Experiment - Electronic Instrumentation, ECE, EEE, ENGG., SEMESTER - Computer Science Engineering (CSE) PDF Download

 Page 1


Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 1 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Experiment 4 
Op-Amp Circuits 
 
Purpose: In this experiment, you will learn about operational amplifiers (or op-amps).  Simple circuits containing 
operational amplifiers can be used to perform mathematical operations, such as addition, subtraction, and 
multiplication, on signals.  They can also be used to take derivatives and integrals.  Another important application of 
an op-amp circuit is the voltage follower, which serves as an isolator between two parts of a circuit. 
Several op-amp chips have the same pin configuration, two examples are the 741 op-amp and the LF351 op-amp.  
The demo version of Capture doesn’t have the LF351 version available, so the modeling will be done using the 741 
op-amp (a.k.a. µ741 op-amp.)  The LF351 is somewhat better for our application and will extend the battery 
lifetime.  Model the experiment in PSpice using the 741 but build it using the LF351 if you experience problems 
with the 741 in the circuit. 
 
Equipment Required: 
 
• DMM  (HP 34401A 6-1/2 Digit Multimeter) 
• Rensselaer IOBoard Rev D (with Mobile Studio Desktop) 
• DV Voltage Source (Rensselaer IOBoard, and two 9V batteries)  Students in JEC 4107/4104 should use the HP 
power supply, and avoid battery issues. 
• Analog I/O (Rensselaer IOBoard) 
• Protoboard 
• Some Resistors (50, 1k, 10k and 100k?) 
• uA741 or LF351 op-amp (LF351 is used if uA741 in kit is bad) 
 
Helpful links for this experiment can be found on the links page for this course: 
http://hibp.ecse.rpi.edu/~connor/education/EILinks.html#Exp4 
 
 
Part A – Introduction to Op-Amp Circuits 
 
Background 
 
Elements of an op-amp circuit: Figure A-1 below is a schematic of a typical circuit built with an op-amp. 
U1
LF351/NS
+
3
-
2
V+
7
V-
4
OUT
6
B2
5
B1
1
U3
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V+
9V
V-
-9V
V+
9V
V-
-9V
0 0
Vout Vout
Rf eedback
10k
R1 1k R1 1k
Rf eedback
10k
Rload Rload
0 0
Vin Vin
0
0
 
Figure A-1. Drawn with both 741 and a LF351 op-amps (Rload ˜ 1k?). 
 
The circuit performs a mathematical operation on an input signal.  This particular op-amp circuit will invert the 
input signal, Vin, and make the amplitude 10 times larger.  This is equivalent to multiplying the input by -10.  Note 
that there are two DC voltage sources in addition to the input.  These two DC voltages power the op-amp.  The 
circuit needs additional power because the output is bigger than the input.  Op-amps always need this additional pair 
Page 2


Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 1 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Experiment 4 
Op-Amp Circuits 
 
Purpose: In this experiment, you will learn about operational amplifiers (or op-amps).  Simple circuits containing 
operational amplifiers can be used to perform mathematical operations, such as addition, subtraction, and 
multiplication, on signals.  They can also be used to take derivatives and integrals.  Another important application of 
an op-amp circuit is the voltage follower, which serves as an isolator between two parts of a circuit. 
Several op-amp chips have the same pin configuration, two examples are the 741 op-amp and the LF351 op-amp.  
The demo version of Capture doesn’t have the LF351 version available, so the modeling will be done using the 741 
op-amp (a.k.a. µ741 op-amp.)  The LF351 is somewhat better for our application and will extend the battery 
lifetime.  Model the experiment in PSpice using the 741 but build it using the LF351 if you experience problems 
with the 741 in the circuit. 
 
Equipment Required: 
 
• DMM  (HP 34401A 6-1/2 Digit Multimeter) 
• Rensselaer IOBoard Rev D (with Mobile Studio Desktop) 
• DV Voltage Source (Rensselaer IOBoard, and two 9V batteries)  Students in JEC 4107/4104 should use the HP 
power supply, and avoid battery issues. 
• Analog I/O (Rensselaer IOBoard) 
• Protoboard 
• Some Resistors (50, 1k, 10k and 100k?) 
• uA741 or LF351 op-amp (LF351 is used if uA741 in kit is bad) 
 
Helpful links for this experiment can be found on the links page for this course: 
http://hibp.ecse.rpi.edu/~connor/education/EILinks.html#Exp4 
 
 
Part A – Introduction to Op-Amp Circuits 
 
Background 
 
Elements of an op-amp circuit: Figure A-1 below is a schematic of a typical circuit built with an op-amp. 
U1
LF351/NS
+
3
-
2
V+
7
V-
4
OUT
6
B2
5
B1
1
U3
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V+
9V
V-
-9V
V+
9V
V-
-9V
0 0
Vout Vout
Rf eedback
10k
R1 1k R1 1k
Rf eedback
10k
Rload Rload
0 0
Vin Vin
0
0
 
Figure A-1. Drawn with both 741 and a LF351 op-amps (Rload ˜ 1k?). 
 
