AC bias

RobD

Joined Dec 14, 2013
12
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

#12

Joined Nov 30, 2010
18,076

RobD

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

#12

Joined Nov 30, 2010
18,076
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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RobD

Joined Dec 14, 2013
12
does the 2v by the bottom resistor denote that it would have 2v across it? and why put an input capacitor?

#12

Joined Nov 30, 2010
18,076
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.

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#12

Joined Nov 30, 2010
18,076
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.

RobD

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

Kermit2

Joined Feb 5, 2010
4,158

#12

Alec_t

Joined Sep 17, 2013
10,267
If the circuit supply voltage is stabilised you can derive the 2V from it simply with a resistive divider (two resistors).

inwo

Joined Nov 7, 2013
2,416
My first thought would be further back in the ops circuit.
Get the offset at the generation of the ac signal.

#12

Joined Nov 30, 2010
18,076
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.

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tubeguy

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

'facepalm' first new word for 2014 ???

#12

Joined Nov 30, 2010
18,076
"facepalm" is a word I learned on internet joke sites.
Just before this post, I used, "meh".