I am a student who is currently struggling to make an op-amp amplify the input voltage. I am using an op-amp (uA741C). The following is a link to the spreadsheet of the op-amp. On page two, there is a diagram of the op-amp I am using (it is the one with 8 pins). http://www.jameco.com/Jameco/Products/ProdDS/830522.pdf.
See attachment for a written diagram of the circuit that I have put together. Something is not working, and my methods of troubleshooting have failed.
Explanation of the Circuit Diagram:
I have 1.5V inputted into the non-inverting input (+) and the inverting (-) to the ground. I also have +12 volts inputted in the VCC+ and -12V inputted in the VCC-. The first resistor is 330k?. The second resistor (1M?) runs from the output back to the non-inverting input. According to my research, this should magnify the voltage by the ratio of the 2nd resistor to the first (1M?/330k?) plus 1, resulting in approximately 6V.
I could easily be wrong about this because I know essentially nothing about op-amps.
However, I don't think you will ever get a stable output with the +ve feedback arrangement that you have there.
Consider the initial condition:
You add +1.5 to the + input. That makes the differential between + & - as +1.5V, so the amp outputs something +ve.
At the next moment, the + input sees 1.5V plus some of the output from the amp. So the new output is more +ve. And the cycle repeats.
I cannot see how your amp can fail to saturate under these conditions, whatever the resistor values. You don't say what you see but I guess that you get an output matching the +ve rail.
My guess (and it is a guess) is that you need to attach the feedback to the inverting input in order to generate a stable output.
I would try your 330K from +1.5 to +ve and your 1M from output to -ve with another 330K from -ve to ground. That way you make a voltage divider on the -ve input which might give you a stable output (as the output voltage goes higher, so the -ve feedback increases).
There must be someone who knows more about this than me, however.
but that did not solve my problem. Any other suggestions?
I suggest that you explain what your problem is with the circuit that CrossRoads posted. Forget your circuit that is all wrong, as explained above.
I would also suggest that you scale back on the absolute values of the resistors, drop them all by 10 times. The 741 op amp has been around nearly as long as I have in the electronics scene, and like me it is starting to creak.
If your schematic is an accurate representation of what you have constructed, you need to swap the + and - inputs of your op-amp. However, 330K and 1M are rather large resistor values to use for a 741 (as Mike says, it's a very old op-amp) so don't expect the output voltage to be exactly -3x the input voltage (it's -3 not 3 because it will be an inverting configuration).
There's a Java circuit simulator that's really easy to use. Design your op-amp circuit on that. Put a wire going nowhere off the output side, right-click it, and set up a scope view to watch it. Tweak your values until you get what you want.
It's a simple and rather limited simulator (ideal components, so sometimes you have to do things like add 0R1 resistors to keep from approaching inifinite current and such) but it has allowed me to build actual things that work on the first try, rather than blowing up parts because I did something bone-headed.
BTW, you can download a .ZIP file with the applet and all its demos so you don't have to run it from the page.
