Circuits
ECSE-2010
Fall 2014
LABORATORY 4: Amplifier models, Comparators, Summers,
Inverting/Non-inverting Amplifiers
Note: If your partner is no longer in the class, please talk to the instructor.
Material covered:
 TL072CP op-amp chip
 Saturation
 Gain
 Dependent sources
Overall notes:
TL072CP chip (dual op-amp):
The data sheet for the chip can be found online from any number of sites. One is
provided below (it is long and contains several chips)
http://www.ti.com/lit/ds/symlink/tl071.pdf
A copy of the pin connections is shown below
There are two op-amps on the chip, indicated by the ‘1’ and the ‘2’ pin labels. For
example, 1IN+ is the V+ and 1IN- is the V- of the first op-amp, with 1OUT being
the Vout. Power connections are +Vcc at pin 8 and –Vcc at pin 4.
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
In PSpice, you can use either the “opamp” component or the “uA741” op-amp
component. The “opamp” component does not have power levels and is assumed
ideal, so will not have saturation effects and the output voltage has infinite range. It
is useful for simplified drawings, but your simulations will not be the same as the
experiments. As such, please use the “ua741” component, with PSpice details
shown below.
3
U1 7
+
V+
OS2
OUT
2
-
4
uA741
OS1
V-
5
6
1
A summary of the connections for PSpice uA741 component:
 1: OS1, floating (no connection)
 2: input,  3: input, +
 4: V-: Negative power, 9 V
 5: OS2, floating (no connection)
 6: Vout, output voltage
 7: V+: Positive power, 9 V
The DC power sources will be the 9 Volt batteries that you have in your kit. Note
the orientation of the batteries when you connect the leads. In PSpice simulations,
the offsets should also be floating. The TL072CP chip does not have offset
connections.
9
U17 V+
+ OS2
OUT
2
- 4 OS1
uA741 V3
0
5
6
1
9
0
Again, for PSpice simulations, the circuit on the following page indicates how to
power a uA741 op-amp. The input and output connections depend on the circuit.
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
An example of an op-amp reaching saturation is shown below. The input is a
sinusoidal. If the op-amp was ideal, the output would also be a sinusoid with a
scaled amplitude. However, saturation occurs and the output voltage cannot exceed
(positive or negative) the source voltages.
10V
5V
0V
-5V
-10V
0s
0.5ms
1.0ms
1.5ms
2.0ms
2.5ms
3.0ms
3.5ms
4.0ms
V(U1:OUT)
Time
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
Dependent sources:
This section is included if you want to use PSpice to check your homework
problems.
Dependent sources are part of the PSpice library. You can find them in the parts
list:
 E/Analog: Voltage controlled voltage source (VCVS)
 F/Analog: Current controlled current source (CCVS)
 G/Analog: Voltage controlled current source (VCCS)
 H/Analog: Current controlled voltage source (CCVS)
The components have four connections, two that are a ‘probe’ and two that
represent a source. The simple model of the op-amp we saw in class is shown
below. It includes a voltage dependent voltage source.
Rs
Rout(small)
+
Vin
V3
Rin(large)
Vx
Rload
A(Vx)
-
0
In PSpice we can implement this circuit using the E/Analog component. The
schematic should look like
Rs1
Vin
Rout(small)1
+
Rin(large)1
Vx
E1
+
-
+
-
Rload1
E
-
0
Op-Amp Model
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
The gain term, A, is set by double clicking on the part. A column appropriately
labeled gain lets you choose the value. If you read through the spec sheet for the
TL072CP amplifier, it indicates the following specifications:
 based on a load resistance of 2kΩ
 a large signal gain of ~200000 with (A in the circuit model on the
previous page)
 input resistance of 10E12Ω, Rin
 you can set an output resistance, Rout, of 20Ω
 common practice is to add a resistor at the input to an op-amp, Rs in
the circuit above, a 1k resistor is reasonable.
The above is a simple model for op-amp circuits. Information that is good to know,
but not something that you will implement in the laboratory.
