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Do active filters even care about input/output impedances?

Discussion in 'Electronic Basics' started by billcalley, Aug 7, 2005.

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  1. billcalley

    billcalley Guest

    Hi All,

    I've been looking at a lot of active filter design software and
    design formulas, and none seem to care too much about the input and
    output impedances that the filter will want to "see" when placed in a
    circuit. When designing *passive* filters, we would typically specify
    these impedance values as a matter of course (normally 50 ohms). Why
    don't active filter design programs even ask what the input and output
    impedances are that the filter will have to work with, nor state what
    it is after the circuit is synthesized? Does it even matter; or will
    the frequency response and gain just not be affected by most normal
    values? Or is it assumed that the active filter will be placed between
    certain impedance values? If not, then how can I tell what the
    "optimal" impedance values should be for an active filter? This has me
    baffled!

    Thanks,

    -Bill
     
  2. Ban

    Ban Guest

    You have to consider that active filters are usually used for *low*
    frequencies from DC to maybe 10MHz, where the wavelength is much longer than
    the mechanical dimensions of the components. We do not have reflections and
    the capacitors are much bigger than the parasitic circuit capacitances. That
    is why it's possible to have relativly high impedances involved, in order to
    decrease capacitor size.
    Active filters do not use inductors (coils) as do passive filters.
    It is also a convention that these filters are used with 0 ohms driving and
    affect freq. response or gain, because usually the values are in the 3k3 to
    100k region, but for a precision measurement device real values have to be
    plugged in. Many programs not only neglect the in/out impedances, but are
    based on an ideal opamp model, with infinite gain and node impedances.
    Though a good simulation program like spice can take all these into account.
     
  3. Bob Eldred

    Bob Eldred Guest

    Most active filters are designed to be driven by constant voltage meaning
    nearly zero impedance. This is the case when they are driven from an op-amp
    in the circuit before the filter. It's output impedance is nearly zero ohms.
    If there is much resistance or impedance in the driving circuit, it will
    affect the response of the active filter and must be included in the
    response calculations. Think of it this way: Suppose the input resistor on a
    certain filter is 10K. Now suppose you drive it with a 5K source, the real
    input resistance is now 15K not 10k of the original design and it affects
    the response. Driving from the low impedance output of an op-amp alleviates
    this issue.

    The output side of active filters is usually the output of an op-amp and,
    therefore, provides nearly zero impedance to the following circuits. In
    other words the active amplifiers "buffer" the circuits from one another.

    Passive filters usually do not have amplifier buffering and cannot be
    designed for near zero impedance. Therefore, they are designed for a
    specific impedance, say 50 ohms. But, because this is not zero ohms it must
    be included in the filter's calculations.
    Bob
     
  4. billcalley

    billcalley Guest

    Thanks Bob and Ban for the great explanations! Now I understand much
    more about what active filters like to see impedance wise.

    Best Regards,

    -Bill
     
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