Operational Amplifiers 2 Active Filters

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Lab Summary

In this lab you will build two active filters on a breadboard, using an op-amp, resistors, and capacitors, and take data for the magnitude frequency response (Bode plot) of the filters. To further develop your understanding of analog circuits and expand your analytical skills, you will compare your measured results with theoretical calculations and simulations (using LTspice) of each of the three circuits.

Lab Preparation

Please review the following material from previous and current labs before attempting this lab:

1. Review the Introduction to Lab Equipment for operational instructions of the Rigol Triple Output Power Supply.

2. Review Active_Filter_I_Notes.pdf for more background on analog filters.

3. Familiarize yourself with the LME49710NA OP Amp datasheet as it will be

referred to throughout the lab.

Videos: Operational Amplifiers 2 – Active Filters.mp4

Lab Supplies

You will need the following supplies for the lab.

1. Bread board and jumper wire kit 2. The following components

Component Name Quantity Markings

Op-Amp LME49710NA 1 LME49710NA

Resistors 4.99K 2 Ylw-Wht-Wht-Brn

100 pF Ceramic 1 101

Capacitors 0.1 uF Ceramic 3 104

1.0 uF Tantalum 2 105, +

Figure 1 Breadboard

3. (1) BNC to BNC Cable 4. (2) BNC to Dual Minigrabber Cables 5. (3) Banana to Minigrabber Cables

(1) Red for +VCC (1) Green for -VCC (1) Black for Ground

(In electronics, Black is ALWAYS ground)

The breadboard used in this lab is shown here:

Figure 2 LME49710NA Operational Amplifier

The pin numbers and functions for the LME49710NA operational amplifier are shown below. The pin below the notch is pin 1 when the IC is oriented with the notch to the left. There is often a small dot in the plastic indicating pin 1.

Figure 3 Recommended Grabber Connections

Figure 4 Recommended bus strips rail layout

Part 1

Procedure 1.1 - Wiring

1) Use the longer jumper wires from your kit to connect the +VCC, -VCC and GND rails to the DC Power Supply.

Fig. 4 shows the bus strips recommended +VCC, -VCC and GND (ground) rails.

Longer jumper wires can be used to connect the clip of the Banana to Minigrabber cables from the power supply to the correct power bus.

Figure 5 Part 1 Filter Schematic

2) Wire the following circuit on your breadboard.

Figure 6 Part 1 Filter

Use the jumper wires in your kit to make connections. You should make an effort to keep all the connections FLAT on the surface of the breadboard by using the correct length jumper.

Procedure 1.2 - Measurements

1) Always monitor your input signal on CH1 of the oscilloscope. Use a BNC to BNC cable to connect the Waveform Generator output from the BNC T connector to the oscilloscope. Use a BNC to Dual Minigrabber cable to connect from the BNC T connector to the circuit input and also ground (black clip!).

2) Use a second BNC to Dual Minigrabber cable to connect your op-amp output (and

ground!) to CH2 of your oscilloscope.

3) Set the DC power supply to VCC+ = +5V and VCC- = -5V.

4) Set CH1 of the Waveform Generator to a 5 volt peak-to-peak sine wave. Don't forget

to output enable CH1 of the Function Generator.

Vary the Function Generator frequency take enough data to characterize the filter. You should be able to easily identify the pass band, cutoff frequency (-3dB) and stop band with your data set. The table Frequency Data, available for download on the website, may be helpful. Be sure to take more points near the cutoff frequency in order to identify this frequency more accurately. Recall that the formula for decibels is:

Figure 7 Part 1 Filter

Figure 8 Bode Plot

5) Plot the Magnitude Gain as a Bode Plot in MATLAB. There is a bode( ) command that may be helpful. The Y axis is in decibels (it is on a linear scale since the dB unit is already logarithmic). The X axis is logarithmic with irregular spacing that repeats every Decade. The axes for the Bode Plot look like this:

NOTE: A complete Bode Plot ALWAYS includes Magnitude and Phase. For purposes of comparisons we have chosen to focus primarily on the Magnitude portion of the filter frequency response.

