ECE 125 Laboratory Project 4
Op-Amp Amplifier Circuits and Use of the Oscilloscope
The purpose of this lab is to help you become familiar with op amps and with integrated
circuits (ICs) in general.You will learn how to use a breadboard to create a circuit with an op
amp and how to configure a dual power supply to produce positive and negative voltages
referenced to a virtual ground.You will design and build an op-amp amplifier circuit and use it to
amplify music from an iPod or Walkman-type CD player.You will also see how an oscilloscope
is used to display time-varying signals.
II. Lab Preparation
A. Using an op amp, 10-kΩ resistor, and 100-kΩ resistor, design an inverting amplifier circuit
with a gain of 10. Draw your circuit below.
B. In the lab you’ll construct the amplifier circuit you just designed.You will use your
breadboard, resistors, a 741 op-amp integrated circuit (included in your toolkit), and an
Agilent E3620A Dual Power Supply.
There are two 741 op-amp integrated circuits (ICs) in a small plastic bag in your toolkit.
Remove one of the op amps, and inspect it closely. The circuit is embedded inside a small,
rectangular black plastic block (which surrounds a “chip”) with eight metal legs, which are
oriented in two columns. A package like this, with two columns of legs pointing downward,
is called a Dual In-Line Package, or DIP. Because the 8-pin DIP is the smallest of this type
of package it’s often called a minidip. Note that the DIP has a small circular indentation in
one corner (or a small notch in the center of one side). If you orient the DIP with the
indentation in the upper left corner (or the notch on the top side) and the legs facing
downward, the legs are numbered as shown in Figure 1.
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1 8 Metal leg
Figure 1. Profile sketch for the LM741 op amp in an 8-pin DIP.
In order to use the op amp, we need to know what each of the eight legs are connected to
inside the package. Inspecting the top of the op amp, you will see a stylized “N”. This is the
trademark for National Semiconductor, one of the manufacturers of 741 op amps. You will
also see the part number LM741CN. You can find information on the op amp by going to
the National Semiconductor Web site (www.national.com) and searching on the key words
741 op amp. Once the Web site returns the results, look for the selection “741 – See the
LM741 Product Folder”. Select this result.
Now select or download the datasheet for the 741 op amp (you will want the datasheet for
the regular 741 op amp, not any of the mil-aero datasheets). This 7-page data sheet contains
a lot of information. Most of what we’re interested in is on the first page. Inspecting this
first page, note that the 741 op amp comes in many different packages (metal can, DIP, and
flatpak). The package in your tool box is an 8-pin DIP. Using the corresponding diagram on
the datasheet, label the pin assignments for the 8 pins on Figure 2. You will recognize some
of the assignments, like INVERTING INPUT, NONINVERTING INPUT, and OUTPUT,
but not others, like V+ and V– (these are power pins, as we shall see shortly), NC (“no
connect”, an extra, unused pin), and OFFSET NULL (these are offset pins that we will not
Figure 2. Pin assigments for the LM741 op amp ( 8-pin DIP).
C. Look at page 4 of the data sheet. This page contains the miniaturized, actual circuit
implemented inside the DIP to produce the op amp (you can see the individual resistors and
transistors in the circuit on the datasheet). How much do you think such a circuit should
D. Go back to the LM741 product folder page on the National Semiconductor Web site. Find
and record the price of the LM741CN op amp. This is the op amp that you have in your
Lab #4, p. 2
toolkit. The CN suffix is used to denote the plastic DIP package for the IC (as opposed to a
ceramic DIP package, which can withstand much higher temperatures, or a metal can
package). Note that the price is specified for “Qty: 1K+”, meaning that National
Semiconductor is providing you with the per-unit price if you are willing to buy at least
1000 of the ICs (suitable for manufacturing, but not too suitable for development or for
small user applications). To find pricing for fewer parts, you can use a vendor such as
Digikey or Allied Electronics. Go to the Digikey Web page, and find and record the price
for a single LM741CN op amp.
E. For the op amp to work, we need to provide power as shown in Figure 3. The two 15-V
sources are independent and will be provided in the lab by the Agilent E3620A Dual Power
Supply. Note that one of the power inputs to the op amp is labeled V+ and the other power
input is labeled V–. The node at the bottom of Figure 3 is referred to as “virtual ground.”
Figure 3. Power and virtual ground for the LM741 op amp.
Using the information from Figures 2 and 3, go to Figure 4, and draw the necessary
connections with the power supply and the 8-pin DIP to provide power to the op amp and
to tie the NONINVERTING INPUT on the op amp to virtual ground. Be sure to also
connect the GND input on the Agilent Power Supply to virtual ground.
Agilent E3620A Dual Power Supply
V1+ V1- GND V2+ V2-
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Figure 4. Power and Virtual Ground Connections for the LM741 Op Amp
F. In Figure 5, repeat the power and virtual ground connections you made in Figure 4. Now,
adding the 10-kΩ and 100-kΩ resistors, show the connections necessary to have the op-
amp input the iPod music signal, amplify it by a gain factor of 10, and output it to the
speaker. The fact that the op-amp circuit inverts the signal (as well as amplifying it) will
not affect the quality of the music.You will need to refer back to your circuit in Part
A.Leave the NC or OFFEST NULL pins unconnected.
G. Review the breadboard construction (by TA).
Agilent E3620A Dual Power Supply
V1 + V1 - GND V2 + V2 -
Portable MPEG-3 or 2 7
CD player Speaker
Figure 5. All Interconnections for the Op Amp amplifier circuit.
A. Your lab instructor will demonstrate how to mount ICs on a breadboard and will show you
the cable we will use to output the music signal from the iPod or Walkman CD. Your lab
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instructor will also connect a speaker directly to the iPod or Walkman and show you that
the iPod or Walkman cannot provide enough power to properly energize the speaker.
B. After your lab instructor has completed the demonstration, use the breadboard, op amp, and
power supply to build the amplifier circuit you designed in Figure 5. You will need to read
the color codes on the resistors in your toolkit to find a 10-kΩ resistor and a 100-kΩ
resistor. Before you connect the Agilent Dual Power Supply to the circuit, set each voltage
output to 15 V, and then turn the power supply off again. Now make all necessary
connections for your circuit, including the speaker and iPod or CD player cable, but do not
turn the Agilent Dual Power Supply on again yet and do not plug the cable into the iPod or
Walkman CD player yet. When you finish, ask the lab instructor to inspect your circuit.
C. When the lab instructor has completed inspecting your circuit, turn on the Agilent Dual
Power Supply, turn on the iPod or Walkman, and enjoy the music. Note that you may not
have concert quality music since the speaker is of rather low quality and since we are only
amplifying and playing one channel (rather than two) of the stereo signal.
D. After all the groups have built and tested their amplifier circuits, the lab instructor will
demonstrate how an oscilloscope is used to display time-varying waveforms, like the music
being output from the Walkman and the amplified music being input to the speaker. After
the demonstration, spend a little time on your own experimenting with the oscilloscope.
You will be using this instrument extensively in your ECE 225, 332, 333, and senior-
Lab #4, p. 5