voltage adder

Discussion in 'General Electronics Chat' started by cmoxey, Jun 6, 2008.

  1. cmoxey

    Thread Starter New Member

    Jun 6, 2008
    3
    0
    Hello,
    I have a project that I am working on that needs some help. What i have is a oscillating voltage varying from .2 volts to .9 volts. I would like to be able to add in a voltage of .2 to .9 volts... I know that this is possible, but I am just not that versed in the circuit design.

    Thanks in advance
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Perhaps you can sketch and post a rough diagram of the signal in and the signal out.

    hgmjr
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Take a look at this chapter in this site's E-book:
    http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
    It describes averaging and summing circuits using opamps. If your AC input is very low in frequency or has a DC level component, you'll need to use something like an opamp summer/summing circuit.

    If your AC input is a moderate to high frequency and you don't care about a DC level component, you could use something like the attached circuit.
     
  4. cmoxey

    Thread Starter New Member

    Jun 6, 2008
    3
    0
    thanks for the info... I guess I need to give you a little more info... What I have is a dc voltage that varies between .2 and .9 volts.. and I would like to be able to ad a user specified voltage somewhere between 0 and .9 volts dc to be able to bring the original voltage up a little bit. Here is a picture of what I am looking for... Thanks
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Attached is one possibility, in simplified form.

    R1 and R2 set the gain for the op amp at 3. Gain = (R1+R2)/R1 = (10k+20k)/10k = 30/10 = 3/1 = 3
    R3 and R4 are the summing resistors.
    Note that R4 actually represents two resistors; slightly less than 10k to Vee, and slightly more than 10k to Vcc. The adjustment will be quite sensitive. You may wish to use a 21-turn trim pot, or perhaps a 1k pot with 9.5k resistors on either side of it.

    The precision of the resistors is important.

    You will need a bipolar supply, or a rail-to-rail input/output op amp in order to use this circuit.
    The LF353 op amp is roughly equivalent to a TL072 low-noise JFET-input dual op amp.
    The TL082 might also be used, but it will add more noise to the signal.
    All three of these opamps are basically audio-spectrum amplifiers.
    You haven't mentioned the frequency you're dealing with.
     
    Last edited: Jun 6, 2008
  6. cmoxey

    Thread Starter New Member

    Jun 6, 2008
    3
    0
    Thanks,
    I will try that and see how it works.
     
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