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EXPERIMENT 1
Operational Amplifiers
Objectives
 To construct and operate a comparator circuit using an operational amplifier.
 To construct and operate an Inverting operational amplifier.
 To construct and operate a summing operational amplifier.
Required Materials
(1) Dual power supply (+15V and -15V)
(1) 2kΩ resistor
(4) 20kΩ resistors
(3) 10kΩ potentiometers
(1) 741 linear integrated circuit
(3) 10 kΩ resistors
(1) 24kΩ resistor
(1) Voltmeter
Introduction
Digital circuits function by using signals that are at one of two distinct voltage levels. The
other classification of electronic circuits is analog. These devices function by operating
with signals that vary anywhere between the low and high extreme voltages supplied by
the power supply. Since these devices produce an output that is exactly proportional to
the input, they are known as linear circuits.
One common type of linear circuit is the operational amplifier. Also known as an op amp,
it is capable of performing many functions. Two functions, comparing and amplifying,
will be examined. Figure 1-1 shows the symbol of an op amp. It has two inputs. One of
them is called an inverting input and is labeled with a minus sign. The other is called a
non-inverting input and is labeled with a positive sign. The output lead is located at the
apex of the triangle.
Figure 1-1 Operational Amplifier (a) Standard schematic symbol (b) Pin diagram
Background Information—Comparator
The op amp in Figure 1-2(a) is capable of comparing the voltage applied to one input to
the voltage applied to the other input. When the voltage at the non-inverting terminal is
greater than the voltage at the inverting terminal, the output will go to approximate
positive 5 volt saturation potential. When the voltage at input (–) is greater than the
voltage at input (+), the output will go to an approximate negative 5 volt saturation level.
When the voltage at both input terminals are the same, the output will go to 0 volts.
Procedure
Step 1
Assemble the circuit shown in Figure 1-2(a).
Step 2
Fill in the output portion of the table in part (b) by applying the voltages listed in the
input section.
Background Information—Inverting operational amplifier
Figure 1-3(a) shows an inverting op amp. An input resistor (R IN) is connected between
the input terminal and the (–) op amp lead, and a feedback resistor (RF) is connected
between the output and the (–) input. The name of the op amp circuit is derived from the
way in which it operates. When a voltage is applied to the (–) input lead, a voltage of the
opposite polarity develops at the output. The gain of the inverting op amp is determined
by the resistance ratio of RF compared to RIN. The larger RF becomes compared to RIN,
the larger the gain.
Step 3
Assemble the circuit in Figure 1-3(a).
Step 4
Fill in the output portion of the table in part (b) by applying the voltages listed in the
input section. Use the formula to verify that the measured voltages are correct.
Step 5
Change RF in Figure 1-3(a) to 25kΩ and RIN to 10kΩ.
Step 6
Fill in the output portion of the table in part (c) by applying the voltages listed in the
input section. Use the formula to verify that the measured voltages are correct.
Background Information—Summing Amplifier
Figure 1-4(a) shows an op amp circuit with more than one input tied at its inverting input
lead. Called a summing amplifier, it is capable of adding the algebraic sum of all the
input voltages applied. The sum of these voltages is inverted to the opposite polarity.
Each of the summing input resistors are tied together and a 20kΩ feedback resistor is
used. The following calculations show how to determine the voltage at the output
terminal:
Step 7
Assemble the circuit shown in Figure 1-4(a).
Step 8
Fill in the output portion of the table in part (b) labeled measured by applying the
voltages listed in the input section.
Step 9
Using the formula to determine the output of a summing amplifier, verify that the
measured values are correct.
Results:
COMPARATOR
INPUTS
V1
V2
+4V
+1V
+2V
+3V
+1V
0V
+4V
+4V
0V
+1V
+3V
+2V
VOUT
(V)
INVERTING OPERATIONAL AMPLIFIER
RIN= 10kΩ
RF = 24kΩ
RIN= 2kΩ
RF = 10kΩ
VIN
+2V
+0.2V
-0.4V
0V
+0.32V
VOUT
VIN
+2V
+0.3V
-0.15V
-2.0V
+0.4V
VOUT
Use the formula below to verify that the measured voltages are correct.
VOUT= VIN x RF
RIN
CALCULATIONS/ANALYSIS
Determine the output voltage Vout based on the value given below:
RIN= 2 kΩ,
RF = 10kΩ
For VIN= +0.2V, VOUT=
For VIN= -0.4V, VOUT=
For VIN= 0V, VOUT=
For VIN= 0.32V, VOUT=
RIN= 10kΩ,
RF=24kΩ
For VIN= +0.3V, VOUT=
For VIN= -0.15V, VOUT=
For VIN= -2.0V, VOUT=
For VIN= +0.4V, VOUT=
Question 1: From your results, is the statement in the background information “When the
voltage is applied to the (-) input lead, a voltage of the opposite polarity develops at the
output” true? _______
Question 2: Are your measured and calculated results approximately the same?_______
Question3: Comparing between the results VIN= 2V for RIN = 2kΩ, RF=10kΩ and VIN=
2V for RIN= 10kΩ, RF= 24kΩ, how can you relate in term of gain and resistance ratio?
Prove it with calculations?
SUMMING AMPLIFIER
Input Voltage
V1
V2
V3
+1V
+1V
+1V
+1V
-1V
-1V
+2V
-1V
-1V
-3V
-1V
+3V
+1V
+2V
-1V
Output Voltage
Measured
Calculated
Using the formulas below to determine the output of a summing amplifier, verify that the
measured values are correct.
IR = VR
R
IRF = IR1 + IR2 + IR3
VOUT = IRF x RF
CALCULATIONS/ANALYSIS
Determine the currents in respective areas given below:
For V1=1V, V2=1V and V3=1V
IR1=
IR2=
IRF=
VOUT=
For V1=1V, V2=-1V and V3=-1V
IR1=
IR2=
IRF=
IR3=
VOUT=
For V1=2V, V2=-1V and V3=-1V
IR1=
IR2=
IRF=
IR3=
IR3=
VOUT=
For V1=-3V, V2=-1V and V3=3V
IR1=
IR2=
IR3=
IRF=
VOUT=
For V1=1V, V2=2V and V3=-1V
IR1=
IR2=
IR3=
IRF=
VOUT=
Question 1: Is it true that all of the voltages obtained from your calculations are inverted
to the opposite polarity if compared to your measurements ?__________
Experiment Questions
1. A/n _______ (analog, digital) signal can vary at any value between the low and
high voltage range supplied by the power supply.
2. A ________ (linear, digital) circuit produces signals that are analog.
3. When the voltage applied to the non-inverting terminal of an op amp comparator
is ________ (less, greater) than the voltage at the inverting input, the output will
be driven into positive saturation.
4. If +2 volts is applied to an inverting op amp that has an input resistance of 2kΩ
and a feedback resistance of 6kΩ, the voltage at the output is _____ volts, and the
polarity is ______ (–, +).
5. The summing op amp in Figure 1-4 has the following voltages applied to the
inputs: +2V, +3V, and 0 V. What is the output voltage?