Introduction: Inverting Amplifier Circuit
This instructable will show you step by step how to build an inverting amplifier circuit. To do this I will use a very common Operational Amplifier (or Op Amp for short) the UA741CD. The main purpose of an inverting amplifier circuit is to take an input signal and increase it by the gain value that you have chosen as well as to shift the signal by 180 degrees with respect to the input signal. This circuit takes about 10-15 minutes to build and test and it requires a beginner level experience to build this circuit. The inverting and non-inverting amplifier circuits are the basis for a wide variety of circuit and they are fun to build.
Step 1: Figuring the Gain of Your Circuit
Before building your inverting circuit you must first decide on the value of the gain that you wish to amplify your input signal by. Since the equation for the gain of this circuit is very simple you can obtain your desired gain value by setting values for Rf and Rin. Once you have chosen values for these resistors you can multiply the gain value by the amplitude of your input signal and obtain the value of Vout from the amplified circuit.
Example: If Rf = 100,000 ohms and Rin = 10,000 ohms then the values of the gain would be: gain = -Rf/Rin = -(100,000)/10,000 = -10 With an input signal with an amplitude of 1V then the output voltage would be: Vout = Vin*gain = 1*(-10) = -10V
Notice that the output voltage is negative, this is because we are inverting the signal of the input by 180 degrees out of phase as stated before.
Caution: You cannot amplify the output voltage to be anything you want. The value of Vout will be restricted by the biasing of the op amp that will be discussed Step 4.
Step 2: Gather Materials
There are a variety of materials that can be used to create an inverting amplifier but these are the ones that you will use for the experiment:
- 1x 100,000 ohm resistor (to be Rf)
- 1x 10,000 ohm resistor (to be Rin)
- 1x UA741CD op amp
- 1x wire
- 1x breadboard
- 5x red connector cords
- 4x black connector cords
- 1x Tetratonix AFG3012B Function Generator
- 1x Agilent E3631A Power Supply
- 1x Agilent InfiniiVision DSO-X 2024A Oscilloscope
Note: The Equipment pictures will be shown with their corresponding steps.
Step 3: Building the Circuit
Tip: Use the beveled edge of the op amp as the top.
- Place UA741CD op amp in the middle of the breadboard so that it has its pins inserted on both halves of the breadboard.
- Insert wire from the third pin (+) of the op amp to the ground terminal of the breadboard.
- Insert Rin (10,000 ohm resistor) from the second pin (-) of the op amp to an arbitrary point on the breadboard that is not connected to anything.
- Connect one end of Rf (100,000 ohm resistor) to the second pin (-) of the op amp with the other end connected to the sixth pin (output) of the op amp.
Caution: Make sure none of the pins on the op amp are bent or it could be damaged when an input signal is applied.
Step 4: Biasing Your Op Amp
With the input signal established you now need to bias your op amp so that it will not be overloaded. The best piece of equipment to do this is the Agilent E3631A Power Supply. Using the power supply you can set the biasing voltages to be what you want but for this experiment chose them to be +15V and -15V. The biasing will restrict the output signal from going above them. With the power supply set up you can now connect it to the circuit.
Caution: Do not turn on the output of the power supply until you are at the end of the instructions and you are sure you have everything hooked up correctly or you could burn out your op amp.
- Connect the +25V port of the power supply with a red cord to the seventh pin (+V) of the op amp.
- Connect the -25V port of the power supply with a red cord to the fourth pin (-V) of the op amp.
- Connect the COM port of the power supply with a black cord to the ground strp of the breadboard.
Step 5: Applying an Input Signal
Now that the circuit is built you will add an input signal to the circuit for it to amplify and invert and you will use the Tetratonix AFG3012B Function Generator to do so. The input signal that you will establish for this experiment is a 1V peak to peak Sine wave with a 1000Hz frequency. With the new input signal you will connect the function generator to the breadboard.
Caution: Do not turn on the output of the function generator until you are at the end of the instructions and you are sure you have everything hooked up correctly or you could burn out your op amp.
- Connect the positive cord (red) from the function generator into the arbitrary pin that was established for the end of resistor Rin
- Connect the ground cord (black) from the function generator to the ground terminal of the breadboard.
Step 6: Connecting Output/Input to Oscilloscpe
With all the input signal and bias voltages hooked up to the circuit the last thing to connect are the cord to measure your output signal. The piece of equipment you will use to do this is the Agilent InfiniiVision DSO-X 2024A Oscilloscope. You will also be hooking the function generator up to the oscilloscope so that you can view the input and output signals at the same time.
- Repeat Step 4 with the connections being from the oscilloscope instead of the function generator.
- Connect the red output cord from the oscilloscope to the sixth pin (output) of the op amp.
- Connect the black output cord from the oscilloscope to the output strip of the breadboard.
Step 7: Viewing the Results
Now that everything is connected to your circuit and that it is correct you can turn on your input signal and your bias voltages. Once this is done hit the auto scale button on the oscilloscope and view your results. Notice that the amplitude of the output signal (green wave) is about ten times that of the input signal (yellow wave) and that it is the inverse wave of the input. Congratulations you have just built an inverting amplifier circuit and can now go on to building bigger and more complicated circuits.
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