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Do active filters even care about input/output impedances?

Discussion in 'Electronic Design' 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 "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. Andrew Holme

    Andrew Holme Guest

    The output of an active filter usually comes from the output of an op-amp.
    This can drive any load impedance within reason. It doesn't change the
    output significantly. In this respect, it's no different from an op-amp
    inverting or non-inverting amplifier.

    The input needs to be driven by a source impedance that's significantly less
    than the input impedance of the filter, if it's not to upset the response.
    The design software may assume zero source impedance i.e. a perfect voltage
    source. If you have a resistor at the filter input, you can reduce it by
    the size of the source impedance, since they are effectively in series. In
    this way the non-zero source impedance is "absorbed" by the input resistor.
     
  3. Pooh Bear

    Pooh Bear Guest

    All active filters require to be driven from a low impedance. This is kinda
    'assumed to be known'.

    Any active filter using op-amps or even a discrete active gain block will
    normally have a low output impedance and therefore doesn't care much about
    the load - within the normal limits of what the op-amp etc can drive.

    Graham
     
  4. Reg Edwards

    Reg Edwards Guest

    Active circuits design assumes that the driving impedance is zero and
    that the terminating impedance is infinite.

    It is up to follow-on circuit designers to include analysis of the
    effects of the surrounding impedances in which a paricular circuit is
    embedded.

    If you are not capable of doing this then you are not qualified to do
    the job anyway. Pass it on to somebody who knows what he is doing.
     
  5. Pooh Bear

    Pooh Bear Guest

    Hi Reg,

    most ' kids out of college / university ' aren't taught to do much that's
    useful or practical these days that you and I would probably have taken
    for granted.

    A sad reflection on the state of education.

    Graham
     
  6. billcalley

    billcalley Guest

    Thanks guys -- this has really cleared up my questions on active
    filters.

    Best Regards,

    -Bill
     
  7. John Larkin

    John Larkin Guest

    You can't learn everything in four years. I think it's better to stick
    to fundamentals (math, physics, circuit theory, signals+systems,
    control theory) in school, and learn the practical stuff on the job.
    Heck, most profs don't know the practical stuff anyhow.

    Anybody can learn the practical stuff and be a tech, but to design
    serious electronics you need some theory.


    John
     
  8. Chris Carlen

    Chris Carlen Guest

    Right on, John. The thing that limits me is theory, since I have a
    Chemistry degree instead of EE. (Why did I do that?!?!)

    Theory takes a lot of time to learn, and few people can do it on their
    own outside of formal instructional environments.

    There are things which just can't be done without it.

    I'm back to a designing a new PLL motor control project, so I know.
     
  9. Pooh Bear

    Pooh Bear Guest

    True. I guess. I'm suppose I'm unusual. I started learning theory and practice
    from about age13 on out of personal interest.

    Much of what I see taught gives me little confidence though.

    Graham
     
  10. Mac

    Mac Guest

    I agree with you. Another point is that universities are supposed to teach
    fundamentals, not solely prepare people for work. Preparing people for a
    particular trade is what trade schools are for.

    I got my BSEE fairly recently (2000), and many times I heard students
    complain that the EE curiculum wasn't practical enough. But the professors
    (and, I guess, the accrediting folks) place a lot of emphasis on EE
    fundamentals.

    --Mac
     
  11. Ben Bradley

    Ben Bradley Guest

    You're doing well to have observed this enough to ask the question,
    OTOH if you really knew your circuits analysis (and had tough EE profs
    like I did) you'd know how to analyze the circuits and decide what
    impedances would affect the filter.
    But there's a way you can redeem yourself...
    Read Don Lancaster's "Active Filter Cookbook." (Do Not Pass Go, Do
    Not Collect $200) All your questions will be answered, and no, he's
    not paying me to say this.
    This is NOT an AC circuits analysis or design book, though it does
    have the formulas and can allow a working engineer to fake a lot of
    good knowledge in a lot of situations.
     
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