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What's wrong with my circuit simulation?

by red123
Tags: circuit, simulation
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Dec12-12, 07:56 PM
P: 22
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.

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Dec12-12, 08:16 PM
P: 166
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.
Dec12-12, 08:26 PM
Sci Advisor
HW Helper
P: 7,147
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.

Dec12-12, 09:05 PM
P: 22
What's wrong with my circuit simulation?

What am i doing wrong? Still no observed gain.

Dec12-12, 09:09 PM
P: 166
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.
Dec12-12, 09:22 PM
P: 22

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?
Dec12-12, 09:53 PM
P: 166
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.
Dec12-12, 10:41 PM
berkeman's Avatar
P: 41,012
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...
Dec13-12, 05:35 AM
P: 22

Like this? Output is square wave and amplitude is 6.618V, gain is still not equal to R2/R1.
Dec13-12, 06:13 AM
Sci Advisor
P: 4,031
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.
Dec13-12, 06:36 AM
P: 22
Thanks everyone, the circuit finally works.

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