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Capacitor on the output

Discussion in 'Electronic Basics' started by Ed, Jul 28, 2004.

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  1. Ed

    Ed Guest

    I've noticed on some circuits that op-amps sometimes have a capacitor on the
    output (in series). For example, the circuit I'm looking at has an LM741
    op-amp with a 0.02uF capacitor on the output. What is the purpose of this
    capacitor? Is it just to filter out noise? How do you calculate the value
    of the required capacitor (does some magic formula exist)?
  2. A capacitor in series with the signal path acts as a high pass filter,
    blocking any DC in the signal from passing through while letting
    frequencies of interest an easy path.
  3. Bob Masta

    Bob Masta Guest

    The value of the capacitor depends upon the input impedance of
    the circuit that follows it, and the frequencies that you want to
    allow to pass. Normally, we consider the "cutoff" frequency to
    be that where the output is reduced by 3 dB (0.707). At lower
    frequencies, the output is reduced even more, proportional
    to the frequency: Every halving of the frequency below cutoff
    results in a halving of the output level (-6 dB / octave).

    The formula for the cutoff is F3 = 1 / (2 * pi * R * C)
    where F3 is the cutoff frequency in Hertz,
    R is the following stage input impedance
    in ohms, and C is the capacitance in *Farads*.
    You can rearrange this to C = 1 / (2 * pi * F3 * C)
    when you are trying to pick a C from design
    considerations. In your example case, if the
    following stage happens to have an input
    impedance of 100K the F3 would be
    F3 = 1 / (2 * pi * 100000 * 0.02x10^-6) = 79.6 Hz.

    Hope this helps!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  4. Activ8

    Activ8 Guest


    C = 1 / (2 * pi * F3 * R)
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