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Building and testing the simple amplifier Op-AMP Circuits

The Op-Amp Circuits under test.

We have talked a lot about Op-Amps and DC electricity. It is now time to build something and test it. Hopefully no smoke is released! Before talking about the circuits I have realized I did not describe how to use the test board I recommended so it is time to back up and do that.

This is all written with the assumption that you have been following all the previous posts.  If you are just starting, I recommend you click on the category “Electricity” and read the previous posts on this subject.

Internal connections on the test board.

The test board is designed so a DIP packaged integrated circuit (I.C.) can be inserted so it straddles the center gap area of the board and and several connectors are dedicated to each pin.  I attempt to show some of the internal connections by drawing lines on a picture of the board.  In the central area of the board each line of 5 socket holes is wired together.  On each side of the board there is  2 rows of sockets all wired together.  However , note there is a break in these connections in the center of each row.  I have installed a jumper to continue each row.  The rows are used for common connection points for the +Vcc, -Vcc and 0 Volt connections.  (Some people call the 0 Volt connection ground but it may or may not be connected to earth ground so I prefer to call it 0 Volt, or Reference connection.)

Adjusting the pins on an I.C.

Before inserting an I.C. into the connector it is necessary to make the pins perpendicular to the body of the I.C. package so they will fit into the socket holes correctly.  Not doing this can end up with bent and possibly broken pins.  I do this straightening by placing all the pins against a flat surface and then bending them all at one time by moving the I.C. body perpendicular to the table.

Two I.C.’s inserted into the board and ready for wiring.

When the I.C.’s are inserted into the board it will look like I have shown in the picture to the right.

The next thing I need to talk about is the size code on resistors.  Since resistors are very small devices, it would be impossible to write the resistance value on the body of the resistor in a readable way.  A series of colored bands hare painted on the resistors and a color code has been developed  to describe the size of the resistors.

Resistor Color Code.

One of the colored bands will be closer to one end of the resistor. This is the first band and will represents the first digit based upon the color code shown to the left.  The 3rd band represents a multiplier and the 4th band will represent the tolerance of the resistor.  For example, if a resistor is red, violet, orange, gold, the resistor will be 27,000 ohms with a tolerance of +/- 5%.

This brings us to 2 more important tools, especially if you are older like I am.  A magnifying glass and a flashlight are very helpful.  This is especially the case when reading  numbers printed on the I.C’s and some other components.  It is also helpful when determining the actual color of the bands.

The actual wired circuit.

After all of this is understood, it is time to do the actual wiring.  It is sometimes helpful to put the pin number for the connections to the I.C. on the schematic drawing as I have done in the first picture and the header picture for this post. Once all the wiring and parts are inserted in the test board it will look similar to my example.  It is possible to make neater layouts than what I did, but I knew I would be changing connections throughout this testing so I did not take the time to make it pretty.

Resistor Values used for the Actual Op-Amp Circuit.

I used the values shown to the left to set the gain and load on the Op-Amp circuits. These values were also chosen to be “nice” values that worked close to the ideal Op-AMP calculated values We will be creating problems for ourselves soon enough.

Voltage Divider used to obtain Vin from the +Vcc or -Vcc power.

To obtain a Vin, I used a simple Voltage divider circuit as shown in the picture to the right.  We will be adjusting Vin by changing the value of R2.  Vcc in this picture can be either +Vcc or -Vcc.

Invertng Amplifier Test Values.





Both circuits were tested with both negative and positive values. You will notice that at smaller Vin values, the ideal calculated values are reasonably close to the measured values.  However, the larger values “maxed out” the op-Amp.  this is called “saturating” the amplifier.  It reached a value close to the + or – Vcc value.  It could not go all the way to the Vcc value because the electronics within the I.C, consumed some of this voltage.

Non-Inverting Amp test values.

This is the first of many limits and problems we will have to work around when using the circuit.  These limits were shown on the graphs of the datasheet. (“Positive output voltage swing versus load resistance” and “Negative output voltage swing versus load resistance” graphs.)

In the next post on electronics we will create more problems for ourselves and  learn of new limitations and how to predict those based upon the datasheet inormation

As you go through all of this it would be good to get a pencil and paper and a calculator and see if you can determine how I did calculations and why the actual values did not match the calculations.   Remember, the resistors are /- 5% tolerance resistors.

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