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Constructing a +/- 10 Volt Reference

Introduction

When we experimented with a basic summing amplifier circuit in the previous experiment, we found it necessary to use a fixed reference voltage from which to develop our experimental inputs. Lacking any alternative, we used the +5 volt power supply for the purpose. Unfortunately, this was less than an ideal choice.

The +5 volt supply, while entirely satisfactory for powering digital ICs and circuits, is not particularly precise. In addition, it does not have a negative counterpart, so it is difficult to generate negative input signals. Yet another problem is that op amps of this class operate over a ±10 volt range, not merely ±5 volts. To enable us to provide input signals over the full ±10 volt range, we need accurate reference voltages of +10 volts and -10 volts, which we can then use in future experiments.

These reference sources need not be major power supplies, although they will be similar in design and operation. The amount of current to be drawn from the reference sources will not exceed the output capacity of a 741 op amp. We will construct such a reference source in this experiment.



Schematic Diagram

Schematic diagram of the +/-10 volt reference source.

The ±10 volt reference is based on the same +3 volt reference that we established earlier for op amp control of the ±12/15 volt power supplies. However, for the ±10 volt references, we won't use external circuitry such as a series pass transistor to control large amounts of load current. Instead, as shown in the schematic diagram to the right, we will simply use the outputs from the two operational amplifiers as our voltage references.

The most difficult part of designing this circuit is the need to find the proper resistor values to give us the voltage ratio we need for the positive reference. Since we are starting with a +3 volt reference and want a final output of +10 volts, the feedback voltage divider for the positive reference must drop 7 volts across the upper resistor (9.1K in the schematic) and 3 volts across the lower resistor (3.9K). This gives us a voltage ratio, and therefore a required resistance ratio, of 7/3 or 2.333. We must find a pair of resistors with legitimate 5% values that will maintain this ratio with each other.

Checking the allowable resistor values and doing a bit of aritmetic (or by using the resistance ratio calculator), we find that there are two possible pairs of resistor values we can use here: 56/24 and 91/39. Unfortunately, both the 24 and 91 values are conspicuously absent from the Radio Shack resistor package. However, either can readily be purchased separately for this experiment, at a reasonable price. We chose the ratio of 91/39, using 9.1K and 3.9K resistors, because the feedback resistor of 9.1K is then close to 10K, keeping similar magnitudes of current flowing through the feedback networks of both op amps in this circuit.

The -10 volt reference is simply a mirror of the +10 volt reference, so that the two references will be matched to each other for use in balanced applications. And of course, as always with op amp circuits, we can get better precision by using 1% or 0.1% tolerance resistors.



Parts List

To construct and test the circuit on your breadboard, you will need the following experimental parts:



Constructing the Circuit

This circuit will just fit in the remaining space on the left half of your breadboard socket. Make sure there are no experimental parts installed here, but that all power supply components from prior experiments are installed as described there. Then refer to the image and text below and install the parts as shown.



Circuit Assembly

Start assembly procedure




Starting the Assembly

The assembly diagram to the right shows the regulated and op-amp controlled ±12/15 volt power supply you installed earlier. The jumper designated "A" is Jumper A, which must be in place to set the power supplies to ±15 volts, or removed to set them to ±12 volts. You'll build the ±10 volt references in the remaining open space shown here.

Click on the `Start' button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart' button that will replace the `Start' button.

0.3" Black Jumper

Locate or construct a 0.3" black jumper, using the same methods you've employed in the past. Install this jumper on your breadboard socket, in the location indicated in the assembly diagram.

Click on the image of the jumper you just installed to continue.

0.5" Orange Jumper

Locate or construct a 0.5" orange jumper, and install this jumper in the location indicated to the right.

Again, click on the image of the jumper you just installed to continue.

0.5" Blue Jumper

Locate or construct a 0.5" blue jumper, and install it as indicated in the assembly diagram. Note that this jumper must be installed on a diagonal to reach an available contact point on the -12/15 volt bus.

As before, click on the image of the jumper you just installed to continue.

0.3" Yellow Jumper

Locate or construct a 0.3" yellow jumper and install it on your breadboard socket as indicated to the right.

As usual, click on the image of the jumper you just installed to continue.

0.4" Brown Jumper

Construct a 0.4" brown jumper and install it in the location indicated in the assembly diagram.

Once more, click on the image of the jumper you just installed to continue.

1458 Dual 741 Op Amp

Locate a 1458 (or 5558) dual 741 op amp IC. Make sure the pins are all straight, and align them with their contact holes on the breadboard socket as shown to the right, with the pin 1 indicator notch oriented to the left. Gently press the IC down until it is fully seated in place.

Click on the image of the IC you just installed to continue.

