Download Lab 2: Op-Amp Parameters

UNIVERSITI MALAYSIA PERLIS
ANALOG ELECTRONICS II
EMT 212 2009/2010
EXPERIMENT # 2
OP-AMP (PARAMETERS)
MARKS
T1
T2
G1
T3
G2
T4
T5
Q
C
Total
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3
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NAME
signature
MATRIK #
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PROGRAMME
GROUP
DATE
Analog Electronics II (EMT212) 2009/2010
Laboratory Module
EXPERIMENT 2
Op-Amp (Parameters)
1.
OBJECTIVE:
1.1
To investigate the bandwidth of an operational-amplifier as a function
of gain.
1.2
To determine the slew rate of an operational-amplifier.
1.3
To investigate the output offset voltage at the output of an operationalamplifier.
2.
INTRODUCTION:
The operational amplifier has a few limitations. This experiment will
investigate the operational-amplifier’s bandwidth as a function of gain, the slew rate
of the operational-amplifier, and output offset voltages. Unlike a real operationalamplifier, the ideal operational amplifier has an infinite voltage gain, perfectly
matched internal transistors, and no output offset voltage.
The bandwidth of an operational-amplifier is inversely proportional to the closed-loop
gain of the amplifier. The following equation shows the relationship between
bandwidth and the feedback ratio,
 : fT 
where
BWCL

(2.1)
fT is the gain-bandwidth product or unity-gain frequency
BWCL is the closed-loop bandwidth of the amplifier
 is the feedback ratio:

