# THE INTEGRATED CIRCUIT

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```					Physics 340                                                                                         Laboratory #6

THE INTEGRATED CIRCUIT
OPERATIONAL AMPLIFIER

Textbook Readings: Diefenderfer and Holton, Chapter 9

1. Objectives

In this experiment you will investigate negative feedback applied to a high gain DC amplifier known as an
operational amplifier (OP AMP). You will use the #741 integrated circuit (IC) OP AMP universally accepted
as the standard general purpose OP AMP. You will verify the theoretical expressions for closed loop gain and
then construct several circuits using the OP AMP to perform mathematical operations. These operations are
performed on data represented by electric potentials and the result (OP AMP output) is also a potential. You
are, in effect, representing the real number scale by DC potentials and thus you are making analog
computations.

2. OP AMP Familiarization and Multiplication
Offset Null     1                        8   No Connection   The 741 OP AMP package pin
connections are illustrated at left. The
table below lists the values of various
Inverting Input     2                        7   V
parameters that govern the operation of
a 741 OP AMP.

Non-Inverting Input     3                        6   Output

V      4                        5   Offset Null

741 OP AMP PARAMETERS
Parameters            Minimum         Typical              Maximum

AVOL                  Open loop voltage gain          50,000         200,000

Vio                    Input offset voltage                          1 mV                  6 mV

IB                        Input bias current                       80 nA                 500 nA

Iio                   Input offset current                         20 nA                 200 nA

Ro                        Output resistance                         75 

CMR (dB)                 Common mode rejection              70 dB       90 dB

Ri                        Input resistance          300 k         2 M

Name: __________________________________________

Partner: __________________________________________

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Physics 340                                                                                       Laboratory #6

A. Select an OP AMP from the storage cabinet in the back of the lab and record the complete chip
identification information. This information could be critical in the future.

Op Amp information: ______________________________

Using the circuit breadboard, setup the following circuit. Be sure to measure and record ALL of your
resistors (except the pot) and power supplies!

5.6 k (5%)

2.7 k (5%)

2.7 k

10 k pot

The 10 k pot (potentiometer) between pins 1 and 5 is used to zero the output (vout) when there is zero
input (vin). This is called an offset null adjustment and must be made in order to ensure that the curve of
vout vs. vin goes through the origin (see figure below). The 10 k pot is located at the bottom-center of
your breadboard. Connect pins 1 and 5 of the Op Amp to the holes on opposite ends of the pot. Connect
the –15V to one of the four center holes of the pot. You should note this general setup of the
potentiometer for future reference.

without
offset
nulled

offset
nulled

B. Before proceeding further, connect vin to ground (zero input) and adjust the 10 k pot until vout reads zero.

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Physics 340                                                                                       Laboratory #6

C. Now connect vin to a DC power supply and adjust vin to 1 volt. Measure vout and compute the closed loop
gain including sign.

Vin=                      1.0 V

Vout=

Ac=vout/vin=

D. Change vin to 5 volts and again measure Vout and Ac.

Vin=                      5.0 V

Vout=

Ac=vout/vin=

E. Vary vin over the range 0 to 8 volts and record vout. Include a plot of vout vs. vin in your final writeup and
note any significant deviations from linearity.

The closed loop gain for an OP AMP with feedback resistor Rf is given, theoretically, by

Rf
As  -
Rin

QUESTION 1

How well do your measurements agree with theory? Be quantitative.

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Physics 340                                                                                         Laboratory #6

QUESTION 2

What precautions must you take to ensure that your amplifier always operates in the linear region?

3. Addition with an OP AMP

A. Setup the following circuit to perform addition with multiplication. Leave the power supply connections
and the offset null adjustment pot connected as for the previous section

5.6 k (5%)

Rf
Rin
2.7 k (5%)
V1

V2

From this circuit, derive the following expression

 Rf       Rf      
vout = -
 R * v1 +    * v2 .

 in       R2      

This circuit adds two numbers (potentials) with weights given by the ratio of feedback to input resistors.
Note that the final result should be rounded to two significant figures, due to the tolerance of the resistors.
Be sure to measure and record your actual resistances.

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Physics 340                                                                                    Laboratory #6

B. Use two power supplies to provide v1 and v2. Set up the following conditions and measure vout.

R2              v1                 v2               vout (Measured)           vout (Calculated)

2.5 V              1.0 V

5 k
2.5 V              -1.0 V

-0.5 V             -1.5 V

3.0 V              -6.0 V
1.9 k
4.0 V              5.0 V

QUESTION 3

How do your measured values of vout compare with calculated values? Be quantitative and explain all
discrepancies.

QUESTION 4

On a separate page, design an OP AMP circuit that would give the average of three numbers (potentials). You
do not have to build this circuit, but be sure you have a complete, properly labeled diagram.

QUESTION 5

Assume you have at your disposal a 6 Volt DC power supply and a square wave generator operating at 1 kHz
with Vmin=0 volts and Vmax=3 volts. How could you mix these, using an OP AMP adder so that the output was
a square wave (at 1 kHz) with Vmin=2 volts and Vmax=+4 volts? Explain your solution and draw your circuit
for this below.

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Physics 340                                                                                       Laboratory #6

Construct the preceding circuit and verify that it works. Use a power supply set to 6 volts for the DC voltage;
for the square wave, adjust the DC offset and the amplitude so that the square-wave output goes from 0 to +3 V
data. Try to use only 5% resistors or your (1%) decade box.

4. Calculus with an OP AMP

A. Setup the following OP AMP integrator. Don’t forget to measure your resistances and capacitances.

1 F

C

11.2 k

2.7 k

2.7 k
2.7 k

Rin

The 11.2 k feedback resistor is present only to prevent any small DC level present at vin from saturating
the output.

B. Apply square waves of amplitude ~5 Vpt (you may need a smaller value if your OP-AMP output saturates)
to the input and record both vin and vout for a range of square wave frequencies from 10 Hz to 10 kHz.
plots of your data in your final write-up as part of QUESTION 7 below.

QUESTION 6

Describe quantitatively what you see over the range. Are the output waveforms what you expect from the
following equation?
t
1
Rin C f 
 out               in dt
0

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Physics 340                                                                                    Laboratory #6

QUESTION 7

From the previous equation you can compute the peak-to-peak output potential for a given input peak-peak
potential. How does theory agree with experiment at f = 10 kHz? Be quantitative. Plot all of the data you

C. Reverse the feed back capacitor and the input resistor to form the following circuit.

(2 2.7 kin parallel)
1.35 k

1 F

2.7 k

Use a square wave input of approximately 4 Vpt and vary the frequency through the range 10 Hz to 500
Hz. Record both vin and vout using your digital oscilloscope and include plots of your data in your final
writeup.

QUESTION 8

What does the output waveform approximate? Plot and discuss your measurements.

7

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