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Summing Amplifier

Introduction

One of the most common applications for an op amp is to algebraically add two (or more) signals or voltages to form the sum of those signals. Such a circuit is known as a summing amplifier, or just as a summer.

The source of these signals might be anything at all. Common input sources are another op amp, some kind of sensor circuit, or an initial constant value. Since we don't have the first two available at this time, we'll use the third source for this experiment.

The point of using an op amp to add multiple input signals is to avoid interaction between them, so that any change in one input voltage will not have any effect on the other input. In this experiment, we will determine whether or not this is the case, and whether the op amp will correctly generate an output voltage that reflects the arithmetic sum of the input voltages.



Schematic Diagram

A two-input analog summing amplifier.

The schematic diagram for our experimental circuit is shown to the right. We will use the +5 volt power supply as our initial reference so that the sum of the two input voltages will not exceed the output range of our op amp. The circuit itself is a very basic op amp summing circuit, with two independent inputs.

Since the two input resistors and the feedback resistor are all 10K, the input voltages will be inverted as they are added, but neither input will be increased or decreased in magnitude; they will be faithfully reproduced. The 10K potentiometers don't figure into the calculations; they'll be used only to set various input voltages applied to the input resistors. As a result, resistive loading and non-linear pot characteristics don't matter. You'll be setting the potentiometers to produce specific output voltages under these loaded conditions, and therefore directly compensating for any such errors.

For this experiment, you won't really need to set precise voltages, although you will need to accurately measure the input and output voltages you use. Therefore, you can use standard, 1-turn, 10K potentiometers here if you wish. Small 1-turn trimpots are available that can be mounted on a breadboard socket without doing any damage. Or, you can simply use two of the same sort of 15-turn trimpot you have used in previous projects and experiments. Our assembly diagrams will assume these 15-turn trimpots, so that you won't have to purchase additional components that may never again be used.



Parts List

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



Constructing the Circuit

Select an area on your breadboard socket that is clear of other circuits. You'll need most of the space displayed in the assembly diagram below for this project. Then refer to the image and text below and install the parts as shown.



Circuit Assembly

Start assembly procedure















Starting the Assembly

This project can be placed anywhere on the right side of your breadboard socket. We selected the right end simply for convenience, but you can move it if you like.

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 a leftover 0.3" black jumper, or else make a new one using the same method you've used in past experiments. Install this jumper on your breadboard socket, in the location indicated in the assembly diagram to the right.

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

0.3" Black Jumper

Locate or make a second 0.3" black jumper, and install it in the location indicated in the assembly diagram.

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

0.3" Red Jumper

Locate or make a 0.3" red jumper, using the usual method. Install this jumper as indicated to the right.

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

0.3" Red Jumper

Locate or make a second 0.3" red 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.

0.5" Orange Jumper

Locate a 0.5" orange jumper left over from previous experiments, or else create a new one in the usual manner. Place this jumper in the location indicated to the right.

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

0.5" Blue Jumper

Locate or prepare a 0.5" jumper, and install this jumper in the location shown in the assembly diagram.

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

0.2" Bare Jumper

Locate a 0.2" bare jumper, or else make a new one by removing insulation from one end of a length of hookup wire and then forming the exposed end to the required dimensions. Install this jumper as indicated to the right.

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

0.1" Bare Jumper

Locate a 0.1" bare jumper, or else form one from either hookup wire or a clipped component lead. Then, install this jumper in the location indicated in the assembly diagram.

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

741 IC Op Amp

Locate a type 741 IC and carefully install it in the location indicated to the right. Make sure that all eight pins are inserted properly into the breadboard socket, and that none of them are bent or folded up under the body of the IC. Also make sure the notch at one end of the IC package is oriented to the left, so that pin 1 will be the bottom left pin on the installed IC.

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

10K Trimpot

Locate a 10K, 15-turn trimpot (you can substitute a different type if you wish), and install it as shown in the assembly diagram. Be sure that the three pins that form the electrical connections fit into the specific breadboard socket connections indicated by the gold squares in the diagram. Each pin will connect to a contact column that already has a jumper in place.

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

10K Trimpot

Locate a second 10K, 15-turn trimpot (again, substitute if you wish), and install it as shown to the right. As before, this trimpot should make electrical contact with three separate jumpers already installed.

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

3.3K, ¼-Watt Resistor

Locate a 3.3K, ¼-watt resistor (orange-orange-red) and form its leads to a spacing of 0.4". Then clip the leads to a length of ¼", and install this resistor in the location shown in the assembly diagram.