R1
R1  RF
(2.2)
Another factor that limits the high frequency response of an operational-amplifier is
its maximum permissible slew rate. The maximum slew rate is the maximum value of:
S
where
V
t
(2.3)
V is a change in output voltage
t is the time interval over which the output voltage changes
The slew rate limits the high frequency response because at high frequencies there
is a large rate-of-change of voltage. The maximum sinusoidal frequency at which an
operational-amplifier having slew rate S can be operated without producing distortion
is:
f S (max) 
S
2K
where
fS (max) is the maximum frequency imposed by the slew rate limitation.
K is the peak value of the output waveform.
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(2.4)
Analog Electronics II (EMT212) 2009/2010
Laboratory Module
Output offset voltage is the dc voltage that appears at the output when both inputs
are zero volts. The output offset voltage of an operational-amplifier is caused by input
offset voltage, due to slightly mismatched transistors in the differential-amplifier input
stage, and differences in input bias currents, I- and I+. The output offset voltage due
to mismatched bias currents I+ and I- can be reduced by connecting a compensating
resistor, RC, in series with the non-inverting input. This resistor does not affect the
closed-loop gain of the amplifier. The optimum value of RC is :
(2.5)
RC  R1 RF
When this compensating resistor is used, the magnitude of the output offset voltage
due to input offset current is:
(2.6)
V os  ( I   I  ) R F  I io R F
where
Vos is the magnitude of the output offset voltage
I
I
I io
is the input bias current at the inverting terminal
is the input bias current at the non-inverting terminal
is the input offset current
Note that Vos may be either positive or negative, depending on which of I+ and I- is
the larger.
The 741 operational-amplifier has externally-accessible terminals that can be used to
null, or balance, the amplifier, i.e. to adjust the output offset to zero when the inputs
are zero. A potentiometer is connected across pins 1 and 5 for this purpose, as
shown in Figure 4.5.
3.
COMPONENTS & EQUIPMENTS:
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Resistors:
3.1.1 1 M (2)
3.1.2 470 k
3.1.3 100 k
3.1.4 47 k
3.1.5 10 k (2)
Variable Resistor :
3.2.1 100 k
LM 741 OP-AMP
DC Power Supply
Function Generator
Oscilloscope
Breadboard
Digital multimeter
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Analog Electronics II (EMT212) 2009/2010
4.
Laboratory Module
PROCEDURE:
4.1
To measure the unity-gain frequency of the operational-amplifier,
connect the following circuit as shown in Figure 4.1. Apply a 100 Hz,
50 mVpk sinewave input signal to the circuit.
+15V
_
7
2
741
3
+
6
4
-15V
Vout
Vs
GND
Figure 4.1 Unity-Gain Frequency Measuring Circuit
4.2
Using a dual-trace oscilloscope observe Vs and Vout. Increase the
frequency of the signal generator, and record the frequency where Vout
decreases to 0.707 times its value at 100 Hz. This frequency is the
unity-gain frequency of the amplifier, or its gain-bandwidth product.
Complete TABLE 1.
4.3
To demonstrate that the gain-bandwidth product is constant, connect
the following circuit as shown in Figure 4.2.
RF
+15V
10 k
_
7
2
741
3
Vs
+
6
4
-15V
Vout
GND
Figure 4.2 Gain-Bandwidth Product Measuring Circuit
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Analog Electronics II (EMT212) 2009/2010
4.4
4.5
Laboratory Module
Apply a 100 Hz, 50 mVpk sinewave input signal to the circuit. With RF
= 47 k, measure and record the output voltage, Vout and complete
TABLE 2. Increase the frequency of the signal generator until the
output voltage decreases to 0.707 times its value at 100 Hz. Remove
the signal generator and make sure its output voltage has not
changed. Record this frequency in TABLE 2. Repeat this procedure
with RF = 100 k.
To measure the slew rate of the operational-amplifier, connect the
circuit in Figure 4.3. Apply a 1 kHz, 1Vpk square-wave input signal to
the circuit.
100 k
+15V
10 k
_
7
2
741
Vs
3
+
6
4
-15V
Vout
GND
Figure 4.3 Slew Rate Measuring Circuit
4.6
With a dual-trace oscilloscope, measure the slew rate in the following
manner. Adjust the time-base of the oscilloscope so that only one
changing edge of the output waveform Vout can be viewed (either a
low-to-high voltage change or a high-to-low change). Expand the time
base on the oscilloscope so that the change in time t can be
observed. Record this waveform in GRAPH 1. Then measure both the
change in voltage V and the change in time t. Use these values to
calculate the slew rate S 
4.7
V
. Record the result in TABLE 3.
t
Calculate the maximum frequency imposed by the slew rate using the
equation in the Introduction part. Now, apply a 1 kHz, 10Vpk sinewave
input signal and use a 10 k resistor for RF. Increase the signal
generator frequency beyond this calculated maximum frequency and
note the changes in the waveform of Vout. Record this waveform in
GRAPH 2.
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Analog Electronics II (EMT212) 2009/2010
4.8
Laboratory Module
To measure the total output offset voltage of the operational amplifier,
connect the following circuit as shown in Figure 4.4(a).
1 M
+15V
1 M
_
7
2
741
3
+
6
4
Vout
-15V
GND
Figure 4.4(a) Output Offset Voltage Measuring Circuit
4.9
Using a digital voltmeter, measure and record the dc output voltage Vout in
TABLE 4.
4.10
Now replace the short-circuit to ground on the non-inverting terminal with a
470 k resistor to ground as shown in Figure 4.4(b). Repeat procedure step
4.9.
1 M
+15V
1 M
_
7
2
741
3
470 k
+
6
4
-15V
Vout
GND
Figure 4.4(b) Total Output Offset Voltage Measuring Circuit
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Analog Electronics II (EMT212) 2009/2010
Laboratory Module
4.11
To demonstrate how a 741 amplifier can be balanced, connect a
potentiometer as shown in Figure 4.5.
4.12
While measuring Vout with a digital voltmeter, adjust the 100 k potentiometer
until the output offset voltage is as close to 0 V as possible. Measure and
record this total output offset voltage in TABLE 5.
1 M
+15V
1 M
_
7
2
741
3
5
+
4
1
470 k
6
-15V
Vout
100 k
RC
GND
-15V
Figure 4.5 Balancing The Total Output Offset Voltage Circuit
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Analog Electronics II (EMT212) 2009/2010
Laboratory Module
Name
:_____________________________
Matric No.
: _______________________________
5.
Date : _______________
RESULTS:
TABLE 1
Voltage
(calculated)
0.707Vout
Frequency
(2 marks)
TABLE 2
RF
Vout
Voltage
(calculated) Frequency
0.707Vout
47 k
100 k
(6 marks)
GRAPH 1
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(1 mark)
Analog Electronics II (EMT212) 2009/2010
Laboratory Module
Name
:_____________________________
Matric No.
: _______________________________
Date : _______________
TABLE 3
V (Volt)
t (sec)
S
(3 marks)
GRAPH 2
(1 mark)
TABLE 4
Vout (V)
Without 470 k
With 470 k
(2 marks)
TABLE 5
Vos (V)
(1 mark)
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Analog Electronics II (EMT212) 2009/2010
Laboratory Module
Name
:_____________________________
Matric No.
: _______________________________
6.
Date : _______________
QUESTION:
mark
Q1
Using the results of procedure steps 4.2 – 4.4, verify that the gain-bandwidth product is
constant. Compare this constant to the manufacturer’s specifications.
A1
(3)
A2
Compare the slew rate calculated in procedure step 4.6 with the manufacturer’s
specifications. Using the results of procedure step 4.7, describe the effects that exceeding
the maximum frequency that the slew rate imposed had on the output waveform Vout.
Q3
Which is better op-amp when it comes to slew rate?
Q2
(3)
(2)
A3
Q4
A4
Compare the total output offset voltage measured in procedure step 4.9 with the
manufacturer’s specifications. How well did the modification of procedure step 4.9
decrease the total output offset voltage? Explain why.
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(3)
Analog Electronics II (EMT212) 2009/2010
Laboratory Module
Name
:_____________________________
Matric No.
: _______________________________
7.
Date : _______________
CONCLUSION: (6 marks)
Based on your measurement data and graph, make an overall conclusion by
referring to the objective of this experiment in terms of:
1. Bandwidth and gain?
2. The relationship between slew rate and frequency?
3. The effect of output offset voltage of an op-amp?
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