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Date ______________
Laboratory
5
Operational Amplifier (II)
Preparation
/ Reading:
Text: Chapter 5.
(or reference [1]: Chapter 4)
Appendix 5
Lecture notes.
Questions marked (*) in this lab script.
OBJECTIVE
To study and compare some of the real operational amplifier
properties to their ideal counterparts. In addition, three very useful
OP-Amp circuits, Differential amplifier; Active integrator, and Active
rectifier will be examined.
EQUIPMENT REQUIRED
Oscilloscope (Tektronix TDS210, dual channel, 60 MHz)
Function generator (BK Precision 4011A, sine, triangle, square, logic
level; to 5 MHz)
Breadboard (PB-503, powered, with a built-in function generator)
Digital multimeter (DMM) (FLUKE 187)
LM741CN op amp
LF411CN op amp
1N914 diodes (2)
0.01 uF capacitor
10 M resistor
1 k resistor (2)
10 k resistor (2)
100 k resistor (3)
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Introduction:
The Op-Amp Difference Amplifier Circuit
In an earlier experiment you studied the inverting and non-inverting operational amplifier
circuits. It is natural to ask if the two circuit types could be combined to perform subtraction or to
find the difference between two voltages. A method of obtaining the difference between two
voltages using one amplifier uses the circuit illustrated in Fig 5.1.
Fig. 5.1
The expression relating input and output voltages for this amplifier may be found from first
principles as was done earlier. However, we can save some work by making use of expressions
derived previously and using the principle of superposition. First assume V2 = 0, then Vo1 = -(Rf
/R1) V1). Now assume v1 = 0. In this case, the voltage at the non-inverting input is
(1)
The output voltage is related to the voltage at the non-inverting input by
(2)
Combining equations 1 and 2 gives
(3)
Now, if both v1 and v2 are present, then applying superposition
(4)
True differential operation results when we require that Vo = 0, when V1 = V2. It is easily shown
that this requires R1R3 = R2Rf. It is easy to understand that, if this requirement is not satisfied
exactly, the output will not be zero even when both inputs are equal. If the stated condition is
met, then the differential voltage gain becomes
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(5)
The quality of the difference amplifier is expressed in terms of a quantity called the Common
Mode Rejection Ratio (CMRR). The CMMR of a differential amplifier is defined by the ratio
where the common mode gain is given by
Lab 5-1 studies the differential voltage gain and CMRR of a differential amplifier. As in
earlier experiments, no particular effort has been made to match components. In practice (but not
in this lab), with careful component selection, a CMRR of 120 dB is achievable.
-------------------------------------------------------------------------------------------------------------------
Lab 5-1: Some properties of a real differential amplifier --- To examine the differential
voltage gain and CMRR of a differential amplifier.
1. Connect the gain-10 differential amplifier shown in Fig. 5.2. The pin identification for the op
amp can be found in the data sheet in Appendix 5.
Fig. 5.2
2. Measure the gain (Vo/V2) when V1 = 0, at 10 Hz, 100 Hz, 1 kHz, and 10 kHz. (Note the gain
measurements should include the phase as well). What is the gain at 1kHz? Why is the gain
lower than expected at 10 kHz?
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3. Now measure Vo/V1 when V2=0 at 1 kHz. Be sure to observe and record the phase of the
output as compared to the input.
4. Measure the common mode (V1=V2) voltage gain and phase, describe your method and
results.
5. Calculate CMRR as differential gain/common mode gain.
6. Compare your measurement with that given on the specification sheet. Can you account for
the poorer-than-expected CMRR?
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7. With V1=V2=0 volts, measure the DC output voltage. Determine the input offset voltage of your
amplifier. Compare your result to that given in the data sheet.
Lab 5-2: Active Integrator
* 1. Construct the active integrator shown in Fig. 5.3. The feedback resistor does no appreciable
harm above some low frequency. From the component values given in the circuit, estimate the
low frequency limit.
Fig. 5.3
* 2. Apply a square wave of 2 Vp-p at 500 Hz to the input. Predict the peak-to-peak triangle wave
amplitude at the output. Then try the experiment, record the input and output waveforms below.
(This circuit is sensitive to small DC offsets of the input waveform (its gain at DC is 100). If the output appears
to go into saturation near the 15 V supplies, you may have to adjust the function generator’s OFFSET control.)
500 Hz
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* 3. What is the function of the 10 Meg resistor? What would happen if you were to remove it? Try
it, and discuss your result.
(Now have some fun playing with the function generator’s DC offset, the circuit will give you a real feeling for the
meaning of an integral).
Lab 5-3: Active Rectifier
1. Construct the active rectifier shown in Fig. 5.4.
Fig. 5.4
2. Apply a relatively slow sine wave, say 50 kHz. Examine carefully the input and the output
waveforms on the scope. (Note that the output of the circuit is not taken at the output of the opamp.) Is there a “glitch”? Sketch the waveforms.
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3. What causes the “glitch”? Use the scope to look at both input and the op-amp output, sketch
the waveforms, then explain.
4. What happens at higher input frequencies? Explain.
Lab 5-4: Improved Active Rectifier
1. Construct an improved circuit shown in Fig. 5.5.
Fig. 5.5
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2. Redo previous lab exercise, i.e., apply a sine wave at 100 Hz. Examine carefully the input and
the output waveforms on the scope. Then examine carefully the input and the op-amp output
waveforms on the scope. Write down your observations.
* 3. The glitch should be much diminished. Explain how the circuit works for the improved
performance.
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------------------------------------------------------------------------------------------------Note : The “Instrumentation Amplifier”
The difference amplifier discussed above in Lab 5-1 is unsuitable for many applications because of
its low input impedance which is determined by the input resistors themselves. The instrumentation amplifier
shown below is a clever interconnection of input buffer amplifiers and a difference amplifier. The circuit has
the advantages of high input impedance at the two inputs and the large common mode rejection of the
difference amplifier.
Instrumentation amplifiers are available in the form of integrated circuit modules. The AD620 from
Analog Devices (http://www.analog.com/) is such an integrated circuit. This device is characterized by
having low noise, high gain accuracy, a low gain temperature coefficient and high linearity.
To illustrate the use of an instrumentation amplifier, an example is given in APPENDIX 8 for
building a very simple electrocardiograph. The basic design is from an article in Scientific American June
2000. Instead of the AD620 a more sophisticated device, AD624 (see APPENDIX 7) which allows the
addition of a third terminal is used.
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