Introduction: How to Use OrCAD Design and PSpice Simulator to Build an Op-Amp
This Instructable was created in fulfillment of the required coursework for the course Communications for Engineers at the University of South Florida.
Members: Cole Andrew, Frederick Ngoiya, Diego Serrano Reinel
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BEFORE YOU START: Don't be discouraged if you don't know what an Op-Amp (Operational Amplififer) is. This tutorial assumes no previous knowledge on the concepts behind an Op-Amp and is just an example for the purposes of demonstrating the use of OrCAD Capture and PSpice.
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PSpice is a circuit simulator within OrCAD Capture that allows designing and simulating circuits before actually building them. These are products from Cadence and are powerful tools that will help you prevent extensive troubleshooting once you have built you circuits or non-working circuits at all. Common mistakes like wires not connected correctly can be prevented by having a blueprint of the desired circuit, which can be achieved with OrCAD Capture. Furthermore, A PSpice simulation will even calculate all the current, voltages and power values that you should expect from your circuit.
Step 1: Place Two Resistors in the Schematic
*Note: For this step, a new blank project of type Analog or Mixed A/D is assumed to have been created by the user. Component libraries are also expected to be added. These are the libraries that you will need for this tutorial:
- Analog
- Source
- Eval
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Open the Place Part menu and select the library ANALOG. Add two resistors to the schematic. The resistors that you will use for this tutorial are listed as R. If you can't find it, just type R in the field at the top.
To drag and drop a part to the workplace, double click it first.
Step 2: Place an Op-Amp in the Schematic
You will now add the Op-Amp to the schematic. Select the library EVAL and type LM324 in the top field. Place this part in the workplace.
Once again, remember that to drag and drop the part to the schematic you have to double click it first.
Step 3: Mirror the Op-Amp Vertically
This step is meant to introduce you to the placement options that the designer has. Right click on the selected Op-Amp and click Mirror Vertically.
The reason to do this is because most schematics will show the non-inverting terminal at the bottom and the inverting one at the top.
Step 4: Place an AC Source in the Schematic
We will use VSIN to simulate an AC source. VSIN is a part that inputs a sinusoidal signal which is also what an AC signal is. However, with VSIN we can control the frequency that we use. Select the library SOURCE and type VSIN in the top field. Place this part in the workplace.
After this tutorial you can try the same circuit with the part VAC to see the differences.
Step 5: Place Three Ground Parts
Click the icon in the images and a new menu with many types of ground parts will be opened. Select the one named 0/CAPSYM and place three of these in the workplace.
The parts that you have placed in the workplace so far can be seen in the second image.
Step 6: Place Two DC Sources in the Schematic
Select the library SOURCE and type VDC in the top field. Place this part in the workplace.
Note that after this step you have all the parts that you will need in the workplace. Now you will learn how to connect these.
Step 7: Connect One Ground in Series With the AC Source and R1
This step uses the Place Wire tool. This is your go-to tool to connect components. Click the icon in the images and click on the workplace to start placing a wire.
Step 8: Connect R2 to the Inverting Input of the Op-Amp and to the Output
The step described in the image should suffice this step.
If you are interested in what this actually means: You are setting up a negative feedback from the output to the inverting input. We recommend this link for a "user-friendly" explanation.
Step 9: Orient the Two DC Sources to Properly Wire the Voltage Ports of the Op-Amp
Connect the VDC parts to the V+ and V- terminals (one source for each terminal). This step is shown with more detail in the images.
This step is very important. It will determine whether your Op-Amp works or not. In real life, if this connection is made incorrectly, you can permanently damage the Op-Amp.
Step 10: Connect the Two DC Sources to a Common Ground
Setup a common ground for the two VDC sources that you just connected to the Op-Amp. Follow the steps in the images.
Step 11: Complete the Circuit
You are almost done with designing the circuit. This step consists of:
- Connecting the "branch" that you made in Step 7 to the inverting input
- Connecting a ground element to the non-inverting input
Step 12: Determine and Change the Values of Your Resistors
To change the value of an element, double click the value next to the element.
For the purposes of this instructable, you will use these values for the following elements:
R1 = 1 kOhm (1x10^3) R2 = 4 kOhm (4x10^3)
Read this only if you want to understand why: The circuit that we are using has an inverting configuration. In simple terms, the voltage source is connected to the inverting terminal (the one labeled with a - sign). The gain/amplification, since operational amplifiers (op-amps) amplify, is defined by:
-R2/R1
Here we are trying to achieve a gain of 4, and therefore we divide 4 kOhm by 1 kOhm. The negative sign flips the sinusoidal wave.
Step 13: Determine and Change the Values for Your AC and DC Sources
These are the values that we recommend for the AC (VSIN) source:
- VOFF = 0
- VAMPL = 1
- FREQ = 300
- AC = 0
For both DC sources:
- V = 15
Remember that you double click the value next to the element to change it.
Step 14: Create and Configure a PSpice Simulation Profile
In order to start a simulation, you need to create a simulation profile. Click the icon shown in the picture and the New Simulation menu will open. It can have any name.
To configure: As soon as you create the profile, a panel with settings will appear. Make sure to select the following settings:
- Analysis type: Time Domain (Transient)
- Options: General Settings
- Run to time: 0.006667
- Maximum step: 0.0001 or 0.001
Step 15: Attach Voltage Probes to Measure the Gain in Voltage
Select the icon shown in the first image in order to place the Voltage Probes as demonstrated in the second image.
Step 16: Perform a PSpice Simulation
Now everything is ready for you to run the simulation and see the Input Voltage increase by a gain of 4. Since the Input Voltage is 1 Volt, the Output Voltage will be 4 Volts.
Click the run simulation icon as demonstrated in the first image and you will obtain a plot similar to the second image.
This concludes the tutorial, please feel free to ask any questions or to leave us feedback in the comments below!