ENGR 43
Lab Activity
Student Guide
LAB 14 – Op-Amp Filters and Oscillators
Student Name: ___________________________________________________
Overview
Getting Started
In the first task, you will construct an opamp filter circuit, measure frequency
response, and create a Bode plot. In the
second task, you will simulate a Wienbridge oscillator and observe the need for an
automatic gain control (AGC) circuit to
maintain circuit stability.
Lab Activity and Deliverables:
It should take students approximately 2
hours to complete the lab activity, and 1
hour of homework time to complete the lab
report.
Before Starting This Activity
The student should review the textbook
sections on filters and oscillators.
Learning Outcomes For Activity
Relevant knowledge (K), skill (S), or
attitude (A) student learning outcomes
Equipment & Supplies
Item
LM358 Op-Amp
10 kΩ resistors
(brown, black, orange)
Resistor, value TBD
0.01 µF capacitors
0.1 µF capacitors
NI-ELVIS trainer
Quantity
1
3
1
2
2
1
K1. Calculate filter critical frequency
Special Safety Requirements
K2. Identify number of poles in a SallenKey filter.
None
K3. Describe the need for an AGC circuit in
a feedback oscillator circuit.
Lab Preparation
S1. Measure filter response
S2. Chart filter response on a Bode plot
S3. Measure oscillator distortion in a circuit
simulation
Download the MultiSim files
LOW_PASS.ms10,
WIEN-BRIDGE1.ms10, and
WIEN-BRIDGE 2.ms10
from the ENGR 43 web page or the
GoogleDocs website
(http://tinyurl.com/engr43-lablinks).
A1. Appreciate the applications of op-amps
for multiple circuit configurations
Lab 14 – Op-Amp Filters and Oscillators
ENGR 43
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© 2012
ENGR 43
Lab Activity
Student Guide
Task #1 – Low-Pass Filter
d) What is the gain in the pass-band?
____________________
(Hint: At low frequencies, capacitors act
as open circuits, and the filter resistors
RA and RB are in series with the very
high input resistance of the noninverting input.)
4. Apply a ±1Vpp sine wave to the input.
Start at 500 Hz, and measure the peakto-peak output of the filter.
Figure 1
1. Refer to the data sheet for the LM358.
Identify the power connections and the
input and output pins for each op-amp in
the package. Connect the +V to +15
volts, and the –V to –15 volts. Connect
bypass capacitors (0.1 µF) between each
power connection and ground near the
op-amp.
2. To generate a Butterworth filter
response, we need a damping factor of
1.414. The damping factor is set by the
gain resistors R1 and R2 according to the
formula:
5. a) What is Vout/Vin?__________
b) How does this compare to the
calculated gain in step 3d?
6.
7.
DF = 2 – R1/R2
With R2=10kΩ, find R1=_________kΩ
3. Construct the low-pass filter shown in
figure 1. With R=10 kΩ and C=0.01 µF,
calculate the critical frequency.
a) fc=1/(2πRC)=__________
b) How many poles?________
8.
9.
______________________________
Increase the frequency by 100 Hz.
Measure the output voltage. Repeat by
100 Hz steps up to 2 kHz. Enter the data
into an Excel table (input frequency and
output voltage).
Measure the output voltage from 2 kHz
to 10 kHz in 1 Khz steps. Add this to
your Excel table. Make a graph with
frequency on the X axis and output
voltage on the Y axis. This type of graph
is also known as a Bode plot. Change the
scale of both X and Y axes to a
logarithmic scale (click on the scale,
Format menu, Selected Axis, Scale).
Open the MultiSim file
LOW_PASS.ms10. Use the bode plotter
instrument to compare the simulation
response to your measured data. You
may also export and add this data to your
Excel bode plot.
Print the data table and the graph and
include with this activity guide.
c) What is the roll-off of the filter?