The circuit performs a mathematical operation on an input signal.  This particular op-amp circuit will invert the 
input signal, Vin, and make the amplitude 10 times larger.  This is equivalent to multiplying the input by -10.  Note 
that there are two DC voltage sources in addition to the input.  These two DC voltages power the op-amp.  The 
circuit needs additional power because the output is bigger than the input.  Op-amps always need this additional pair 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 2 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
of power sources.  The two resistors R
feedback
 and R1 determine how much the op-amp will amplify the output.  If we 
change the magnitude of these resistors, we do not change the fact that the circuit multiplies by a negative constant; 
we only change the factor that it multiplies by.  The load resistor R
load
 is not part of the amplifier.  It represents the 
resistance of the load on the amplifier. 
 
Powering the op-amp: The two DC sources, (labeled as
 
V+ and V-, but also often labeled as ± V
CC
), that provide 
power to the op-amp are typically set to have an equal magnitude but opposite sign with respect to the ground of the 
circuit.  This enables the circuit to handle an input signal which oscillates around 0V, like most of the signals we use 
in this course.  (Note the signs on the sources in the circuit above.)  The schematic in Figure A-2 shows a standard ± 
V
CC
 configuration for op-amps.  The schematic symbols for a battery are used in this schematic to remind us that 
these supplies need to be a constant DC voltage.  They are not signal sources. 
 
 
Figure A-2. 
 
Students in JEC 4201 use two 9V batteries for power.  Batteries are self explanatory. 
Students in JEC 4107/4104 should use the HP E3631A supplies for power.  The HP E3631A power supply 
provides two variable supplies with a common ground (for ±V
CC
 ) plus a variable low voltage supply (not used in 
this lab).  The power supply jack labeled "COM" between the V
CC
 supplies should be connected to circuit ground.  
When you supply power to the op-amp, adjust the two voltage levels so that +V
CC
 and -V
CC
 are equal, but opposite 
in sign, at 9V.  These are independent and adjusted separately.  Note that in PSpice, there are two ways to represent 
a source with a negative sign.  Figure A-3 shows the two options: you can either set the voltage source to a negative 
value, or you can reverse the polarity of the source. 
=
0
V1
-15V
V2
15V
0
 
Figure A-3. 
 
The op-amp chip: Study the chip layout of the LF351 and 741 op-amps is shown in Figure A-4.  Both have the same 
pin numbers and names.  Either op-amp can be used without changing the circuit.  The standard procedure on DIP 
(dual in-line package) "chips" is to identify pin 1 with a notch in the end of the chip package.  The notch always 
separates pin 1 from the last pin on the chip.  In the case of the LF351, the notch is between pins 1 and 8.  Pin 2 is 
the inverting input.  Pin 3 is the non-inverting input, and the amplifier output, V
O
, is at pin 6.  These three pins are 
the three terminals that normally appear in an op-amp circuit schematic diagram.  The +V
CC
 and -V
CC
 connections (7 
-9V 
9V 
Page 3


Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 1 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Experiment 4 
Op-Amp Circuits 
 
Purpose: In this experiment, you will learn about operational amplifiers (or op-amps).  Simple circuits containing 
operational amplifiers can be used to perform mathematical operations, such as addition, subtraction, and 
multiplication, on signals.  They can also be used to take derivatives and integrals.  Another important application of 
an op-amp circuit is the voltage follower, which serves as an isolator between two parts of a circuit. 
Several op-amp chips have the same pin configuration, two examples are the 741 op-amp and the LF351 op-amp.  
The demo version of Capture doesn’t have the LF351 version available, so the modeling will be done using the 741 
op-amp (a.k.a. µ741 op-amp.)  The LF351 is somewhat better for our application and will extend the battery 
lifetime.  Model the experiment in PSpice using the 741 but build it using the LF351 if you experience problems 
with the 741 in the circuit. 
 
