PHYS 3322 Modern Laboratory Methods I
Op Amps I
Purpose
This lab course reinforces your understanding of basic op amp behavior by utilizing simple op
amp circuits.
Equipment
•
Digital multimeter
•
•
•
Function generator
o 680 Ω
o 10 nF
•
Oscilloscope
o 3300 Ω
o 0.1 µF
•
Bipolar power supply
o 4700 Ω
•
Proto-board
o 10 kΩ
•
Wire jumper kit
o 47 kΩ
Resistors
Capacitors
Procedure
The lab consists of three parts: the inverting amplifier, high and low filters, and the band pass
filter. The theoretical expressions for each circuit, assuming ideal op amp behavior, were
obtained in “Transfer Functions of Op Amp Active Filters”. As a comparison to the theoretical
expressions, you will experimentally measure the magnitude of the transfer functions and, at
most for the inverting amplifier, the phase relationship as a function of frequency for each
circuit.
For your convenience, questions regarding each circuit are included within each section to assist
in making and entering your observations in the lab notebook. These questions are the bare
minimum to consider and answer and should by no means be the minimum you, as the student
should consider. The answers to the questions, in report form, must be included in the lab write
up. It is also advisable to complete the data acquisition, plotting, etc., for each section before
continuing on to the next.
Obtain the circuit elements listed above and use an appropriate multimeter to determine the exact
values, resistance and capacitance, of each element.
The op amp you will be using is an LM741. This general-purpose op amp comes in a dual-inline 8-pin package. The pin connections are denoted below.
OFFSET NULL
1
8
NC
INVERTING INPUT
2
7
V+
NON-INVERTING INPUT
3
6
OUTPUT
V-
4
5
OFFSET NULL
LM741
Revised: 16 January 2003
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Op Amps I
THE INVERTING AMPLIFIER
R1 :
10 kΩ
R2 :
47 kΩ
R2
R1
VIN
_
A
VOUT
B
+
Figure 2. Set up for measuring magnitude and phase of VOUT/VIN
Set up the circuit shown in Figure 2. Use the +12/-12 V terminals of the power supply for the op
amp power. All ground connections are to be made to the ground connection of the power
supply. In this circuit as well as all subsequent circuits, the output of the function generator
(VIN) is read from the oscilloscope via channel 2 and the output of the op amp is read from the
o-scope via channel 1.
At least initially, set the function generator to output a sinusoidal wave at a frequency of 200
kHz. Observing the output signal of the op amp on the o-scope set the amplitude of VIN such
that VOUT is a sinusoidal wave.
•
Determine the magnitude of the transfer function VR/VIN as well as the phase difference
between the input signal (VIN) and output of the op amp (VOUT) as a function of frequency
over the range 20 Hz to 200 kHz.
•
On the same graph, plot |VR/VIN| as well as a theoretical curve for |VR/VIN| versus f.
•
On a separate graph, plot the phase difference as well as a theoretical curve for the phase
difference between the input signal and the output signal as a function of frequency.
•
Using two other resistor combinations and an appropriate frequency determined from the
above to verify that the inverting amplifier does indeed invert and amplify the input signal.
Print the o-scope traces.
Questions:
Compare the results of your measurements with your theoretical predictions.
THE HIGH PASS AND LOW PASS ACTIVE FILTERS:
Set up one and only one of the following circuits:
Circuit 1: The high pass filter
Circuit 2: The low pass filter.
.
0.1 µF
3300 Ω
3300 Ω
680 Ω
VIN
_
0.1 µF
VOUT
VIN
680 Ω
_
VOUT
+
+
Revised: 16 January 2003
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Op Amps I
For the circuit of your choice:
•
Over the range 20 Hz to 5000 Hz obtain the magnitude of VOUT/VIN.
•
For both the measured and theoretical values plot |VOUT/VIN| versus log f.
•
On the graph mark the cutoff frequency.
Questions: The high pass or low pass filter
Compare the results of your measurements with your theoretical predictions. Discuss any
discrepancies and consider possible sources of error.
Does the circuit possess any filtering capabilities? What is the cutoff frequency? How does the
cutoff frequency compare to the ideal cutoff frequency?
THE BANDPASS ACTIVE FILTER CIRCUIT:
C2
Set up the circuit using both cases
below in turn.
Case I
R2
Case II
R1 :
680 Ω
4700 Ω
C1 :
0.1 µF
0.1 µF
R2 :
3300 Ω
10 kΩ
C2 :
0.1 µF
10 nF
C1
VIN
R1
_
VOUT
+
For each case,
•
Over the range 20 Hz to 5000 Hz obtain the magnitude of VOUT/VIN.
•
For both the measured and theoretical values plot |VOUT/VIN| versus log f.
•
On the graph mark the cutoff frequency.
Questions:
Compare the results of your measurements with your theoretical predictions. Discuss any
discrepancies and consider possible sources of error.
Does this circuit possess any filtering capabilities? What are the cutoff frequencies? How do the
cutoff frequencies compare to the ideal cutoff frequencies?
What is the range of frequencies in the pass band? What are the experimental and calculated Q
values and how do they compare?
Revised: 16 January 2003
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