2.7K, ¼-Watt Resistor

Locate a 2.7K, ¼-watt resistor (red-violet-red) and form its leads to a spacing of 0.5". Clip the ends to a length of ¼" and install this resistor as shown in the assembly diagram.

Click on the image of the resistor you just installed to continue.

3.9K, ¼-Watt Resistor

Locate a 3.9K, ¼-watt resistor (orange-white-red) and form its leads to a spacing of 0.3". Clip the ends to a length of ¼" and install this resistor as indicated to the right.

Again, click on the image of the resistor you just installed to continue.

5.1K, ¼-Watt Resistor

Locate a 5.1K, ¼-watt resistor (green-brown-red) and form its leads to a spacing of 0.3". Clip the ends to a length of ¼" and install this resistor as shown in the assembly diagram.

As before, click on the image of the resistor you just installed to continue.

9.1K, ¼-Watt Resistor

Locate a 9.1K, ¼-watt resistor (white-brown-red). This resistor must be connected to adjacent contact columns on your breadboard socket, so form the leads as shown in the pictorial below. Then install this resistor on end, in the location indicated to the right.

Again, click on the image of the resistor you just installed to continue.

10K, ¼-Watt Resistor

Locate a 10K, ¼-watt resistor (brown-black-orange) and form its leads for 0.1" spacing as you did before. Clip the leads to ¼" below the body of the resistor, and install this resistor as indicated in the assembly diagram.

As before, click on the image of the resistor you just installed to continue.

10K, ¼-Watt Resistor

Locate a second 10K, ¼-watt resistor (brown-black-orange), and form the leads to a spacing of 0.5". This resistor must fit over the top of the IC you installed earlier, so clip the leads to ½" as shown in the pictorial, and then install the resistor in the location indicated to the right.

One more time, click on the image of the resistor you just installed to continue.

Assembly Complete

Since there are no more bus strips available on your breadboard socket, we'll use the contact columns immediately to the right of the 1458 IC as the reference sources for +10 and -10 volts, as shown now in the assembly diagram.

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

Restart assembly procedure
Continue assembly procedure


Performing the Experiment

Set your voltmeter to measure dc voltage in the range of ±20 volts. Connect the ground lead of your voltmeter to the ground bus strip on your breadboard socket (or to the grounded end of one of the power supply reservoir capacitors). Then, turn on power to your experimental circuit.

Verify that Jumper A is in position as shown in the assembly diagram above (just to the right of the trimpot controlling the +3 volt reference voltage), and that the main power supplies are correctly set to ±15 volts.

Measure the output voltage of the +10 volt reference source at the righthand end of the yellow jumper you installed in this experiment. Record your measured voltage in the upper left corner of the table to the right. Next, move the meter probe to the right-hand end of the brown jumper you installed in this experiment. This is the -10 volt reference. Measure this voltage and record it in the cell to the right of your first entry.

Remove Jumper A and set it aside for the moment. Verify that your power supply voltages have dropped to ±12 volts. Now measure the +10 volt reference output voltage again. Is it the same as before? Record this voltage in the lower left cell of the table to the right. Measure the -10 volt reference output voltage as well, and record this output voltage in the remaining cell of the table.

Does your ±10 volt reference circuit maintain correct output voltages whether the main power supplies are operating at ±12 volts or ±15 volts?

When you have completed your measurements and recorded your results, turn off the power to your voltmeter and experimental circuit, restore Jumper A to its place, and compare your results with the discussion below.

Power
Supply
Voltage
Reference Output
+10 V -10 V
±15 V
±12 V


Discussion

You should have found that your ±10 volt reference sources were accurate to within the tolerances of the resistors you used in this experiment. Our test circuit, using 5% resistors, had output voltages of ±10.30 volts, which constitutes a 3% error. Precision resistors would have reduced this error significantly.

If your reference output voltage was less than about ±10.5 volts, you should have found that it remained unchanged when you reduced the power supply voltages to ±12 by removing Jumper A. This worked because the 741 op amp can produce an output voltage as close as about 1.5 volts from the power supply before its output becomes saturated. Thus, even with ±12 volt power supplies, an accurate ±10 volt reference source of this type will remain stable. This is highly useful in some circumstances.

If your reference voltages were too high, or if they changed when you removed Jumper A, try other resistors of the same nominal values, to get your reference voltages to be as nearly correct as possible. If you can find higher-precision resistors for use in this circuit, by all means use them. The 9.1K and 3.9K resistors are the critical components for the +10 volt reference, while the two 10K resistors are critical for the -10 volt reference. The more precise these resistors are, the more accurate your reference voltages will be.

When you have completed this experiment, make sure power to your voltmeter and experimental circuit is turned off, and restore Jumper A to its place if you have not already done so.


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