Equipment Required: 
 
• DMM  (HP 34401A 6-1/2 Digit Multimeter) 
• Rensselaer IOBoard Rev D (with Mobile Studio Desktop) 
• DV Voltage Source (Rensselaer IOBoard, and two 9V batteries)  Students in JEC 4107/4104 should use the HP 
power supply, and avoid battery issues. 
• Analog I/O (Rensselaer IOBoard) 
• Protoboard 
• Some Resistors (50, 1k, 10k and 100k?) 
• uA741 or LF351 op-amp (LF351 is used if uA741 in kit is bad) 
 
Helpful links for this experiment can be found on the links page for this course: 
http://hibp.ecse.rpi.edu/~connor/education/EILinks.html#Exp4 
 
 
Part A – Introduction to Op-Amp Circuits 
 
Background 
 
Elements of an op-amp circuit: Figure A-1 below is a schematic of a typical circuit built with an op-amp. 
U1
LF351/NS
+
3
-
2
V+
7
V-
4
OUT
6
B2
5
B1
1
U3
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V+
9V
V-
-9V
V+
9V
V-
-9V
0 0
Vout Vout
Rf eedback
10k
R1 1k R1 1k
Rf eedback
10k
Rload Rload
0 0
Vin Vin
0
0
 
Figure A-1. Drawn with both 741 and a LF351 op-amps (Rload ˜ 1k?). 
 
The circuit performs a mathematical operation on an input signal.  This particular op-amp circuit will invert the 
input signal, Vin, and make the amplitude 10 times larger.  This is equivalent to multiplying the input by -10.  Note 
that there are two DC voltage sources in addition to the input.  These two DC voltages power the op-amp.  The 
circuit needs additional power because the output is bigger than the input.  Op-amps always need this additional pair 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 2 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
of power sources.  The two resistors R
feedback
 and R1 determine how much the op-amp will amplify the output.  If we 
change the magnitude of these resistors, we do not change the fact that the circuit multiplies by a negative constant; 
we only change the factor that it multiplies by.  The load resistor R
load
 is not part of the amplifier.  It represents the 
resistance of the load on the amplifier. 
 
Powering the op-amp: The two DC sources, (labeled as
 
V+ and V-, but also often labeled as ± V
CC
), that provide 
power to the op-amp are typically set to have an equal magnitude but opposite sign with respect to the ground of the 
circuit.  This enables the circuit to handle an input signal which oscillates around 0V, like most of the signals we use 
in this course.  (Note the signs on the sources in the circuit above.)  The schematic in Figure A-2 shows a standard ± 
V
CC
 configuration for op-amps.  The schematic symbols for a battery are used in this schematic to remind us that 
these supplies need to be a constant DC voltage.  They are not signal sources. 
 
 
Figure A-2. 
 
Students in JEC 4201 use two 9V batteries for power.  Batteries are self explanatory. 
Students in JEC 4107/4104 should use the HP E3631A supplies for power.  The HP E3631A power supply 
provides two variable supplies with a common ground (for ±V
CC
 ) plus a variable low voltage supply (not used in 
this lab).  The power supply jack labeled "COM" between the V
CC
 supplies should be connected to circuit ground.  
When you supply power to the op-amp, adjust the two voltage levels so that +V
CC
 and -V
CC
 are equal, but opposite 
in sign, at 9V.  These are independent and adjusted separately.  Note that in PSpice, there are two ways to represent 
a source with a negative sign.  Figure A-3 shows the two options: you can either set the voltage source to a negative 
value, or you can reverse the polarity of the source. 
=
0
V1
-15V
V2
15V
0
 
Figure A-3. 
 
The op-amp chip: Study the chip layout of the LF351 and 741 op-amps is shown in Figure A-4.  Both have the same 
pin numbers and names.  Either op-amp can be used without changing the circuit.  The standard procedure on DIP 
(dual in-line package) "chips" is to identify pin 1 with a notch in the end of the chip package.  The notch always 
separates pin 1 from the last pin on the chip.  In the case of the LF351, the notch is between pins 1 and 8.  Pin 2 is 
the inverting input.  Pin 3 is the non-inverting input, and the amplifier output, V
O
, is at pin 6.  These three pins are 
the three terminals that normally appear in an op-amp circuit schematic diagram.  The +V
CC
 and -V
CC
 connections (7 
-9V 
9V 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 3 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
and 4) MUST be completed for the op-amp to work, although they usually are omitted from simple circuit 
schematics to improve clarity. 
 
 
Figure A-4. 
 
The balance (or null offset) pins (1 and 5) provide a way to eliminate any offset in the output voltage of the 
amplifier.  The offset voltage (usually denoted by Vos) is an artifact of the integrated circuit.  The offset voltage is 
additive with V
O
 (pin 6 in this case).  It can be either positive or negative and is normally less than 10mV.  Because 
the offset voltage is so small, in most cases we can ignore the contribution V
OS
 makes to V
O
 and we leave the null 
offset pins open.  Pin 8, labeled "NC", has no connection to the internal circuitry of the 741 or LF351, and is not 
used. 
 
