What's wrong with my circuit simulation?

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Discussion Overview

The discussion revolves around troubleshooting a circuit simulation involving a 741 operational amplifier, specifically focusing on the observed output gain and issues related to circuit connections and configurations. Participants explore various technical aspects and potential errors in the setup.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that pin 3 of the operational amplifier should not be left hanging, suggesting a wiring issue.
  • Another participant points out that the output (pin 6) is incorrectly connected directly to ground, which would result in a 0V output.
  • A participant mentions that the power supply voltage is not properly referenced to ground, recommending the use of two 7.5V batteries to avoid the op amp "locking up."
  • One participant expresses confusion about the lack of gain despite corrections, indicating ongoing issues.
  • Another participant reiterates that the output is still connected to ground, questioning the expected behavior of the circuit.
  • A later post reports that a gain is now observed, but it does not match the intended R2/R1 ratio, prompting further inquiry into the setup.
  • Participants suggest that the oscilloscope connections may be incorrect, advising to measure output voltage against ground rather than the input voltage.
  • One participant advises breaking the output connection to compare it with the input on different channels to analyze gain properly.
  • Another participant explains that the circuit is being overdriven, providing calculations for the expected output based on input voltage and suggesting a reduction in input voltage to achieve the desired gain.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the circuit's configuration and the reasons for the lack of expected gain. There is no consensus on a single solution, as multiple issues are identified and discussed throughout the thread.

Contextual Notes

Participants highlight several potential limitations, including incorrect wiring, oscilloscope connection issues, and the relationship between supply voltage and output capabilities. These factors contribute to the uncertainty in achieving the intended gain.

red123
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What's wrong with my circuit simulation? No gain is observed at the amplifier output. The intent is for the output gain of the 741 operational amplifier to be equal to R2/R1.

2kok1x.jpg
 
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You can't just leave pin 3 hanging. You have a wire wrong there, and when you look at what pin3 should do in your test circuit, you should see which one it is.
 
As well as the wrong inputs, the output (pin 6) is corrected direct to ground, so what would you expect its voltage to be except 0V?

The oscilloscope is connected straight across the AC voltage generator, so that is what it will display.

A more subtle problem is that your 15V power supply isn't related to your "ground" voltage. The simplest way to fix that is use two 7.5V batteries between ground and pin 4, and ground and pin 7. if you don't do that, the op amp will probably "lock up" with the output stuck at one of the supply voltages, even when you fix the other problems.
 
What am i doing wrong? Still no observed gain.

1t4tp2.jpg

1jtbg9.jpg
 
You still have your output connected directly to ground... not sure what you expect that to do except act as an expensive but inefficient heater.
 
5vnpra.jpg

Ok, a gain is observed now. Output amplitude is 8.032V but a gain equal to R2/R1 is intent, what am I doing wrong?
 
That looks better, you still have the scope lead hooked up wrong. You want to measure output voltage vs. ground (I imagine), not output voltage vs. oscillating input voltage.
 
Your output and input are both connected to Channel A input. Break the output connection and reconnect it to the Channel B input. Then you can compare Channels A & B to see the gain...
 
juy793.jpg

Like this? Output is square wave and amplitude is 6.618V, gain is still not equal to R2/R1.
 
  • #10
You are just overdriving the circuit.

1 volt RMS sinewave is 2.8 volts peak to peak.

So to have an undistorted gain of 10, the output would be 28 volts peak to peak.

But the supply is only 15 volts total, so the opamp can't possibly produce 28 volts out.

Try reducing the input to 0.1 volts RMS. This is 0.28 volts peak to peak, so a gain of 10 would mean 2.8 volts peak to peak.
 
  • #11
Thanks everyone, the circuit finally works.
 

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