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Old 27th March 2011, 06:05 PM   #1
yakideo is offline yakideo  Poland
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Default op-amp with virtual ground in SPICE

Hello,

I just made a simple SPICE simulation with an op-amp and virtual ground (read: single supply). However it doesn't seem to work. The output of the op-amp gets distorted and inverted for some unknown reason to me. Have I made some fundamental error in the circuit or is SPICE just kidding with me?

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Old 27th March 2011, 06:19 PM   #2
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The circuit should provide an amplifier output, but the input needs to be "centered" at 6 volts. The equivalent circuit to the left of R2 is 6 volts in series with 2.35k. The gain is 1+10k/(1K+2.35k) and the output will be 6 volts when the input is 6 volts. Do you have the input centered at 6 volts?
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Old 27th March 2011, 07:03 PM   #3
yakideo is offline yakideo  Poland
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Quote:
Originally Posted by sawreyrw View Post
The circuit should provide an amplifier output, but the input needs to be "centered" at 6 volts. The equivalent circuit to the left of R2 is 6 volts in series with 2.35k. The gain is 1+10k/(1K+2.35k) and the output will be 6 volts when the input is 6 volts. Do you have the input centered at 6 volts?
By using a voltage divider I get 6V connected to the op-amp (-) input.
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Old 27th March 2011, 07:38 PM   #4
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What is the voltage on the positive input? AC component and DC component?
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Old 27th March 2011, 08:44 PM   #5
yakideo is offline yakideo  Poland
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Quote:
Originally Posted by sawreyrw View Post
What is the voltage on the positive input? AC component and DC component?
The signal AC voltage is at 0.71V and the DC is at 0V.

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Old 27th March 2011, 09:07 PM   #6
cbdb is offline cbdb  Canada
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Try biasing the + input to 6 volts too.
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Old 28th March 2011, 04:53 PM   #7
yakideo is offline yakideo  Poland
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Originally Posted by cbdb View Post
Try biasing the + input to 6 volts too.
How do I do that?
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Old 28th March 2011, 05:29 PM   #8
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Your virtual ground should be the node at were r3 and r4 are connected. jer
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Old 28th March 2011, 09:10 PM   #9
cbdb is offline cbdb  Canada
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Dont know the software your using. Try adding a dc offset to the signal generator.
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Old 28th March 2011, 09:28 PM   #10
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#7 Click the image to open in full size. yakideo Click the image to open in full size.
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Quote:
Originally Posted by cbdb Click the image to open in full size.
Try biasing the + input to 6 volts too.

How do I do that?
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by putting a resistor from the node of r3 and r4 to the +input on the opamp.
And disconnect r2 from the same node and it should work (i hope). jer

Last edited by geraldfryjr; 28th March 2011 at 09:31 PM.
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