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

10K, ¼-Watt Resistor

Locate a 10K, ¼-watt resistor. Normally, this should be a 1% resistor (color code brown-black-black-red), but can be a 5% resistor (brown-black-orange) if you don't have 1% resistors available.

If you don't already have such a resistor formed to a spacing of 0.5", select a new resistor and form its leads top this spacing. Then clip the leads to a length of ¼" and install this resistor in the location shown to the right.

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

10K, ¼-Watt Resistor

Locate a second 10K, ¼-watt resistor. Again, a 1% resistor (brown-black-black-red) is preferred, but a 5% resistor (brown-black-orange) may be used instead. Form its leads, if necessary, to a spacing of 0.5", then clip the ends to a length of ¼". Install this resistor in the location indicated in the assembly diagram.

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

10K, ¼-Watt Resistor

Locate one more 10K, ¼-watt, 1% resistor (brown-black-black-red), or substitute a 5% resistor (brown-black-orange) for it as before. Form its leads for a spacing of 0.5". This time, however, clip the ends to a length of ½" to hold the body of this resistor up from the surface of the breadboard socket. Install this resistor diagonally across the 741 IC, from pin 2 to pin 6, as shown to the right.

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

Assembly Complete

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

Turn on power to your experimental circuit, and set your voltmeter to measure voltages in the ±20 volt range. Connect its common lead to ground on your breadboard socket. Now connect the other voltmeter lead to the bare jumper that is partly under the lefthand trimpot. This is Input A for this experiment; adjust the trimpot so that Input A is at 0.00 volts.

Next, connect your voltmeter input to the bare wire just below the righthand trimpot. This will be Input B. Adjust Input B for an input voltage of 0.00 volts. Now, move your voltmeter probe to pin 6 of the 741 (the upper end of the last 10K resistor you installed) and measure the output voltage at this point. Record this voltage in the upper left corner of the table below, in the cell corresponding to 0 volts for both inputs.

Next, set Input A to +1.00 volt. Again, measure the output voltage and record this value in the next cell to the right, in the table. Continue to set Input A to 2.00 volts, 3.00 volts, 4.00 volts, and 5.00 volts in turn. If your power supply won't quite reach 5.00 volts, set the input as high as you can. In each case, record the output voltage in the appropriate cell of the result table.

Now set Input B to +1.00 volts and record the measured output voltage in the rightmost cell of the second row of the table. Reduce Input A volt by volt and record the output voltage at each step. Continue in this way until you have recorded the output voltage you measure for each combination of inputs.

When you have recorded your results, turn off the power to your experimental circuit and compare your results with the discussion below.

VOUT Input A
0 1 2 3 4 5
I
n
p
u
t
 
B
0
1
2
3
4
5


Discussion

You should have found that the output voltage for any combination of inputs was equal to the sum of the two input voltages, A and B, to within the tolerances of the 10K resistors you used in this experiment. In addition, as you would expect from this circuit, the output voltage polarity was negative, showing that this is indeed an inverting amplifier.

If you used the recommended 1% resistors, the maximum error for each input was 1.01/.99 = 1.020202 on the high side, or .99/1.01 = 0.98019802 on the low side. Our test circuit produced an output voltage of -4.03 volts when both inputs A and B were +2.00 volts, corresponding to an overall gain of 1.0075, or 0.75% high. This is well within the tolerances of 1% resistors and is therefore an accurate and satisfactory result.

If you used 5% resistors, your total error could have been anywhere within about 10.5% of the expected output voltage: from .95/1.05 = 0.9047619 to 1.05/.95 = 1.1052632.

Also, although mathematically 2 + 2 = 3 + 1 = 1 + 3, you are not likely to get precisely the same results from this circuit. The test circuit gave the following results, with Input A listed first, then Input B:

In the last case, the volmeter kept shifting back and forth between -4.02 and -4.03, indicating an actual reading between these two. Thus, we see that the two input resistors were very similar but not identical, and the feedback resistor was a bit higher in value than either input resistor. Your results should have been within the tolerance ranges indicated above, and similar to ours.

Note that in order to get more precise results, you would have to use resistors of even closer tolerances, which makes them much more expensive than the resistors we're using in these experiments. To get even greater precision, it is necessary to use digital circuitry rather than analog circuitry. This is the essential distinction between these two types of circuitry.

When you have completed this experiment, make sure power to your experimental circuit is turned off. Remove all experimental components from your breadboard socket and put them away for use in future experiments.


Prev: Balancing the Input Offset Next: Constructing a +/- 10 Volt Reference

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