1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quality factor of any circuit

  1. Jul 7, 2012 #1
    Well If we talk about the quality factor of any circuit, we say that:

    It is the ratio of natural frequency of any circuit and the band width.

    Q = ω0 / Δω

    Somewhere I read : It is the ratio of the P.D. across the capacitor or the inductor with the P.D. across the resistor in any circuit.

    Q = P.D. across L or C / P.D. across R

    My confusion is that what does Quality factor actually signifies ?
  2. jcsd
  3. Jul 7, 2012 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    It signifies how 'sharp' the resonance curve is. Bigger Q is sharper / narrower band. Also Q gives a measure of how fast the oscillations will die down because it represents (the inverse of) the fraction of energy lost each cycle.
  4. Jul 8, 2012 #3


    User Avatar

    Staff: Mentor

    The term probably originated in the early days of radio and tuned circuits. The higher the Quality of an experimenter's tuned circuit, the better the ability of his circuit to separate or discriminate between adjacent or interfering stations.

    Mathematical analysis will turn up various equivalent statements of this measure, but you'll find they all relate back to the basic ratio of energy store to energy loss in a resonant system, whether electrical, mechanical or optical.
  5. Jul 8, 2012 #4
    i.e. someone should choose a circuit having high quality factor because it will have greater efficiency of energy storage in its inductor or capacitor. ?
  6. Jul 8, 2012 #5


    User Avatar

    Staff: Mentor

    The fact that a tuned circuit has a high Q means that its impedance has a very sharp peak (or dip) at the circuit's resonant frequency. It has this response entirely due to having a high ratio of energy stored to energy loss. How to reduce energy loss? -- reduce the resistive losses.
  7. Jul 10, 2012 #6
    Thank you very much friend.
    The confusion has been cleared.
  8. Aug 30, 2013 #7
    I am reading about quality factor. I have searched a lot but all books and sites I read only talking about Q of a component (inductor, capacitor), or a resonant circuit (RLC in series and RLC in parallel). I don't see anywhere they mention about quality factor of an arbitrary circuit. Could you tell me why? Is that all other circuits can be transformed to said circuits above?
  9. Aug 30, 2013 #8


    User Avatar
    Science Advisor
    Gold Member

    The concept of a quality factor only makes sense for some circuits, typically band-pass filters (where it defines how "sharp" the filter is) or resonators (where it tells you how quickly the resonance dies out). It is simply not useful for a generic circuit.

    Also, note that an ideal inductor or capacitor does not have a Q value. However, real components are not ideal and it turns out that you can create useful circuit models of both inductors and capacitors, and the Q value tells you something about that circuit model (mainly how dissipative the element is).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook