Q of R+LC Circuit: Understanding Q & Phasor Analysis

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In summary: Well, nothing too unusual: ##Q=\dfrac{\omega_\text{resonance}}{B_\text{3dB}}## where ##B_\text{3dB}## is the 3dB bandwidth.I find your question a little odd though; aren't all definitions equivalent up to a factor of ##2\pi##. It's been years since I looked at this stuff in detail, but I don't recall any defn that would give a substantially different result.Maybe I forget?
  • #1
strauser
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TL;DR Summary
Derive Q of R in parallel with tank circuit
I've been experimenting with an LC tank circuit in series with a resistance R, and I've noted that the Q seems to increase with R. I've tried to derive this result via phasor analysis, but I'm not sure if my expression is correct.

To make things clear, I'm talking about the circuit with impedance ##Z=R+jX_L || X_C=R+j(\dfrac{\omega L}{1-\omega^2 LC}) ##

The only thing I've found via google is this:

https://electronics.stackexchange.com/questions/108788/voltage-output-from-a-tank-circuit

where the first answer suggests that ##Q=R\sqrt{\dfrac{C}{L}}## which at least agrees with my measured results. I've found however that ##Q=R\sqrt{\dfrac{C}{L+4R^2C}}##

So which result, if either, is right? I note that mine approximates the quoted result if ##L \gg 4R^2C##.
 
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  • #2
At first you have to convince yourself about the DEFINITION of the quantity you call "Q".
What is your definition?
 
  • #3
LvW said:
At first you have to convince yourself about the DEFINITION of the quantity you call "Q".
What is your definition?
Well, nothing too unusual: ##Q=\dfrac{\omega_\text{resonance}}{B_\text{3dB}}## where ##B_\text{3dB}## is the 3dB bandwidth.

I find your question a little odd though; aren't all definitions equivalent up to a factor of ##2\pi##. It's been years since I looked at this stuff in detail, but I don't recall any defn that would give a substantially different result. Maybe I forget?
 
  • #4
Well, may be that my question appears to you "a little odd" - nevertheless, would you mind to tell us HOW you have found the expression for Q you have mentioned?
According to system theory, the Q value of a frequency-dependent netork is defined using the pole position in the compex s-plane - and only for some special cases this value is identical to the ratio "resonant frequency/bandwidth".
More than that, for R approaching infinity your Q expression would be close to Q=0.5.
This is not correct. In contrast, Q must be very large...
 
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  • #5
strauser said:
Maybe I forget?
In addition to what LvW said it’s critical to precisely define what is the input and output variable. For example are you looking at voltage or current? Across or through what? Etc.
 
  • #6
LvW said:
Well, may be that my question appears to you "a little odd" - nevertheless, would you mind to tell us HOW you have found the expression for Q you have mentioned?
According to system theory, the Q value of a frequency-dependent netork is defined using the pole position in the compex s-plane - and only for some special cases this value is identical to the ratio "resonant frequency/bandwidth".
More than that, for R approaching infinity your Q expression would be close to Q=0.5.
This is not correct. In contrast, Q must be very large...
I don't have time to reply to this fully today, but your final observation suggests that I've indeed effed up somewhere.

I'll put up the details tomorrow.
 
  • #7
strauser said:
Summary: Derive Q of R in parallel with tank circuit

I've been experimenting with an LC tank circuit in series with a resistance R, and I've noted that the Q seems to increase with R. I've tried to derive this result via phasor analysis, but I'm not sure if my expression is correct.

To make things clear, I'm talking about the circuit with impedance ##Z=R+jX_L || X_C=R+j(\dfrac{\omega L}{1-\omega^2 LC}) ##

The only thing I've found via google is this:

https://electronics.stackexchange.com/questions/108788/voltage-output-from-a-tank-circuit
where the first answer suggests that ##Q=R\sqrt{\dfrac{C}{L}}## which at least agrees with my measured results. I've found however that ##Q=R\sqrt{\dfrac{C}{L+4R^2C}}##

So which result, if either, is right? I note that mine approximates the quoted result if ##L \gg 4R^2C##.
If you have L, C and R all in parallel, the Q is approx R/X, where X is the reactance of either L or C, which are the same when Q is greater than about 2. For the case when R, L and C are in series, Q= X/R exactly. The topic is actually a bit intricate for parallel circuits, because max voltage and zero phase do not quite coincide for low Q values. It is covered in the very old book Radio Engineering, by Terman.
 

FAQ: Q of R+LC Circuit: Understanding Q & Phasor Analysis

1. What is the Q of an RLC circuit?

The Q of an RLC circuit, also known as the quality factor, is a measure of the circuit's ability to store energy and oscillate without dissipating it. It is defined as the ratio of the reactance to the resistance in the circuit. A higher Q value indicates a more efficient circuit with less energy loss.

2. How is Q calculated in an RLC circuit?

The Q value can be calculated using the formula Q = ωL/R, where ω is the angular frequency, L is the inductance, and R is the resistance. It can also be calculated using the formula Q = 1/R√(C/L), where C is the capacitance. