# Lab 2 Circuit Simulation

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```					                        Circuit Construction using Integrated Circuits

Objective
In this experiment we‟ll look at how to construct circuits using an IC (integrated circuit) chip. You‟ll
have an opportunity to practice reading schematics and translating from a manufacturer‟s “pin out”
diagram to the actual wires on your breadboard and circuit. We‟ll continue to practice the model,
measure, repeat strategy as we gain experience with Matlab and the circuit simulation tools in P-
Spice. The specific Chip we‟ll use is the LM741 Op Amp (operational amplifier). We‟ll use PSpice
to simulate circuits, to see what we should expect theoretically, and also Matlab to predict the circuit
output. Once we know what to expect, the actual circuits will be built, and the results compared with
the theory. You‟ll continue to explore the features of the bench test equipment. Many common
circuits require the use of “dual voltage,” a plus voltage and a minus voltage. Op-amps are also very
common ICs. HINT: When you put a sine wave into a linear system, you‟d expect to see a sine
wave come out. If you see a wave form that looks squared off and flat on the top, be prepared to
explain why your model didn‟t predict it.
Concepts
Figure 1 shows a basic inverting amplifier circuit. The triangle is the circuit symbol for the op amp
chip. The „-„input is called the inverting input, and the „+‟ is called the non-inverting input. When an
input signal is applied between R1 and ground, the circuit amplifies it. The circuit also inverts the
signal (the output will have the opposite sign as the input) because the non-inverting input is
grounded, and the input signal goes through R1 to the inverting input.

Figure 1: Inverting amplifier circuit

The voltage gain, Av, is defined as the output voltage divided by the input voltage. For the circuit in
Figure 1, the gain is:

Av = -R2/R1.                                            (1)

Thus the resistors can be manipulated to change the gain. One aspect of the circuit that‟s not shown
on the circuit diagram is the positive and negative 15 V source that is used to power the chip. After
you do part one of the prelab, you‟ll know where these need to be connected.

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Equipment and Components
This experiment will require the use of the Lodestar Power Supply, Fluke Digital Multimeter,
Agilent Oscilloscope, a breadboard, 22 AWG wire, and resistors with nominal values of 10 k, 51
k, 220 k, 560 k, 820 k, and 1 M. The Op Amp we‟ll use is the LM741. We‟ll also use the
computers for PSpice and Matlab.

Prelab (25 points) – Due at the beginning of lab
1. Bring a printout of the National Semiconductor Corp (www.national.com) performance
specifications (data sheet) for the LM741 Op Amp. This gives you the pin connections for the chip so
that you‟ll know how to connect the circuit. Have your TA initial your data sheet for this step. (10
points)

2. Write a paragraph in which you define the following terms common in operational amplifier
component specifications: Common Mode Rejection Ratio, Slew Rate. What are common test
conditions for determining these component characteristics? Use your own words and cite your
sources. (15 points)

Part 1: Inverting Amplifier
DC Input

1. First, for the table on your data sheet, calculate the ideal gain, and write the numbers on the sheet.
Now we‟ll have some idea what kind of numbers to expect.

2. The complete circuit, with positive and negative 15 V bias for the op amp chip power, is shown
in Figure 2. V1 is the input voltage, while V2 and V3 are the power for the op amp chip. Use
PSpice to simulate this circuit for each set of resistance values in the data sheet table, and print
out a copy of the simulation result. Write the output voltages in the table, and calculate the gain.
The op amp part is called uA741. Since this is DC, a Bias Point simulation profile would be most
useful. Write the output voltage, measured from Pin 6 to ground, for each set of resistors on your
data sheet.

0
V2
7

V4                            U1                      15Vdc
0.5Vdc                             3                         5
V+

+             OS2
6
R1                          OUT
2                         1
-    4    OS1
820k                        V-
uA741
V3
15Vdc

R2

1000k

Figure 2: Inverting amplifier circuit with DC bias shown

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3. Now build the circuit on your breadboard, using your pin diagram from the data sheet as a guide.
The +/- 15 V source will come from your power supply, and your DC input will come from your
function generator.
Using both outputs of the power supply to get both positive and negative voltage is known as
dual-ended mode. To operate in this mode, you can connect the power supply as shown in Figure
2. Connect the output probes as you normally would to each power supply output. Now connect
the ground output probe of one side to the high output probe of the other as shown in Figure 3 to
establish the circuit ground (note that the point where the „+‟ side of V3 and the „-„side of V2 are
connected in Figure 2 is grounded). Now the remaining single red probe is your +15 V voltage
and the single black probe is your -15 V voltage. The electrical ground comes from your
connection of the high and low probes of the opposite supply outputs.

Figure 3: Power supply output configuration

Now use your multimeter to verify the input voltage as 0.5 V, and also the +/- 15 V sources. For
each set of resistor values in the data sheet table, measure the output voltage (again between pin 6
and ground) and write it on the sheet. Remember to turn the power supply off while you’re

4. Which value(s) for gain (both PSpice and measured) deviated from the ideal gain? If there was
deviation, why did it occur? With the input voltage of 0.5 V, one gain value should be far off
from the ideal gain.

AC Input

5. Keep the same circuit, but change R1 to 560 k, and keep R2 at 1 M. Instead of a DC input,
we‟ll now use a sinusoidal input of 0.5 Vpk and 5 kHz. Calculate the ideal gain, and write it on

6. Use Matlab to create a plot of two or three periods of the input signal (0.5cos(t)). Then, on the
same graph, plot what the output signal should look like, based on what you know about the gain
of the inverting op amp circuit. Label your graph, axes, and each signal.

7. Now simulate the circuit in PSpice, using a time-domain simulation that shows two or three
periods. If you use a voltage level marker (    ) at pin 6, the trace will automatically show up on

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the graph output. Print out a copy of the result, showing both Vin and Vout. For a smoother trace,
try adjusting the maximum step size in the simulation profile.

8. Construct the circuit on your breadboard, and obtain an oscilloscope plot showing Vin and Vout.

9. Compare the measured gain from your circuit to the ideal gain by answering question 8 on the
data sheet.

Part 2: Summing Amplifier
Now we‟ll look at a simple summing amplifier, shown in Figure 3. This circuit takes two signals and
adds them together. One application of this type of circuit would be an audio mixer.

Figure 4: Inverting Summing Amplifier Circuit

AC and DC input:

1. First construct the circuit in PSpice. Remember that the +/- 15 V power supply is not shown, so
you‟ll have to add it to your circuit. Choose V1 as a 2 V DC source, and V2 as a 0.5 Vpk 5 kHz
sinusoid. Take R1 and R2 to be 10 k, and Rf to be 51 k. Do a time-domain simulation
showing both input voltages and the output. Print out a copy to hand in.

an oscilloscope plot showing the sinusoidal input and the output.

Two AC inputs:

3. Now let‟s see what happens when two AC inputs are used in out summing amplifier. We‟ll take
V1 to be 0.5cos(200t) and V2 as 0.25cos(2000t). First, using the gain of the amplifier and the
assumption that the amplifier adds the two signals, show the input and output signals in Matlab,
then plot what the expected output should be. On the plot, include both inputs and the output,
and label each, along with the title and axis labels.

4. Show the same plots in a PSpice simulation of the circuit, and print out a copy to hand in.

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inputs. Show the low frequency input on one channel, and the output on the other, and include a
printout.

Part 3: Oscillator
Finally, we‟ll look at an oscillator circuit that has no input besides the power to the opamp. The
circuit, shown in Figure 5, has no input signal, instead relying on both positive and negative
feedback. The negative feedback is time delayed by the capacitor in the feedback loop, which
causes the output to switch back and forth between the maximum voltage inputs at a predictable
frequency given in Equation (1).

(1)

Figure 5: Oscillator Circuit

1. First, calculate the frequency for the oscillator shown in Figure 5, and write it on the data sheet,
showing the calculation.

2. Simulate the oscillator in PSpice, keeping in mind that you again need to include the +/- 15 V to
the chip. Include a printout showing the output, and compare the frequency of the simulation to
the ideal calculated quantity.

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3. Build the circuit on your breadboard and include a hard copy of the output. Compare the
measured frequency to that of the PSpice simulation and the theoretical frequency. Also compare
the measured magnitude to that which you got from PSpice. Give reasons for any differences.

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Lab 10: Integrated Circuits

Name_____________________________                                                Section________

Prelab (due at the beginning of lab)
1. Bring a copy of the LM741 data sheet. TA initials: _______

2. Include your paragraph on CMRR, slew rate, and their test conditions.

Part 1: Measurement of Time Varying Signals
1. With input voltage of 0.5 V:

R1             R2          PSpice       Measured        Ideal Gain      Gain in       Measured
Vout (V)      Vout (V)         (R2/R1)        PSpice         Gain
820 k           1 M
220 k           1 M
100 k           1 M
10 k            1 M
2. Include a printout of the DC simulation from step 2.

3. Answer step 4: Which value(s) for gain (both PSpice and measured) deviated from the ideal gain? If
there was deviation, why did it occur?

4. Ideal gain for amplifier in step 5:

5. Include a printout of the input and output signals generated in Matlab in step 6

6. Include a printout of the AC simulation from step 7, showing Vin and Vout.

7. Include an oscilloscope plot showing Vin and Vout.

8.     Measured Vin = _________        Measured Vout = __________ Measured Gain = ___________
compare the measured gain to the ideal gain, and account for any discrepancies between the two.

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Part 2: Summing Amplifier
1. Include a print out of the PSpice AC and DC summing amp.

2. Include an oscilloscope print out of the AC and DC summing amp.

3. Include a Matlab plot of the inputs and output expected with two AC signals.

4. Include a PSpice plot of the inputs and output expected with two AC signals.

5. Include an oscilloscope plot of the input and output with two AC signals.

Part 3: P-Spice Time-Domain Simulation
1. Theoretical oscillator frequency (show the calculation):

2. Include a PSpice printout.

3. What was the frequency according to your PSpice simulation, and how did it compare to the
theoretical number?

4. Include an oscilloscope printout.

5. Compare the measured frequency to that of the PSpice simulation and the theoretical frequency.
Compare the measured magnitude to that which you got from PSpice. Give reasons for any
differences.

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