Intro to circuit Lab

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Inverting AMP
Resistor (ohm) Measured Value (ohm)
Theoritical Value (volts)Experimental Value
10k
10.07k
-6.6
33k
32.78k
6.8k
6.65k
Vo
VA = 2.002 V
Vout = -6.51 V
Inverting Summing AMP
VA = 1.001 V
VB = 0.0557 V
Vout = -5.976 V
USD ELEC 201__________________________________________________________Lab 06
LAB 6: Introduction to Operational Amplifier Circuits
OBJECTIVES
After completing this lab, you should be able to
 Correctly apply DC power to a 741 Op Amp and identify the input and output terminals.
 Use Multisim to simulate DC-excited Op Amp circuits using a 741 Op Amp model.
 Design, simulate, and experimentally demonstrate the use of a 741 Op Amp as an
inverting amplifier, inverting summing amplifier, and difference amplifier.
INTRODUCTION
Operational Amplifiers (Op Amps) are small-scale integrated circuits that form a common
building block for many analog electronic applications. They exhibit high voltage gain, high
input resistance, and low output resistance.
Op Amps can form solutions to many electronic design goals with the simple addition of passive
elements, such as resistors and capacitors, to their terminals. Application of the few constraints
imposed by the characteristics of the Op Amp allows for simple analysis using the node voltage
method and Kirchhoff’s laws to solve for the input/output relationships.
Since the open loop voltage gain, A, is typically >1,000,000 and the input resistance is >1 M,
the two input terminals of an Op Amp are commonly referred to as sharing a “virtual short.”
That is, the two input terminals are essentially at the same voltage, with negligible current
entering the input terminals of the Op Amp. The output terminal can carry more current than the
input terminals since there is current injected into the two power supply terminals.
One of the most popular Op Amps (and the one we will use in ELEC 201) is the µA741 (“741”)
in an 8-pin DIP (Dual In-line Package) package. The terminal designations associated with the
pins on the package are shown in Figure 1.
Offset Null
1
8
No Connection
Inverting Input
2
7
V+ Power
Noninverting Input
3
6
Output
V- Power
4
5
Offset Null
Figure 1. Top view of an 8 pin DIP package: 741 OpAmp.
Calculations
1. Design circuits using an ideal Op Amp to implement the following functions. (Choose 2 out
of 3 from the inverting amplifier, inverting summing amplifier, and difference amplifier circuits
from your textbook and/or class notes.)
______________________________________________________________________________
Based on a lab by Dr. T. Schubert and adopted from Dr. S. Lord
1
USD ELEC 201___________________________________________________________Lab 06
1. Vout = – 4VA (VB = 0) {Inverting Amplifier}
2. Vout = – 4VA – VB = – [4VA – VB] {Summing Amplifier}
3. Vout = 3(VB – VA) {Difference Amplifier}
Draw the circuit diagram and indicate all resistor values. Use +15V for Vcc+ and –15V for Vcc–
. Note that we only have a specific selection of discrete resistors (5% tolerance) available in
Loma 206. Table Lab7-1 lists the multipliers for these values. For example, in the k range, we
have 1.0, 1.2, 1.5, … k resistors. (It is important to minimize currents at input terminals of an
Op Amp. Thus, resistor values of at least 1kare recommended.)
Table Lab7-1: Standard 5% Tolerance Resistance Values Available in L206
10
12
15
18
22
27
30
33
39
47
56
68
75
82
2. Simulation
Verify your solutions using Multisim with VA = 2V and VB = 5V using the A741 model in the
Multisim Library. Note that the feedback should always go to the ‘–‘ (inverting) input.
Also, be careful that you have the +15V connected to VCC+ and the –15V to VCC–. You do not
need to include a load resistor at Vout.
Procedure
Build the 3 circuits that you designed in the prelab. (If you and your partner have different
values, choose one set of values. You do not need to build each circuit twice.) Measure the
output voltage and compare it to your theory using an ideal opamp as well as your simulation
with a 741 Op Amp.
______________________________________________________________________________
Based on a lab by Dr. T. Schubert and adopted from Dr. S. Lord
2
USD ELEC 201__________________________________________________________Lab 06
A. INVERTING AMPLIFIER
Connect the circuit of Figure 2. Apply  15 V to the power terminals of the Op Amp. Knowing
that the voltages at the inverting and noninverting terminals are identical due to the virtual short,
find the expression for Vout as a function of VA: this expression leads to the Voltage Gain =
Vout/ VA. Compare the measured input and output voltages with theory.
33 k
VA
10 k

2V
Vout

Figure 2. Circuit diagram for inverting amplifier.
B. INVERTING SUMMING AMPLIFIER
Connect the circuit of Figure 3. Find an expression for Vout in terms of VA and VB and calculate
the expected value of Vout.
10 k
33 k
VA
VB
6.8 k

1
4V
5V
0.5
V

out
Figure 3. Circuit diagram for inverting summing amplifier.
Confirm your results experimentally. Is the voltage at the inverting terminal identical to the
voltage at the noninverting terminal? If the 33 k resistor is replaced by a 6.8 k resistor, what
would the new value for Vout be?
______________________________________________________________________________
Based on a lab by Dr. T. Schubert and adopted from Dr. S. Lord
3
USD ELEC 201___________________________________________________________Lab 06
C.
DIFFERENCE AMPLIFIER
Connect the circuit of Figure 4: this circuit is intended to amplify the difference between VA and
VB. Find an expression for Vout in terms of VA and VB, and calculate the expected value of Vout.
10 k
33 k
VA
R1
2V
VB

R2
1 k

R3
5V
Vout
3.3 k
R4
Figure 4. Circuit diagram for difference amplifier.
Confirm your results experimentally. Is the voltage at the inverting terminal identical to the
voltage at the noninverting terminal? Is the relationship between the ratios R3/R4 and R1/R2
critical to performing the difference function? Why?
______________________________________________________________________________
Based on a lab by Dr. T. Schubert and adopted from Dr. S. Lord
4

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