ELECTRONIC II LAB 5 (MULTISIM): Operational Amplifiers Applications - Active Filter

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ELEC 161 Electronics II

Module 5 Lab: Op Amp Applications – Active Filters

Introduction.- This lab explores the three types of basic filters that can be built with Op Amps: Low-pass, High- pass and Band-pass filters. We also introduce different methods that can be used to find the transfer function of any circuit, in particular filters.

Procedure

1.- Low Pass Filter

1.1 Build the low-pass filter circuit shown in Figure 5-1

Figure 5- 1: Low-pass filter

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1.2 Calculate the expected cut off frequency for this filter.

1.3 Connect Channel A of the Oscilloscope to the input signal and Channel B to Vout. Make sure both channels of the Oscilloscope are coupled in DC

1.4 Select an amplitude for the input signal that will result on the output signal not being distorted or clipped.

Now, we will measure the gain of the circuit (Av = Vout / Vin) for signals at different frequencies. Complete the table below for each frequency:

Frequency 100 Hz 500 Hz 1 k Hz 5 kHz 10 kHz 15 kHz 20 kHz 50 kHz 100 kHz Gain (Av)

1.5 Plot the gain vs. frequency in Excel or similar software. Include the graph in your report.

1.6 Based on the plot from 1.5, estimate the cut off frequency of the filter. Remember that the cut off frequency is that frequency at which the gain is 0.707 times the gain of the filter in the bandpass.

1.7 Compare the calculated and measured cut off frequencies. Comment on your results.

An alternative method to measure the Transfer Function for a filter is by using the Bode Plotter. The Bode Plotter is located in the Instruments Palette as shown in Figure 5-2.

Figure 5- 2: Bode Plotter

You can also find it from the Main Menu: Simulate  Instruments  Bode Plotter

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1.8 Connect the IN input of the Bode Plotter to the input signal and the OUT input to the Output of the circuit as shown in Figure 5-3. Don’t forget to connect the grounds !

Figure 5- 3: Bode Plotter Connection

1.9 Double clicking the Bode Plotter will bring its configuration menu (don’t forget that pressing F1 at any time an instrument is active, will bring its help menu).

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Figure 5- 4: Configuration Menu for Bode Plotter

For this part of the lab we will select:

- Mode: Magnitude (default) - Horizontal: Log (default) - F: 1 MHz (highest frequency we will measure) - I: 1 Hz (Initial frequency) - Vertical: Log - F: 0 dB (highest gain) - I: - 50 dB (lowest gain) - Reverse: ON

1.10 Run the Simulation. You should see a transfer function similar to the one seen in Figure 5-5. Don’t forget to include your screenshot!

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Figure 5- 5: Output of Bode Plot

1.11 The original position of the cursor is shown in Figure 4-5. Drag the cursor until the amplitude reads – 3dB. This is the cut off frequency.

1.12 Compare the cut off frequency with the cut off frequency you estimated in 1.6 Comment on your results.

2.- High-Pass Filter

2.1 Build the High-pass filter shown in Figure 5-6

Figure 5- 6: High-pass filter

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2.2 Calculate the cut off frequency for this filter.

2.3 Repeating the process used for the low-pass filter, complete the table below.

Frequency 100 Hz 500 Hz 1 k Hz 5 kHz 10 kHz 15 kHz 20 kHz 50 kHz 100 kHz Gain (Av)

2.4 Plot the results and estimate the cut off frequency for this filter.

2.5 Compare this result with the calculated cut off frequency.

2.6 Use the Bode Plot to measure the cut off frequency of this filter (You may have to adjust the parameters for the Bode Plot instrument ). Again, don’t forget to include your screenshot

2.7 Compare the results for the cut off frequency between the two methods.

3. Band-pass filter

3.1 Build the band-pass filter shown in Figure 5-7

Figure 5- 7: Band-pass filter

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3.2 Calculate the high cut off frequency and the low cut off frequency.

3.2 Use the 2 methods that you have practiced in this lab to measure both cut off frequencies. Include the necessary graphs and screenshots.

3.3 Comment on your results.

Laboratory Report Create a laboratory report using Word or another word processing software that contains at least these elements:

- Introduction: what is the purpose of this laboratory experiment?

- Results for each section : Measured and calculated values, calculations, etc. following the outline. Include screenshots for the circuits and waveforms as necessary -- You can press Alt + Print_Screen inside Multisim or if using Windows 7, you can use the “Snipping tool”. Either way, you can paste these figures into your Word processor. Also include here the charts and graphs that you have created with the data you have collected.

- Conclusion : What area(s) you had difficulties with in the lab; what did you lean in this experiment; how it applies to your coursework and any other comments.