______dB/octave, or ______dB/decade
Lab 14 – Op-Amp Filters and Oscillators
ENGR 43
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© 2012
ENGR 43
Lab Activity
Student Guide
7. What is the closed-loop gain at this
setting, and how does this compare to
the textbook description of closed-loop
gain for oscillation?
Task #2 – Wien-Bridge Oscillator
1. Open WIEN-BRIDGE1.ms10 in
MultiSim. Set the potentiometer value to
50% by typing “a” to reduce or “A” to
increase the value.
2. Examine the component values.
3. Calculate the resonant
frequency.
cl
fr=1/(2πRC)
________________________________
________________________________
r
f = _________________
With the 10 kΩ resistor for R2, what is
the total feedback resistance (R2 +
potentiometer) required for Av= 3?
Rf = __________________
4. Double-click on the oscilloscope to open
the o-scope window. Click on the circuit
window to enable control of the
potentiometer. Start the simulation.
What do you see on the o-scope.
5. Verify the Channel A V/div is set to
10V/div. Increase the pot to 55%
(remember to click on the circuit
window to enable control of the pot, then
type “A”). Wait about 10 seconds. Now,
what does the output look like? Describe
it in terms of frequency, peak-to-peak
voltage, and wave shape.
________________________________
8. Stop the simulation. Verify Chan A
V/div=10. Set the pot to 55%. Start the
simulation while monitoring the o-scope
display. Wait until you see the output
voltage start to increase, then quickly set
the pot to 50% (type “a”) before the
waveform reaches ±10 V (two divisions
peak-to-peak). If you are not quick
enough on the keyboard the waveform
will increase until clipping occurs. If this
happens, stop the simulation and start
over. Notice you can now control the
amplitude of the waveform by typing
“A” to increase and “a” to decrease the
voltage. (This is almost impossible to do
in a real circuit.) Set the output voltage
to ±10 V. Describe the voltage in terms
of frequency, peak-to-peak voltage, and
wave shape.
______________________________
________________________________
______________________________
________________________________
______________________________
________________________________
9. Open the Distortion Analyzer
instrument (XDA1). This
measures the sine wave harmonic
distortion (THD). While the
simulation is running, you may
notice the reading fluctuating, so
take several readings and average
the values. After measuring the
distortion, reduce the amplitude to
approximately ±1 V (change the
______________________________
6. Reduce the pot value (“a”) until the
clipping is gone. What was the lowest
percentage that still maintained the
oscillation?
________________________
Lab 14 – Op-Amp Filters and Oscillators
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© 2012
ENGR 43
Lab Activity
Student Guide
V/div on the o-scope as
necessary). Record the distortion
at this point. Reduce the
amplitude to approximately ±100
mV. Record the distortion at this
point. Increase the amplitude until
clipping occurs. Record the
distortion at this point.
13. 12. Set the pot value up to 25%.
Describe the waveform in terms
of frequency, peak-to-peak
voltage, and THD.
___________________________
___________________________
±10V THD__________%
___________________________
±1V THD__________%
___________________________
±100mV THD__________%
___________________________
clipped THD_________%
10. Open the WIEN-BRIDGE2.ms10
file in Multisim. This circuit has
added an AGC circuit. Open the
o-scope window and start the
simulation. Increase the pot value
up to 100%. Describe the
waveform in terms of frequency,
peak-to-peak voltage, and THD.
11. Set the pot value to 50%. Describe
the waveform in terms of
frequency, peak-to-peak voltage,
and THD.
12. Set the pot value up to 30%.
Describe the waveform in terms
of frequency, peak-to-peak
voltage, and THD.
14. What can you conclude about
output amplitude vs. distortion?
___________________________
___________________________
___________________________
___________________________
___________________________
15. How does the range of output
voltages compare with the WIENBRIDGE1.ms10 circuit
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
Deliverable(s)
___________________________
___________________________
Lab 14 – Op-Amp Filters and Oscillators
ENGR 43
Print your Excel table and Bode plot and
save it with this lab activity guide in your
Lab Activity Binder.
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© 2012