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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    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
     
  2. #12

    Expert

    Nov 30, 2010
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    How about a micropower op-amp?
     
  3. RobD

    Thread Starter New Member

    Dec 14, 2013
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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.:D
    Maybe this one?
     
  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.
     
    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
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    Ok, thanks, I will try that first
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    [​IMG]
     
    #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
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    Just curious, what is the p-p voltage of the AC signal?

    'facepalm' first new word for 2014 ???:D:D
     
  14. #12

    Expert

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    "facepalm" is a word I learned on internet joke sites.
    Just before this post, I used, "meh".:D
     
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