ME 530.241: Electronics and Instrumentation
Lab 6: Op-Amp Applications
Louis L. Whitcomb∗
Department of Mechanical Engineering
The Johns Hopkins University
Spring 2010
Apparatus: Multimeter, DC Power Supply, Tektronix TDS220 oscilloscope and manual, Tektronix CFG 253 function generator and manual, Leads, Breadboard
Components: OPA544 Power Op-Amp, INA111 Instrumentation Amplifier, LF411 Op-amp,
resistors, thermistor
1
Pre-Lab Exercises
1. Exercise 8.19 of [1].
2. Exercise 8.44 of [1].
3. Read all the steps in this lab.
4. Draw the connections and components for the circuit shown in Figure 3 using the pin connection diagrams shown in Figure 2. Drawing this as clearly as possible will help you construct
the circuit in lab.
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Instrumentation Amplifiers and Thermistors
1. Thermistors exhibit a large negative temperature coefficient (NTC) or positive temperature
coefficient (PTC) where the resistance varies as a logarithmic function of temperature. The
thermistor you are using nominally exhibits a 1KΩ resistance at 25◦ C. With the thermistor at
room temperature, check its resistance with your voltmeter. Note how the resistance changes
when you touch the thermistor with your hand.
2. Analyze the circuit shown in Figure 1. Derive the formula for iout as a function of Vin and
R1 . Show your work. Note: this computation may be redudant with previous labs you have
done before; that is okay.
3. For the case of Vin =5V, determine the value of R1 required to result in iout = 1 mA.
4. Construct the circuit shown in Figure 1 using a provided 1K thermistor, an LF411 op-amp
(OPAMP-A in the figure), and an appropriate resistor. Note that you will need the following
FOUR voltage levels to construct your circuit:
• +15V
∗
c
2010
Louis L. Whitcomb, Kyle B. Reed, and Noah J. Cowan – do not reproduce without permission.
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Node 2
+
OPAMP-A
1K
Thermistor
Node 1
Figure 1: Thermistor Circuit
• +5V
• 0V (connect 0V to ground)
• -15V
Be careful with your circuit layout. Use the power distribution rails on the prototype board
to distribute these power supply connections.
5. Use your multimeter as an ammeter to verify that your circuit provides 1mA through the
thermistor. Measure and record the actual current. How does it compare to the theoretical
value?
6. Check your circuit.
(a) Use your oscilloscope to measure the voltage at Node 1, and also at the op-amp output
Node 2. Does this agree with your expectations?
(b) What is the voltage across the thermistor? Does this make sense based on what you
know about the thermistor specifications?
(c) What is the power (in Watts) being dissipated in the thermistor?
(d) What is the power (in Watts) being dissipated in the op-amp due to its the 1mA output
current? What assumptions do you have to make to complete that calculation?
7. Based on the specifications of the manufacturers spec sheet and your lecture notes, what is
the thermistor’s resistance as a function of temperature in degrees C? Be specific.
8. Plot this function, i.e. plot the curve of resistance (on the Y axis) as a function of temperature
(on the X axis) for the temperature range 0◦ C to 50◦ C.
9. At 25◦ C, what is the thermistor’s change of resistance per degree (C) of temperature change?
That is, what is the slope of the curve you obtained in the previous question at the point
25◦ C? Show your work. Check your signs.
2
LF411
INA111
Figure 2: Thermistor Circuit Physical Layout
3
+
Node 2
OPAMP-A
-
+
1K
Thermistor
IN111
INA111
Node 1
Figure 3: Thermistor circuit with instrumentation amplifier.
10. For the case of the thermistor being driven with a 1mA current, what is the voltage across
the thermistor as a function of temperature?
11. Plot this function, i.e. plot the curve of thermistor voltage (on the Y axis) as a function of
temperature (on the X axis) for the temperature range 0◦ C to 50◦ C
12. At 25◦ C, when driven by a 1mA current, what is the thermistor’s change of voltage as per
degree (C) of temperature change? That is, what is the slope of the curve you obtained in
the previous question at the point 25◦ C? Show your work. Check your signs.