Op-amp limitations: Just like all real circuit elements, op-amps have certain limitations which prevent them from 
performing optimally under all conditions.  The one you are most likely to encounter in this class is called 
saturation.  An op-amp becomes saturated if it tries to put out a voltage level beyond the range of the power source 
voltages, ±V
CC
, For example, if the gain tries to drive the output above 9V, the op-amp is not supplied with enough 
voltage to get it that high and the output will cut off at the most it can produce.  This is never quite as high as 9V 
because of the losses inside the op-amp.  Another common limitation is amount of current an op-amp can supply.  
Large demands for current by a low resistance load can interfere with the amount of current available for feedback, 
and result in less than ideal behavior.  Also, because of the demands of the internal circuitry of the device, there is 
only so much current that can pass through the op-amp before it starts to overheat.  A third limitation is called the 
slew rate and is the result of limit internal currents in the op-amp.  Delays caused by the slew rate can prevent the 
op-amp circuit from displaying the expected output instantaneously after the input changes.  The final caution we 
have about op-amps is that the equations for op-amps are derived using the assumption that an op-amp has infinite 
intrinsic (internal) gain, infinite input impedance, zero current at the inputs, and zero output impedance.  Naturally 
these assumptions cannot be true, however, the design of real op-amps is close enough to the assumptions that 
circuit behavior is close to ideal over a large range. 
 
The inverting amplifier: Figure A-5 shows an inverting amplifier. 
 
 
Figure A-5. 
+V
CC 
-V
CC 
Page 4


Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 1 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Experiment 4 
Op-Amp Circuits 
 
Purpose: In this experiment, you will learn about operational amplifiers (or op-amps).  Simple circuits containing 
operational amplifiers can be used to perform mathematical operations, such as addition, subtraction, and 
multiplication, on signals.  They can also be used to take derivatives and integrals.  Another important application of 
an op-amp circuit is the voltage follower, which serves as an isolator between two parts of a circuit. 
Several op-amp chips have the same pin configuration, two examples are the 741 op-amp and the LF351 op-amp.  
The demo version of Capture doesn’t have the LF351 version available, so the modeling will be done using the 741 
op-amp (a.k.a. µ741 op-amp.)  The LF351 is somewhat better for our application and will extend the battery 
lifetime.  Model the experiment in PSpice using the 741 but build it using the LF351 if you experience problems 
with the 741 in the circuit. 
 
Equipment Required: 
 
• DMM  (HP 34401A 6-1/2 Digit Multimeter) 
• Rensselaer IOBoard Rev D (with Mobile Studio Desktop) 
• DV Voltage Source (Rensselaer IOBoard, and two 9V batteries)  Students in JEC 4107/4104 should use the HP 
power supply, and avoid battery issues. 
• Analog I/O (Rensselaer IOBoard) 
• Protoboard 
• Some Resistors (50, 1k, 10k and 100k?) 
• uA741 or LF351 op-amp (LF351 is used if uA741 in kit is bad) 
 
Helpful links for this experiment can be found on the links page for this course: 
http://hibp.ecse.rpi.edu/~connor/education/EILinks.html#Exp4 
 
 
Part A – Introduction to Op-Amp Circuits 
 
Background 
 
Elements of an op-amp circuit: Figure A-1 below is a schematic of a typical circuit built with an op-amp. 
U1
LF351/NS
+
3
-
2
V+
7
V-
4
OUT
6
B2
5
B1
1
U3
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V+
9V
V-
-9V
V+
9V
V-
-9V
0 0
Vout Vout
Rf eedback
10k
R1 1k R1 1k
Rf eedback
10k
Rload Rload
0 0
Vin Vin
0
0
 
Figure A-1. Drawn with both 741 and a LF351 op-amps (Rload ˜ 1k?). 
 
The circuit performs a mathematical operation on an input signal.  This particular op-amp circuit will invert the 
input signal, Vin, and make the amplitude 10 times larger.  This is equivalent to multiplying the input by -10.  Note 
that there are two DC voltage sources in addition to the input.  These two DC voltages power the op-amp.  The 
circuit needs additional power because the output is bigger than the input.  Op-amps always need this additional pair 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 2 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
of power sources.  The two resistors R
feedback
 and R1 determine how much the op-amp will amplify the output.  If we 
change the magnitude of these resistors, we do not change the fact that the circuit multiplies by a negative constant; 
we only change the factor that it multiplies by.  The load resistor R
load
 is not part of the amplifier.  It represents the 
resistance of the load on the amplifier. 
 
Powering the op-amp: The two DC sources, (labeled as
 
V+ and V-, but also often labeled as ± V
CC
), that provide 
power to the op-amp are typically set to have an equal magnitude but opposite sign with respect to the ground of the 
circuit.  This enables the circuit to handle an input signal which oscillates around 0V, like most of the signals we use 
in this course.  (Note the signs on the sources in the circuit above.)  The schematic in Figure A-2 shows a standard ± 
V
CC
 configuration for op-amps.  The schematic symbols for a battery are used in this schematic to remind us that 
these supplies need to be a constant DC voltage.  They are not signal sources. 
 
