How to model filter circuits properly (LT..)?

Discussion in 'General Electronics Chat' started by Mike33, Jun 12, 2013.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005

    Just messing around, and trying out some audio filter circuits from books with LT Spice (tone controls). What I'm noticing is that I don't get the same kind of waveforms as shown in the some places they refer to the output impedance of previous gain stages ("assuming a 40K output impedance at the input of the filter..."). What I'm noticing is that my sims are pretty far off, so what I model may or may not correctly represent what I'll get in-circuit!

    How is this modeled the right way in LT? Is it as simple as making your voltage source's resistance equal to the impedance you'd like to present to the filter? (that still results in some 'off' waveforms, at least compared to a couple of circuits I've modeled)

    This could make a difference in real world results, esp. where opamps (as buffer) have a very low output Z, and a BJT stage may only be mediocre...
  2. LvW

    Active Member

    Jun 13, 2013
    Mike, it would be very helpful to show us one of your circuits.
    Otherwise, it is hard to answer your question.
  3. crutschow


    Mar 14, 2008
    If a certain output impedance is expected by the filter than you should add that to the simulation. It can be added to the source or as a separate resistor in series with the source. Generally LTspice will model such filters quite accurately if the proper circuit values are used.

    But as LvW noted, posting your circuit would be a big help in providing answers.