13. Figure 3 shows an instrumentation amplifier connected to the current-driven thermistor to
measure the voltage drop across the thermistor. Using the INA111 spec sheets, what value
of gain resistor is required for a instrumentation amplifier gain of
(a) Gain = 1?
(b) Gain = 5?
(c) Gain = 10?
(d) Gain = 100?
14. Add the INA111 instrumentation amplifier to your circuit as shown in Figure 3, with a gain
of 10. Be sure to hook the Ref pin to ground.
15. With the thermistor at room temperature, measure and note the voltage across the thermistor,
and the voltage vout . Verify that your instrumentation amplifier is providing the desired gain.
16. Monitor vout with your oscilloscope, with the time-base set to 2.5 seconds/division or 5.0
seconds/division.
(a) What happens to vout when you hold the thermistor between your fingers?
(b) What happens to vout when you let go the thermistor between your fingers?
(c) Describe what is going on here quantitatively and qualitatively.
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Node 2
+
OPAMP-B
Pittman
Motor
Node 1
Figure 4: Current Amplifier
3
Power Op-amp Current Amplifier
You will use a pre wired op-amp board which contains an OPA544 op-amp, a 2Ω power resistor,
and a heat sink, as shown in Figure 3. Your TA will explain how to connect this op-amp board to
your power supplies, input signal, and load.
1. What is the formula for iout as a function of Vin and R1 . Show your work.
Configure your signal generator to generate a 0.4V peak-to-peak sin wave with 0V mean and
a frequency of about 1Hz. Connect this signal to Vin of the circuit shown in Figure 4.
• Connect CH 1 of your scope to Vin .
• Connect CH 2 of your scope to NODE 1.
• Connect CH 3 of your scope to NODE 2 (i.e. the op-amp output).
• Set the power supply current limit to 2.0 Amp.
Be careful not to overheat the op-amp - check the temperature frequently by touching the
heat sink. If it is too hot to comfortably grasp then shut it down as follows: (1) shut down
the signal generator, (2) set the power supply voltage to zero, and (3) give it a minute to cool
off. After it cools off, power up in the reverse sequence.
Use your scope to measure and print the input and output waveforms of this circuit.
Explain the plot. What is happening here? Be sure to discuss the relationship between the
voltage at NODE 1 and the current iout ?
How does the circuit’s actual behavior compare to the predicted behavior? Explain any
discrepancies.
2. Try several different waveforms (shape, offset, magnitude) from the signal generator. How
does the circuit’s actual behavior compare to the predicted behavior? Discuss any discrepancies.
As you are doing this, monitor the op-amp temperature - it can heat up very quickly. Be
careful not to overheat the op-amp.
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3. Consider the power consumption of the linear amplifier circuit in Figure 3. Lets look at two
representative cases:
Case A: Suppose the op amp was operating from +/- 15V supplies, with an input voltage,
Vin , of 1V, and an attached motor with resistance 10Ω. The motor shaft is stalled, i.e. it
is held fixed and cannot rotate. Under these conditions, the motor behaves as a resistor.
Calculate how much power is being dissipated in each of the following:
• The motor.
• The circuit’s 2 Ohm power resistor.
• The op-amp.
Case B: Now suppose that the supply voltages are increased to +/- 25V. Compute the power
consumption for the motor, 2 Ohm resistor, and op-amp. How do they compare?
Hint: to compute the power dissipated in the op-amp, remember that Kirchhoff’s Current Law
holds. Therefore, the current going out of the op-amp, iout , must equal the current being supplied
by the ±15V or ±25V supply.
REMEMBER
• Note your lab partner’s secret code on your lab report as well as your lab station number.
• Return all electronic test equipment and components to their proper location.
• Remember to show your work.
• Typewritten reports are not required, but messy or disorganized reports are unacceptable.
• Clean up your workstation to perfection when you are done.
• Get your station checked off by your TA.
References
[1] Giorgio Rizzoni. Fundamentals of Electrical Engineering. McGraw Hill, New York, 2009.
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ME 530.241 Lab 6: Op-Amp Applications
My Secret Code
My Partner’s Secret Code
Lab Date
Lab Station Number
Today’s Date
:
:
:
:
:
You must have the T.A. inspect your lab station at the end of your lab session.
T.A. Signature and Date
:
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