# AC bias

Discussion in 'General Electronics Chat' started by RobD, Jan 1, 2014.

1. ### RobD Thread Starter New Member

Dec 14, 2013
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0
Hello,

I have to bias an AC signal up about 2 volts, but can't think of any way to do it other thank having a 2 volt battery in series with the signal. Is there some way to accomplish this without having to have a battery other than the power source (9v + regulator)?

Best

Nov 30, 2010
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3. ### RobD Thread Starter New Member

Dec 14, 2013
12
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as in use an op amp w/negative feedback and a gain of 1, but have normally grounded input tied to 2v?

4. ### #12 Expert

Nov 30, 2010
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I can't quite see how to do it that way right now. Probably short on coffee today.
Maybe this one?

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5. ### RobD Thread Starter New Member

Dec 14, 2013
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does the 2v by the bottom resistor denote that it would have 2v across it? and why put an input capacitor?

6. ### #12 Expert

Nov 30, 2010
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Maybe you were thinking of this?
Edit: I don't think this will work because, if the - input is at +2V and the + input is ground referenced, the op-amp can't equalize its inputs. That's what I can't seem to get right now. Maybe some other clever person can spot why I can't see this working.

Yes, the 2V marking means, "Find the input impedance of the op-amp and design a voltage divider to give it 2 volts across the bottom resistor".

The capacitor is a, "just in case" to keep the 2 volts from escaping into the AC supply.

• ###### another 2V offset.png
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Last edited: Jan 1, 2014
7. ### #12 Expert

Nov 30, 2010
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Depending on the impedances of the input and output, the first drawing might work without even using the op-amp. Just AC couple the signal to a voltage divider and proceed without using an op-amp as a current buffer.

8. ### RobD Thread Starter New Member

Dec 14, 2013
12
0
Ok, thanks, I will try that first

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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#12 likes this.
10. ### Alec_t AAC Fanatic!

Sep 17, 2013
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If the circuit supply voltage is stabilised you can derive the 2V from it simply with a resistive divider (two resistors).

11. ### inwo Well-Known Member

Nov 7, 2013
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My first thought would be further back in the ops circuit.
Get the offset at the generation of the ac signal.

12. ### #12 Expert

Nov 30, 2010
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facepalm.

Hang on...that won't work with a single 9V battery.
You need to reverse the polarities of the inputs and change the lower section to a simple wire for negative feeedback.
Then it's a variation of what I did in post #4.

Maybe the problem is that there IS a dual supply and I'm missing it.

Last edited: Jan 1, 2014
13. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
Just curious, what is the p-p voltage of the AC signal?

'facepalm' first new word for 2014 ???

14. ### #12 Expert

Nov 30, 2010
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"facepalm" is a word I learned on internet joke sites.
Just before this post, I used, "meh".