op-amp with virtual ground in SPICE
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 27th March 2011, 07:05 PM #1 yakideo   diyAudio Member   Join Date: May 2010 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?
 27th March 2011, 07:19 PM #2 sawreyrw   diyAudio Member   Join Date: Apr 2006 Location: Minnesota 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?
yakideo
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by sawreyrw 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.

 27th March 2011, 08:38 PM #4 sawreyrw   diyAudio Member   Join Date: Apr 2006 Location: Minnesota What is the voltage on the positive input? AC component and DC component?
yakideo
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by sawreyrw 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.

 27th March 2011, 10:07 PM #6 cbdb   diyAudio Member   Join Date: Oct 2008 Location: Vancouver Try biasing the + input to 6 volts too.
yakideo
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by cbdb Try biasing the + input to 6 volts too.
How do I do that?

 28th March 2011, 06:29 PM #8 geraldfryjr   diyAudio Member   Join Date: Feb 2008 Location: Jackson,michigan Your virtual ground should be the node at were r3 and r4 are connected. jer
 28th March 2011, 10:10 PM #9 cbdb   diyAudio Member   Join Date: Oct 2008 Location: Vancouver Dont know the software your using. Try adding a dc offset to the signal generator.
 28th March 2011, 10:28 PM #10 geraldfryjr   diyAudio Member   Join Date: Feb 2008 Location: Jackson,michigan #7 yakideo diyAudio Member Join Date: May 2010 Quote: Originally Posted by cbdb Try biasing the + input to 6 volts too. How do I do that? 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 10:31 PM.

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