Lab Manual
ECE 3765
Electronic System - Lab
(Fall 2026)
Department of Electrical and Computer Engineering
Utah Valley University
Prepared by James McEachern
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Lab 3
COMPARATORS AND SUMMING AMPLIFIERS
THE DURATION TIME OF THIS LAB IS ONE WEEK
INTRODUCTION:
Operational Amplifiers (Op-Amps) are linear devices with the properties required
for nearly ideal DC amplification, making them extensively used in signal
conditioning, filtering, and performing mathematical operations such as addition,
subtraction, integration, and differentiation. An op-amp is fundamentally a voltageamplifying device designed to be used with external feedback components, such
as resistors and capacitors, between its output and input terminals. These
feedback components determine the resulting function or "operation" of the
amplifier. Depending on the feedback configuration—whether resistive, capacitive,
or both—the amplifier can perform various operations, which is why it is called an
"Operational Amplifier."
Commercial vendors offer different shapes for op-amp chips or packages, but the
one we will use in the lab is commonly known as the 741 (LM741/NE741/uA741).
The 741 is an 8-pin op-amp, and the pin distribution and main characteristics for
this device are provided in Figure 1 and Table 1 below.
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Figure 1: OP-Amp LM741 Pin Distribution
Table 1: OP-Amp LM741 Main Characteristics
OPERATION SELECTION:
A. Independent Operation
When the supply is in the independent mode it generates two independent output
voltages or currents. The outputs can be controlled by the master and slave
controllers.
Figure 2: Independent Operation: Two Independent DC Powers Supplies
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B. Series Connection to Act as a Single Power Supply
The Master and Slave terminals can be connected in series to form a Single Power
Supply as shown in Figure 3.
Figure 3: Series Connection to Act as a Single Power Supply
C. Positive and Negative Voltages Supply
When the supply is in the independent mode connection it could be used to
generate positive and negative voltages by connecting the positive side of the
slave part to the negative side of the master part with an external wire. This wire
will be the ground or reference point of the positive and negative output voltages,
this connection is shown in Figure 4.
Figure 4- Positive and Negative Voltages Supply
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OBJECTIVES:
After completing this exercise, you should be able to:
 Analyze the operation of a comparator.
 Analyze the operation of a summing amplifier. output frequency.
THEORY OVERVIEW:
Although both circuits in this lab are built around op-amps, there is no other
significant relationship between them. They are presented in a single exercise
simply for convenience.
A comparator is a circuit used to compare two input voltages and provide a
DC output that indicates which of the two inputs is greater. In most cases, the
comparator is used to compare a changing input voltage to a set DC reference
voltage. The DC outputs from a comparator are usually referred to as high and
low. In the first part of this lab, you will explore the basic operation of an op-amp
comparator.
The summing amplifier is a circuit that produces an output proportional to the
sum of its inputs. The term "proportional" is used because the circuit may or may
not have gain and provides an inverted output. The inputs can be DC or AC values.
In the second part of this lab, you will examine the operation of a two-input
summing amplifier using sine wave inputs.
SOFTWARE REQUIRED
NI Multisim
MATERIALS:
Function generator
Dual-trace oscilloscope
Dual-polarity variable dc power supply
Breadboard
Resistors:
1kΩ
5
10kΩ (3)
22kΩ (2)
KA741 op-amp
PROCEDURE:
Part 1: Comparators
1. Construct the circuit shown in Figure 5. Adjust the function generator to produce a
5V 1 KHz sine wave output.
Figure 5: A comparator circuit.
2. Set up your oscilloscope so that both traces are ground-referenced to the center
of the CRT display. This setup will allow you to use your oscilloscope to measure
the input voltage that causes the op-amp output to change states. To establish the
center of the grid as the ground reference:
 Set both AC/GND/DC switches to the ground (GND) position.
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 Move both traces to the center of the CRT display.
 Set both channels to DC coupling.
3. Observe the circuit's input and output at the same vertical sensitivity (V/Div).
Determine the magnitude of the input voltage at which the output changes state,
and
record
this
value
below.
Vin =
(when Vout changes state)
4. Draw both waveforms on the grid in Figure 6. You may draw the waveforms in a
different program and paste an image into this document. Please do not take a
picture of a screen.
(a) Waveform Vin
(b) Waveform Vout
Figure 6: Comparator waveforms.
5. Modify the circuit as shown in Figure 7. Repeat Steps 3 and 4. Record the
reference voltage value below and draw the waveforms on the grid in Figure 6b.
Vin =
(when Vout changes state)
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Figure 7: Another comparator circuit.
Part 2: Summing Amplifiers
6. Construct the circuit shown in Figure 8. Note that the function generator input is
different than before (2 Vpp @ 300 Hz).
Also note that the signal generator serves as the original source for both inputs of
the summing amplifier (op-amp 2). The op-amp on the left (op-amp 1) is included
solely as a buffer, providing isolation between the two summing amplifier inputs.
7. Measure and record the peak-to-peak output voltage.
Vout peak-to-peak =
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8. Change R1 to 22 kΩ, and measure the peak-to-peak output voltage.
Vout peak-to-peak =
9. Restore R1 to 10 kΩ, and change Rf to 22 kΩ. Measure the peak-to-peak output
voltage.
Vout peak-to-peak =
Figure 8: A two-input summing amplifier.
QUESTIONS & PROBLEMS:
1. Using the nominal values of the resistors in Figure 5, calculate the value of Vout.
Compare this calculated value to the voltage measured in Step 3. Explain any
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significant discrepancies between the two values. The formula is given as:
Vout = +V(R2 / (R1 + R2))
Vout =
2. Refer to the waveforms you recorded in Figure 6. Explain the input/output relation
-ships that you observed for these two circuits.
3. Refer to Step 8. Describe and explain the effect (if any) that increasing R1 had on the
circuit's input/output relationship.
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4. Refer to Step 9. Describe and explain the effect (if any) that increasing Rf had on
the circuit's input/output relationship.
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