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FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS
EXPERIMENT 9
OPERATIONAL AMPLIFIERS
1.0 INTRODUCTION
1.1 OP AMP FUNCTION
The operational amplifier, or op amp, is a high performance linear integrated
circuit. It offers a high gain, high impedance and low output impedance, all
important characteristics for an amplifier.
Modern op amps are typically housed in 8 or 14 pin dual-in-line packs and are
extremely versatile components. Many different op amps are available form a
variety of manufacturers all offering different specifications and operating
characteristics. All however confirm to the same basic format of two inputs,
one labelled “+” and the other “-”, and one output. The symbol for an op amp
is shown in Figure 1.1.
Figure 1.1
The two inputs are known as inverting (-) and non-inverting (+) and under
normal operating conditions, any signal applied to the inverting will undergo a
180 phase shift (i.e. it will be inverted). Figure 1.2 shows the resulting
outputs of various inputs, and the effect of signals being sent to either the noninverting or inverting input.
Figure 1.2
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FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS
The gain, or amount of amplification, is set through the use of external
components, such as resistors.
1.2 NON-INVERTING AMPLIFIER
A typical non-inverting amplifier is shown in Figure 1.3.
RF
+VS
R1
+
Vout
-VS
Vin
Figure 1.3
Note how the signal is applied to the non-inverting (+) input, and that the
inverting (-) input is grounded. Also note the feedback resistor RF, and the
power supply connections +VS and –VS.
The gain (AV), which is the ratio of the output voltage to the input voltage, for
the non-inverting amplifier can be shown to be:
AV  1 
RF
R1
or
 R 
Vout  Vin 1  F 
R1 

1.3 INVERTING AMPLIFIER
A typical inverting amplifier is shown in Figure 1.4.
Note how for the amplifier configured in inverting mode, the input signal is
applied to the inverting (-) input via R1, and that the non-inverting input (+) is
set to ground.
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FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS
RF
+VS
R1
Vin
+
Vout
-VS
Figure 1.4
The gain for this amplifier is
AV  
RF
R1
or
R
Vout  Vin  F
 R1



1.4 OP AMP CHIP
The op amp chip used for the purpose of this experiment is the UA741CN, and
is shown in Figure 1.5. Its advantages are numerous, including having a large
input voltage range and a high gain capability. It is a high performance
operational amplifier constructed on a single silicon chip. It is intended for a
wide range of analogue applications, such as a summing amplifier, voltage
follower, integrator, active filter and function generator.
Figure 1.5
Its pin connections, as viewed from the top of the chip are shown in Figure
1.6. Note that there is a notch or black spot on the top surface of the chip. Held
with this notch or black spot pointing away from you, pin 1 can be identified
as the leg which is in the top left position. The bottom left leg is thus pin 4, the
bottom right is pin 5, and the top right is pin 8.
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FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS
Figure 1.6
Pins 4 and 7 are the power terminals for the chip. These pins are not to be
confused with the input pins (2 and 3). Pins 4 and 7 need to be connected to a
power supply in order for the device to function, just like any other electronic
device. Pin 4 needs to be connected to a negative power source, whereas pin 7
needs to be connected to a positive power source.
2.0 PROCEDURE
2.1 OPERATION AS A NON-INVERTING AMPLIFIER
Connect up the op amp to function as a non-inverting amplifier, as shown in
Figure 1.3. Use RF = R1 = 8.2k. Use the pin layout shown in Figure 1.6 to
make the correct connections. Make sure your chip has been connected in a
way so that the left and right sides occupy different halves of the breadboard,
i.e. make use of the board’s central groove. Pins 4 and 7 will need to be
connected to the DC power supply, which needs to be set up in order to supply
a dual voltage of +5V and –5V. This is done by implemented the wiring
configuration shown in Figure 2.1.
Figure 2.1
Draw your circuit in your lab book, showing components values, pin
connections and the configuration of the dual voltage supply.
Calculate the expected value of non-inverting gain and record your results in
your lab book.
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FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS
Set the output of the function generator to produce a bipolar sine wave output
of 300mV peak-to-peak, at 1 kHz. Connect the signal generator to the input of
your circuit and display your input and output waveforms on the oscilloscope
(using Channels 1 and 2). Measure the peak to peak voltages of VIN and VOUT
from the oscilloscope display and record your results in your lab book. Use
these values to calculate the actual gain of your circuits. Sketch VIN and VOUT
in your lab book, and note down their respective dimensions.
Turn off your circuit and replace the 8.2k resistor used for RF with a 150 k
resistor. Repeat the above procedure, recording your results in your lab book.
Increase the value of VIN to 700 mV. Observe the output on the oscilloscope
and sketch the waveform in your lab book. Explain what has happened to the
output and why. You may want to consider the DC power supply voltages
when considering an explanation for what you have observed.
2.2 OPERATION AS AN INVERITNG AMPLIFIER
Turn off your circuit and reconstruct it as an inverting amplifier, as shown in
Figure 1.4. To begin with, use RF = R1 = 8.2k. Reset your signal generator to
produce a 300 mV peak-to peak sinusoidal output and carry out the same
procedures as laid out in Section 2.1. Record the relevant circuit diagrams,
measurements, oscilloscope displays and calculations in your lab book.
3.0 COMMENTS AND CONCLUSIONS
Comment on the discrepancies (if any) between theory and practice. Also
identify where your waveforms illustrate the difference between inverting and
non-inverting configurations.
Finally, discuss the saturation phenomenon, and explain where your
waveforms show this for positive and negative signals.
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