 
Figure A-2. 
 
Students in JEC 4201 use two 9V batteries for power.  Batteries are self explanatory. 
Students in JEC 4107/4104 should use the HP E3631A supplies for power.  The HP E3631A power supply 
provides two variable supplies with a common ground (for ±V
CC
 ) plus a variable low voltage supply (not used in 
this lab).  The power supply jack labeled "COM" between the V
CC
 supplies should be connected to circuit ground.  
When you supply power to the op-amp, adjust the two voltage levels so that +V
CC
 and -V
CC
 are equal, but opposite 
in sign, at 9V.  These are independent and adjusted separately.  Note that in PSpice, there are two ways to represent 
a source with a negative sign.  Figure A-3 shows the two options: you can either set the voltage source to a negative 
value, or you can reverse the polarity of the source. 
=
0
V1
-15V
V2
15V
0
 
Figure A-3. 
 
The op-amp chip: Study the chip layout of the LF351 and 741 op-amps is shown in Figure A-4.  Both have the same 
pin numbers and names.  Either op-amp can be used without changing the circuit.  The standard procedure on DIP 
(dual in-line package) "chips" is to identify pin 1 with a notch in the end of the chip package.  The notch always 
separates pin 1 from the last pin on the chip.  In the case of the LF351, the notch is between pins 1 and 8.  Pin 2 is 
the inverting input.  Pin 3 is the non-inverting input, and the amplifier output, V
O
, is at pin 6.  These three pins are 
the three terminals that normally appear in an op-amp circuit schematic diagram.  The +V
CC
 and -V
CC
 connections (7 
-9V 
9V 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 3 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
and 4) MUST be completed for the op-amp to work, although they usually are omitted from simple circuit 
schematics to improve clarity. 
 
 
Figure A-4. 
 
The balance (or null offset) pins (1 and 5) provide a way to eliminate any offset in the output voltage of the 
amplifier.  The offset voltage (usually denoted by Vos) is an artifact of the integrated circuit.  The offset voltage is 
additive with V
O
 (pin 6 in this case).  It can be either positive or negative and is normally less than 10mV.  Because 
the offset voltage is so small, in most cases we can ignore the contribution V
OS
 makes to V
O
 and we leave the null 
offset pins open.  Pin 8, labeled "NC", has no connection to the internal circuitry of the 741 or LF351, and is not 
used. 
 
Op-amp limitations: Just like all real circuit elements, op-amps have certain limitations which prevent them from 
performing optimally under all conditions.  The one you are most likely to encounter in this class is called 
saturation.  An op-amp becomes saturated if it tries to put out a voltage level beyond the range of the power source 
voltages, ±V
CC
, For example, if the gain tries to drive the output above 9V, the op-amp is not supplied with enough 
voltage to get it that high and the output will cut off at the most it can produce.  This is never quite as high as 9V 
because of the losses inside the op-amp.  Another common limitation is amount of current an op-amp can supply.  
Large demands for current by a low resistance load can interfere with the amount of current available for feedback, 
and result in less than ideal behavior.  Also, because of the demands of the internal circuitry of the device, there is 
only so much current that can pass through the op-amp before it starts to overheat.  A third limitation is called the 
slew rate and is the result of limit internal currents in the op-amp.  Delays caused by the slew rate can prevent the 
op-amp circuit from displaying the expected output instantaneously after the input changes.  The final caution we 
have about op-amps is that the equations for op-amps are derived using the assumption that an op-amp has infinite 
intrinsic (internal) gain, infinite input impedance, zero current at the inputs, and zero output impedance.  Naturally 
these assumptions cannot be true, however, the design of real op-amps is close enough to the assumptions that 
circuit behavior is close to ideal over a large range. 
 
The inverting amplifier: Figure A-5 shows an inverting amplifier. 
 
 
Figure A-5. 
+V
CC 
-V
CC 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 4 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Its behavior is governed by the following equation: 
in out
V
Rin
Rf
V - = .  The negative sign indicates that the circuit 
will invert the signal.  (When you invert a signal, you switch its sign.  This is equivalent to an180° phase shift of a 
sinusoidal signal.)  The circuit will also amplify the input by Rf/Rin.  Therefore, the total gain for this circuit is –
(Rf/Rin).  Note that most op-amp circuits invert the input signal because op-amps stabilize when the feedback is 
negative.  Also note that even though the connections to V+ and V- (±V
CC
) are not shown, they must be made in 
order for the circuit to function in both PSpice and on your protoboard. 
 
The non-inverting amplifier: Figure A-6 shows a non-inverting amplifier.  Its behavior is governed by the following 
equation: 
in out
V
Rin
Rf
V ?
?
?
?
?
?
+ = 1 . 
 
