question about q-factor

Discussion in 'General Electronics Chat' started by mrozmt, May 28, 2008.

  1. mrozmt

    Thread Starter New Member

    May 28, 2008
    i have an assignment for home and have a question and i don't know where to find the answer i have search alot. Can someone pls help me. the question is "Explain the difference between the q-factor of a circuit and the q-factor of a coil?" thanks for your help
  2. nanovate

    Distinguished Member

    May 7, 2007
    What type of circuit? Q-factor is general refers to ratio of energy stored to energy dissipated at a frequency. For an inductor this is usually omega*L/R and is a measure of how "ideal" (lossless) it is. For a circuit the formula depends on the particular circuit but it still has the form of omega*energy stored/energy lost
  3. bertus


    Apr 5, 2008
  4. silvrstring

    Active Member

    Mar 27, 2008
    Hello mrozmt.

    The Q factor of the circuit is the ratio of reactive power to avrerage power.

    For a series filter at resonance, this would be Q=X(L)/R. For a parallel circuit, it is Q=R/X(L).

    It is important because it determines the selectivity of your filter. The bandwidth of your filter will be f/Q, so the higher your Q factor, the smaller your bandwidth.

    Sometimes, an inductor will have its own Q included in the schematic. You will need to include it in your calculations to obtain Q of the circuit.

    Q of the inductor = X(L) / R of the inductor.

    Example: if Q(coil) = 50, and X(L)=100, then R(coil) = 2ohms, and R(total) = R+R(coil). This in turn effects Z(total) which effects your I result (because I=E/Z) and your V(L) and V(c) result, therefore your average power, and reactive power for the inductor.

    Finally, you can find Qfactor of the circuit by Qseries=Q(L)/P. (Q(L) here is the reactive power of the inductor, not the Qfactor of the inductor).

    Hope this helps.
    Last edited: May 29, 2008