Cancel trace inductance with capacitance?

Discussion in 'General Electronics Chat' started by coinmaster, Jun 6, 2016.

  1. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
    6
    Inductors try to prevent current change by floating voltage and capacitors try to prevent voltage change by floating current.

    Let's say you had a wire with 1mh of inductance from end to end. Could you not use an equivalent capacitance to effectively negate the inductance?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    If you connect the inductor in series with a capacitor then the combined impedance will be zero at one specific AC frequency.
    Resonance.PNG
    In the above example circuit the output voltage has 0dB attenuation at a frequency of about 50kHz.
     
  3. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
    6
    What about in parallel?
    [​IMG]
    Does that look right?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    That only works for one frequency, of course.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    No. The parallel combination has maximum impedance (not zero) at the resonant frequency.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Of course.
    You have to put them in series to get minimum impedance.
     
  7. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
    6
    Except DC can't travel through a cap :(
    Oh well, guess I just have to minimize wire length to begin with.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Or use a controlled impedance trace aka transmission line.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,288
    1,255
    Tell us what you are trying to do or what your concern is. We may be answering the wrong question.
     
  10. coinmaster

    Thread Starter Member

    Dec 24, 2015
    350
    6
    Nothing special.
    Just trying to minimize inductance between components in my amplifier builds. LTspice simulations show that even small inductance can cause big issues.
    It would be nice if I could not have to worry about inductance when laying out components.
     
  11. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    An audio preamp ia a lot different than an FM transmitter output stage. ;)
    Can you tell us more about your amplifiers -- such as frequency and signal/power levels?
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,288
    1,255
    As @RichardO says, it is usually not a problem with low frequency amplifiers. That's because the impedance of the traces is low at low frequencies (Z=2pif).
    But if the frequency is high decoupling is a good idea.
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,288
    1,255
    Here is a calculator so you can see how big the "loop" would need to be to make a large inductor.
    http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml
     
Loading...