Figure A-6. 
 
This circuit multiplies the input by 1+(Rf/Rin) and, unlike the previous op-amp circuit, the output is not an inversion 
of the input.  The overall gain for this circuit is, therefore, 1+(Rf/Rin).  The inverting amplifier is more commonly 
used than the non-inverting amplifier.  That is why the somewhat odd term “non-inverting” is used to describe an 
amplifier that does not invert the input.  If you look at the circuits, you will see that in the inverting op-amp, the chip 
is connected to ground, while in the non-inverting amplifier it is not.  This generally makes the inverting amplifier 
behave better.  When used as a DC amplifier, the inverting amp can be a poor choice, since its output voltage will be 
negative.  However, for AC applications, inversion does not matter since sines and cosines are positive half the time 
and negative half the time anyway. 
 
Experiment 
 
The Inverting Amplifier 
In this part of the experiment, we will wire a very simple op-amp circuit using PSpice and look at its behavior. 
 
• Wire the circuit shown in Figure A-7 below in PSpice. 
U1
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V2
9V
V3 9V
V1
R2
1k
2 1
R3
10k
2 1
0
0
RL
1k
2
1
V
V
Vout Vin
 
Figure A-7. 
 
o The input should have 200mV amplitude, 1kHz and no DC offset. 
o The op-amp is called uA741 and is located in the “EVAL” library. 
Page 5


Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 1 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Experiment 4 
Op-Amp Circuits 
 
Purpose: In this experiment, you will learn about operational amplifiers (or op-amps).  Simple circuits containing 
operational amplifiers can be used to perform mathematical operations, such as addition, subtraction, and 
multiplication, on signals.  They can also be used to take derivatives and integrals.  Another important application of 
an op-amp circuit is the voltage follower, which serves as an isolator between two parts of a circuit. 
Several op-amp chips have the same pin configuration, two examples are the 741 op-amp and the LF351 op-amp.  
The demo version of Capture doesn’t have the LF351 version available, so the modeling will be done using the 741 
op-amp (a.k.a. µ741 op-amp.)  The LF351 is somewhat better for our application and will extend the battery 
lifetime.  Model the experiment in PSpice using the 741 but build it using the LF351 if you experience problems 
with the 741 in the circuit. 
 
Equipment Required: 
 
• DMM  (HP 34401A 6-1/2 Digit Multimeter) 
• Rensselaer IOBoard Rev D (with Mobile Studio Desktop) 
• DV Voltage Source (Rensselaer IOBoard, and two 9V batteries)  Students in JEC 4107/4104 should use the HP 
power supply, and avoid battery issues. 
• Analog I/O (Rensselaer IOBoard) 
• Protoboard 
• Some Resistors (50, 1k, 10k and 100k?) 
• uA741 or LF351 op-amp (LF351 is used if uA741 in kit is bad) 
 
Helpful links for this experiment can be found on the links page for this course: 
http://hibp.ecse.rpi.edu/~connor/education/EILinks.html#Exp4 
 
 
Part A – Introduction to Op-Amp Circuits 
 
Background 
 
Elements of an op-amp circuit: Figure A-1 below is a schematic of a typical circuit built with an op-amp. 
U1
LF351/NS
+
3
-
2
V+
7
V-
4
OUT
6
B2
5
B1
1
U3
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V+
9V
V-
-9V
V+
9V
V-
-9V
0 0
Vout Vout
Rf eedback
10k
R1 1k R1 1k
Rf eedback
10k
Rload Rload
0 0
Vin Vin
0
0
 
Figure A-1. Drawn with both 741 and a LF351 op-amps (Rload ˜ 1k?). 
 
The circuit performs a mathematical operation on an input signal.  This particular op-amp circuit will invert the 
input signal, Vin, and make the amplitude 10 times larger.  This is equivalent to multiplying the input by -10.  Note 
that there are two DC voltage sources in addition to the input.  These two DC voltages power the op-amp.  The 
circuit needs additional power because the output is bigger than the input.  Op-amps always need this additional pair 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 2 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
of power sources.  The two resistors R
feedback
 and R1 determine how much the op-amp will amplify the output.  If we 
change the magnitude of these resistors, we do not change the fact that the circuit multiplies by a negative constant; 
we only change the factor that it multiplies by.  The load resistor R
load
 is not part of the amplifier.  It represents the 
resistance of the load on the amplifier. 
 
Powering the op-amp: The two DC sources, (labeled as
 
V+ and V-, but also often labeled as ± V
CC
), that provide 
power to the op-amp are typically set to have an equal magnitude but opposite sign with respect to the ground of the 
circuit.  This enables the circuit to handle an input signal which oscillates around 0V, like most of the signals we use 
in this course.  (Note the signs on the sources in the circuit above.)  The schematic in Figure A-2 shows a standard ± 
V
CC
 configuration for op-amps.  The schematic symbols for a battery are used in this schematic to remind us that 
these supplies need to be a constant DC voltage.  They are not signal sources. 
 
