfectly symmetrical.