6) Please convert all frequencies to Hertz (Hz) in your plots. Don't forget to label your axes.

Checkpoint 1: Show the magnitude response plot to your monitor

7) Determine the following characteristics of the filter experimentally from your plot and add them to your Results sheet:

Is this a low pass or a high pass filter?

The DC Gain (low frequencies, unitless)

The DC Gain in decibels

The cutoff Frequency in radians/sec and in Hz

The Phase Shift at the Cutoff Frequency

NOTE: There is a Phase function under 'Meas' on the oscilloscope. Careful with it though, If it gives more than 180 degrees, subtract the number from 360 to get a correct value. Assign positive or negative based on your knowledge from Lab 3.

Calculate the theoretical DC gain?

Calculate the theoretical cutoff frequency in Hertz

Procedure 1.3 - LTspice Simulation

Now construct the same circuit in LTspice and do an AC analysis the same way you did in your previous lab. You do not need to add the capacitors at the power supply rails. You may refer to the Phase portion of the graph to help you decide if your measured phase shifts are correct.

What is the simulated DC gain in decibels?

What is the simulated cutoff frequency in Hz?

Figure 9 Part 2 Filter Schematic

Part 2

Procedure 2.1 - Reconfigure

1) Reconfigure the passive components on your breadboard to implement the following circuit. Remove the feedback capacitor (you will need again it in Part 3). Add the 0.1uF capacitor in series with the input resistor as shown. Everything else is the same.

Figure 10 Part 2 Filter

Figure 11 Part 2 Filter

Procedure 2.2 - Measurements

2) Use the same instrument settings, and take enough data to characterize this filter by using another Frequency Data table.

3) Plot the Magnitude Gain in MATLAB as before.

Checkpoint 2: Show the magnitude response plot to your monitor

4) Determine the following characteristics of the filter experimentally and include them in your Results sheet.

Is this a low pass or a high pass filter?

The high frequency Gain (unitless)

The high frequency Gain in decibels

The cutoff Frequency in radians/sec and in Hz

The Phase Shift at the Cutoff Frequency

Calculate the theoretical high frequency gain

Calculate the theoretical cutoff frequency in Hertz

Figure 12 Part 3 Filter Schematic

Procedure 2.3 - LTspice Simulation

1) Construct this circuit in LTspice, perform the AC analysis.

2) What is the simulated high frequency gain in decibels?

3) What is the simulated cutoff frequency in Hz?

Part 3

Procedure 3.1- Reconfigure

1) Reconfigure the passive components on your breadboard to implement the following circuit. All you need to do is replace the 100 pF capacitor that you removed for Part 2.

Figure 13 Part 3 Filter

Figure 14 Part 3 Filter

Procedure 3.2- Measurements 2) Use the same instrument settings, and take enough data to characterize this filter by

using another Frequency Data table.

3) Plot the Magnitude Gain in MATLAB as before.

Checkpoint 3: Show the magnitude response plot to your monitor

What kind of filter is this?

What is the highest gain in the Pass Band?

Find BOTH cutoff Frequencies in radians/sec and in Hz

The Phase Shift at Cutoff Frequency #1

The Phase Shift at the exact middle of the passband

The Phase Shift at Cutoff Frequency #2

Calculate the theoretical maximum gain

Calculate the theoretical cutoff frequencies in Hertz

Procedure 3.3 - LTspice Simulation

1) Construct this circuit in LTspice and perform the AC analysis.

What are the simulated cutoff frequencies in Hz (there are 2)?

What is the gain in decibels at the middle of the passband?

Verify that all of the answers on your RESULT Sheets have been filled in.

Checkpoint 4: Clean up and show your workbench to the lab monitor.