 
Figure A-2. 
 
Students in JEC 4201 use two 9V batteries for power.  Batteries are self explanatory. 
Students in JEC 4107/4104 should use the HP E3631A supplies for power.  The HP E3631A power supply 
provides two variable supplies with a common ground (for ±V
CC
 ) plus a variable low voltage supply (not used in 
this lab).  The power supply jack labeled "COM" between the V
CC
 supplies should be connected to circuit ground.  
When you supply power to the op-amp, adjust the two voltage levels so that +V
CC
 and -V
CC
 are equal, but opposite 
in sign, at 9V.  These are independent and adjusted separately.  Note that in PSpice, there are two ways to represent 
a source with a negative sign.  Figure A-3 shows the two options: you can either set the voltage source to a negative 
value, or you can reverse the polarity of the source. 
=
0
V1
-15V
V2
15V
0
 
Figure A-3. 
 
The op-amp chip: Study the chip layout of the LF351 and 741 op-amps is shown in Figure A-4.  Both have the same 
pin numbers and names.  Either op-amp can be used without changing the circuit.  The standard procedure on DIP 
(dual in-line package) "chips" is to identify pin 1 with a notch in the end of the chip package.  The notch always 
separates pin 1 from the last pin on the chip.  In the case of the LF351, the notch is between pins 1 and 8.  Pin 2 is 
the inverting input.  Pin 3 is the non-inverting input, and the amplifier output, V
O
, is at pin 6.  These three pins are 
the three terminals that normally appear in an op-amp circuit schematic diagram.  The +V
CC
 and -V
CC
 connections (7 
-9V 
9V 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 3 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
and 4) MUST be completed for the op-amp to work, although they usually are omitted from simple circuit 
schematics to improve clarity. 
 
 
Figure A-4. 
 
The balance (or null offset) pins (1 and 5) provide a way to eliminate any offset in the output voltage of the 
amplifier.  The offset voltage (usually denoted by Vos) is an artifact of the integrated circuit.  The offset voltage is 
additive with V
O
 (pin 6 in this case).  It can be either positive or negative and is normally less than 10mV.  Because 
the offset voltage is so small, in most cases we can ignore the contribution V
OS
 makes to V
O
 and we leave the null 
offset pins open.  Pin 8, labeled "NC", has no connection to the internal circuitry of the 741 or LF351, and is not 
used. 
 
Op-amp limitations: Just like all real circuit elements, op-amps have certain limitations which prevent them from 
performing optimally under all conditions.  The one you are most likely to encounter in this class is called 
saturation.  An op-amp becomes saturated if it tries to put out a voltage level beyond the range of the power source 
voltages, ±V
CC
, For example, if the gain tries to drive the output above 9V, the op-amp is not supplied with enough 
voltage to get it that high and the output will cut off at the most it can produce.  This is never quite as high as 9V 
because of the losses inside the op-amp.  Another common limitation is amount of current an op-amp can supply.  
Large demands for current by a low resistance load can interfere with the amount of current available for feedback, 
and result in less than ideal behavior.  Also, because of the demands of the internal circuitry of the device, there is 
only so much current that can pass through the op-amp before it starts to overheat.  A third limitation is called the 
slew rate and is the result of limit internal currents in the op-amp.  Delays caused by the slew rate can prevent the 
op-amp circuit from displaying the expected output instantaneously after the input changes.  The final caution we 
have about op-amps is that the equations for op-amps are derived using the assumption that an op-amp has infinite 
intrinsic (internal) gain, infinite input impedance, zero current at the inputs, and zero output impedance.  Naturally 
these assumptions cannot be true, however, the design of real op-amps is close enough to the assumptions that 
circuit behavior is close to ideal over a large range. 
 
The inverting amplifier: Figure A-5 shows an inverting amplifier. 
 
 
Figure A-5. 
+V
CC 
-V
CC 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 4 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
Its behavior is governed by the following equation: 
in out
V
Rin
Rf
V - = .  The negative sign indicates that the circuit 
will invert the signal.  (When you invert a signal, you switch its sign.  This is equivalent to an180° phase shift of a 
sinusoidal signal.)  The circuit will also amplify the input by Rf/Rin.  Therefore, the total gain for this circuit is –
(Rf/Rin).  Note that most op-amp circuits invert the input signal because op-amps stabilize when the feedback is 
negative.  Also note that even though the connections to V+ and V- (±V
CC
) are not shown, they must be made in 
order for the circuit to function in both PSpice and on your protoboard. 
 
