Operational Amplifiers 2 Active Filters ReadMeFirst 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, +
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 1 Breadboard
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 2 LME49710NA Operational Amplifier
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. Figure 3 Recommended Grabber Connections Fig. 4 shows the bus strips recommended +VCC, -VCC and GND (ground) rails. Figure 4 Recommended bus strips rail layout 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.
2) Wire the following circuit on your breadboard. Figure 5 Part 1 Filter Schematic Figure 6 Part 1 Filter
Figure 7 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:
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: Figure 8 Bode Plot 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?
Procedure 2.1 - Reconfigure Part 2 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 9 Part 2 Filter Schematic 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
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? Procedure 3.1- Reconfigure Part 3 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 12 Part 3 Filter Schematic
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.