What does Q factor mean?

[Rathohan]

Well, during calculation, I found that Q factor can be either negative or positive?

Why? What do the negative Q and the positive Q mean?

 

[FvM]

I would expect negative Q values in an oscillator circuit.

 

[BigBoss]

Q=Quality Factor definition is generally used to define Energy Loss in inductors.Q factor is always positive before Self Resonance Frequency and beyond it may be negative and it doesn't make sense because Inductor wouldn't be "Inductor" anymore beyond Self Resonance Frequency...

 

[mtwieg]

In a very general sense, Q defines the ratio between energy stored to energy dissipated per radian (or per cycle*2pi). In other terms, it's also the magnitude of the ratio of imaginary impedance to real impedance (Q for passive components is always positive).

Q factor can also be used to describe more complex systems, but its exact meaning isn't quite the same...

 

[FvM]

There are previous threads about Q factor, e.g. https://www.edaboard.com/threads/128004/

It happens that the two definitions mentioned by mtwieg result in the same Q value. Both can be also extended to negative Q numbers and describe a resonant circuit with negative real impedance respectively an oscillation with increasing energy.

 

[zorro]

Ha all,

Negative Q means "Q beyond infinity" .

In a resonant circuit, negative Q can be obtained with negative resistance (and positive L and C), as pointed out.
Yet, negative Q could be obtained too with positive R and both L and C negative...

Now I propose a puzzle:
Suppose we have an RLC circuit (series or parallel), and that R, L and C are all negative.
Then Q is positive and the circuit is stable (the poles are in the left half-plane and the impulse response decreases with time).
Now we excite the circuit with a sinusoid at the resonant frequency. I.e., with a voltage source in the series circuit or a current source in the parallel circuit.
How and where is the energy dissipated?

Regards

Z