Newb question on capacitor

Discussion in 'General Electronics Chat' started by Graygem, Jan 27, 2012.

  1. Graygem

    Thread Starter New Member

    Feb 4, 2011
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    Newb question. On the non inverting amp below, what is the capacitor for? I see the capacitor on some circuits but not on others. Does it make a difference if the circuit is single supply or dual supply?
    Thanks.

    Non Inverting OP Amp with Cap.jpg
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    For DC input signals the output follows the input.

    For AC input signals with frequencies greater than 0.333Hz, the output is 100 times greater than the input.
     
  3. Graygem

    Thread Starter New Member

    Feb 4, 2011
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    0
    That makes sense, thank you. So R2 + R1 and C create a high pass filter. Now, I have also seen a small cap, say 22pF, in parallel with the R2 resistor. What does that cap do?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    It decreases the AC gain at high frequencies.
     
  5. Graygem

    Thread Starter New Member

    Feb 4, 2011
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    Thank you. I appreciate your time.
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    Usually that capacitor is to allow offsetting the input DC voltage (typically by 1/2 the supply voltage) to allow an op amp that normally requires dual supply voltages to operate from a single supply. It makes the DC gain equal to +1 but the AC gain is determined by the ratio of the two resistors.
     
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