The non-inverting amplifier: Figure A-6 shows a non-inverting amplifier.  Its behavior is governed by the following 
equation: 
in out
V
Rin
Rf
V ?
?
?
?
?
?
+ = 1 . 
 
Figure A-6. 
 
This circuit multiplies the input by 1+(Rf/Rin) and, unlike the previous op-amp circuit, the output is not an inversion 
of the input.  The overall gain for this circuit is, therefore, 1+(Rf/Rin).  The inverting amplifier is more commonly 
used than the non-inverting amplifier.  That is why the somewhat odd term “non-inverting” is used to describe an 
amplifier that does not invert the input.  If you look at the circuits, you will see that in the inverting op-amp, the chip 
is connected to ground, while in the non-inverting amplifier it is not.  This generally makes the inverting amplifier 
behave better.  When used as a DC amplifier, the inverting amp can be a poor choice, since its output voltage will be 
negative.  However, for AC applications, inversion does not matter since sines and cosines are positive half the time 
and negative half the time anyway. 
 
Experiment 
 
The Inverting Amplifier 
In this part of the experiment, we will wire a very simple op-amp circuit using PSpice and look at its behavior. 
 
• Wire the circuit shown in Figure A-7 below in PSpice. 
U1
uA741
+
3
-
2
V+
7
V-
4
OUT
6
OS1
1
OS2
5
V2
9V
V3 9V
V1
R2
1k
2 1
R3
10k
2 1
0
0
RL
1k
2
1
V
V
Vout Vin
 
Figure A-7. 
 
o The input should have 200mV amplitude, 1kHz and no DC offset. 
o The op-amp is called uA741 and is located in the “EVAL” library. 
Electronic Instrumentation 
ENGR-4300  Experiment 4 
K.A. Connor, S. Bonner, P. Schoch 5 of 16 Revised: 1/13/2008 
Rensselaer Polytechnic Institute  Troy, New York, USA 
o Be careful to make sure that the + and – inputs are not switched and that the two DC voltage supplies have 
opposite signs. 
o Note the location of the input voltage, Vin.  In this case, R2 is the input resistor, Rin,  so the marker goes to 
its left. 
 
• Run a transient simulation of this circuit that displays three cycles. 
o What does the equation for this type of circuit predict for its behavior? 
o Use the cursors to mark the amplitudes of the input and output of the circuit. 
o Calculate the actual gain on the circuit.  Is this close to the gain predicted by the equation? 
o Print out this plot and include it with your report. 
 
• Run a transient of the circuit with a much higher input amplitude. 
o Change the amplitude of the source to 5V and rerun the simulation. 
o What does the equation predict for the behavior this time?  Does the circuit display the output as expected?  
What happened? 
o Use the cursors to mark the maximum value of the input and output of the circuit. 
o What is the magnitude of the output of the circuit at saturation? 
o Print out this plot and include it with your report. 
 
Build an Inverting Amplifier 
In this part of the circuit, you will build an inverting amplifier.  Build the circuit using the 741 or LF351 op-amp.  
Students in JEC 4201 use batteries to provide the +9V and –9V power sources. 
 
• Build the inverting op-amp circuit in Figure A-7 on your protoboard. 
o Don’t neglect to wire the DC power voltages at pins 4 and 7.  Do not connect pin 4 and 7 to ground.  They 
go through the power supply to ground. 
o For students in JEC 4107/4104, do not forget to set both the positive and negative values on the DC power 
supply.  One does not automatically set when you set the other.  Do not forget to attach the common ground 
for the power supply voltages to the ground for the circuit as a whole. 
 
• Examine the behavior of your circuit. 
o Take a picture with the IOBoard software of the input and output of the circuit at 1kHz and 200mV 
amplitude and include it in your report. 
o What was the gain of your circuit at this amplitude and frequency?  [Use the signals to calculate the gain, 
not the values of the resistors.] 
o Saturation: Change R
feedback
 to a 22k? resistor.  Vary the amplitude of the function generator until the op-
amp output starts to saturate.  At about what input amplitude does this happen?  What is the magnitude of 
the output of the circuit at saturation?  How does this compare with the saturation voltage found using 
PSpice? 
 
Summary 
 
As long as one remains aware of some of their limitations, op-amp circuits can be used to perform many different 
mathematical operations.  That is why collections of op-amp circuits have been used in the past to represent dynamic 
systems in what is called an analog computer.  There are some very good pictures of analog computers and other 
computers through the ages at H.A. Layer’s Mind Machine Web Museum.  A link is located on the course links 
page. 
 
 
Part B – Voltage Followers 
 
Background 
 
The voltage follower: The op-amp configuration in Figure B-1 is called a voltage follower or buffer.  Note that the 
circuit above has no resistance in the feedback path.  Its behavior is governed by the equation: 
in out
V V = .