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					Instrumentation Lab
Lab 14: Operational Amplifiers II




                         Purpose of the experiment

             •   Apply the op amp to a real world problem - measuring temperature.
             •   To use an op amp with a transistor to amplify voltage and current.




                                  Operational Amplifiers II - 1
Background Information
As we have seen in Lab 13, an operational amplifier can be used to amplify a voltage. The purpose
of this lab is to see how an op amp can be used with a transducer. A transducer is a device that
takes a physical quantity (force, distance, temperature, magnetic field, etc.) and converts it to a
voltage. The op amp can then be used to amplify that voltage into a readable value. In this lab, we
will limit ourselves to the relationship between op amps and transducers while excluding the
computer. In the next labs, we will be reading these voltages with the computer.


Thermocouples
An interesting and useful property of metals is that a voltage is produced between two junctions of
two dissimilar metals. More importantly, the voltage produced is a function of the temperature of
the junctions. Thermocouples are devices that use this effect to measure temperature with far better
results than a thermistor. The type of metals used in the construction determines the temperature
range and accuracy of the thermocouple. For this lab, we will be using what is called a "K-type"
thermocouple. This thermocouple is made up of nickel-chromium and nickel-aluminum wires and
follows this equation:
                                                ⎛ µV ⎞
                                   V JUNCTION = ⎜ 45 O ⎟(T1 − T2 )
                                                ⎝     C⎠




                                                       Figure A: Thermocouples in series.


VJUNCTION is the voltage produced across both thermocouples in series, and T1 and T2 are the
temperatures of the two thermocouple junctions.


Null Offset




                                       Figure B: Circuit for using the LM741’s null offset.


                                   Operational Amplifiers II - 2
All circuits have small uncertainties. Most op amps are equipped with the ability to correct for a
zero reading offset (i.e. the circuit is not reading zero when you want it to and you want to correct
that problem). The adjustment is called null offset. The adjustment circuit for the LM 741 is shown
above in figure B.


Prelab
Consider the following voltage and current amplifier:




                              Figure C: Voltage and current amplifier.
Analyze the circuit to find VOUT in terms of VIN. Why is the maximum voltage across RL only
about 7.5 V – 0.7 V = 6.8 V?


The Lab

The goal: To apply your knowledge of op amp circuits to a practical physical measurement of
temperature, and to see how to amplify current and voltage simultaneously.
Exercise 14-1: Set up the circuit shown in figure D. Then:
a.)    Calculate the approximate gain needed for an output voltage of 10V at 1000 oC.
b.)     Next, set the zero reference point. Place both thermocouples in the ice bath. Adjust the
offset to zero. Now all temperature readings will be referenced to
0 oC. Remove junction 1 from the bath and leave junction 2 as the zero reference.
c.)    Now, calibrate the setup with a second temperature by placing junction 1 into boiling water.
Adjust the gain until the output reads 1.0 volts. You now have a thermometer that is calibrated to
the Celsius temperature scale. 10 mV/oC.
d.)    Test your thermometer on various temperatures: flame, room temperature, liquid nitrogen (if
available), etc.


                                    Operational Amplifiers II - 3
                   Figure D: Circuit to amplify the voltage from a thermocouple.


Exercise 14-2: The following circuit contains both an op amp and a transistor to amplify both
voltage and current.




   Figure D: Construct and test the circuit with RL = 100 Ω. This amplifier should be capable of
                          delivering 6.8 V/100 Ω = 68 mA (see Prelab).




                                   Operational Amplifiers II - 4

				
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posted:9/16/2011
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