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
Laboratory
Part 1) Comparator
0
V2
9
3
U17
+
V+
OS2
OUT
Vin
2
-
uA741
0
4
OS1
V-
0
V3
9
5
6
Vout
1
RLarge
0
0
Build the comparator circuit shown above. V+ and V- will be your inputs and Vout
will be the output. In Mobile Studio experiments, use the TL072CP chip (you only
need one amplifier for this part). In the PSpice simulations use the uA741/Eval
component.
a) We will use AWG1 and AWG2 for out amplifier inputs. (refer to the
discussion in Laboratory 3 about the function generator channels AWG1 and
AWG2). The Voltmeter channels inputs will act as the RLarge.
a. Connect W1 (yellow wire) to the V+ op-amp input and ground
(orange striped wire) to the V- op-amp input..
b. Ground the V- op-amp input.
c. To compare input voltage to output voltage, use the Voltmeter to
measure the output voltage (refer to Lab 2).
d. Using the Discovery board, set the AWG1 output voltage to DC mode
and check the output voltage of the op-amp for the following input
voltages
Vin [V]
2
1
0
-1
-2
Vout [V]
e. Comment on your results and expectations when Vin = 0 V.
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
b) In PSpice, build the comparator circuit using a uA741 op-amp and using the
part a settings. You will need to add a load resistor at the output node since
PSpice does not allow nodes to float (be unconnected). A 1E6Ω load is fine
(use exponential notation since M in Pspice is 1E-3). Compare the output
voltages between PSpice and Mobile studio. You should see some
differences, what causes these differences?
Vin [V]
2
1
0
-1
-2
Written by J. Braunstein
Rensselaer Polytechnic Institute
Vout [V]
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
0
V2
9
3
U17
+
V+
OS2
OUT
Vin
2
1.5
-
uA741
V4
4
OS1
V-
0
V3
9
0
5
6
Vout
1
RLarge
0
0
c) Remove the ground connection at V- and use AWG2 (W2, yellow striped
wire) to provide a 1.5V input at the V- opamp input. Effectively, your circuit
will behave as if there was a 1V source at the negative input, as shown
above.
If you didn’t use AWG2 for the 1.5V input, what type of circuit can you use
to produce the 1.5V? (Consider the 5V Discovery board connection from
Lab 2.)
a. Repeat the output voltage measurements again
Vin [V]
2
1
0
-1
-2
Vout [V]
d) Again, compare your Mobile Studio experiment to the PSpice simulation.
Vin [V]
2
1
0
-1
-2
Written by J. Braunstein
Rensselaer Polytechnic Institute
Vout [V]
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
Part 2) Inverting Op-amp, Non-inverting op-amp, Summers
+
0
OS2
2
uA741
-
4
OUT
OS1
5
6
1
Vout
V-
Vin
R1
U2
V+
3
7
Build and test the following circuits (in the schematics, the power connections
have been removed to simplify the drawing, they must still be included in the
circuit).
a) Inverting Op-amp with a gain of -2
R2
a. When considering the saturation voltage, what is the maximum Vin
such that Vout = -2 Vin? Choose appropriate resistors.
Vin [V]
5
3
1
0
-1
-3
-5
Vout [V]
b. Build the circuit in PSpice, using the uA741 opamp and verify that
simulation is constant with experiment (within the limits of the
respective saturation voltages).
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA
Circuits
ECSE-2010
Fall 2014
3
U3
+
V+
Vin
7
b) Non-inverting Op-amp with a gain (1+R2/R1) of 3
a. When considering the saturation voltage, what is the maximum Vin
such that Vout = 3 Vin? In other words, at what Vin does the output
reach saturation? (Discovery Board only)
OS2
uA741
-
OS1
6
1
Vout
V-
2
4
OUT
5
R2
R1
0
Vin [V]
5
3
1
0
-1
-3
-5
Vout [V]
c) Design a summing Op-amp circuit such that Vout = V1 - 3V2, where V1
and V2 are the two inputs. Both AWG channels are needed on the Discovery
Board. You will also need more than one op-amp for this circuit.
Remember, each chip has two op-amps and each kit should have two chips.
(Discovery Board only)
a. Verify your design by selecting various combinations of V1 and V2.
b. When V1 is 3 V, what is the range of V2 so that Vout is not in
saturation (positive or negative).
Written by J. Braunstein
Rensselaer Polytechnic Institute
S. Sawyer Fall 2014: 10/2/2014
Troy, New York, USA