Difference amplifier won't work?

Discussion in 'The Projects Forum' started by bart_dood, Aug 27, 2010.

  1. bart_dood

    Thread Starter New Member

    Jul 14, 2010
    22
    0
    Hi Guys, simple difference amplifier. Voltages in are around 3-4 volts. Gain is supposed to be 4.02x (I know it won't be that precise).

    I'm using an LM358 as a test amp, it just doesn't work?? gain is wrong or no gain, just doesn't match what its supposed to do.

    Something missing here?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What are the values of R6 and R7?

    hgmjr
     
  3. bart_dood

    Thread Starter New Member

    Jul 14, 2010
    22
    0
    I can't quite remember, I have a 10k pot in there and I adjust it until I get a certain voltage at the output of the opamp which is about 3-4 volts.

    If the gain was exactly 4.02x then I would need about 1 volt delta on the inputs to get this voltage, however it just doesn't seem to work this way. I get something like an 80mV delta at the input which works out a gain of about 110x, which doesn't make sense.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Can you explain how you got a gain of 110?

    hgmjr
     
  5. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Have you double checked your resistor values?

    You're sure you're measuring the input in the right location?
     
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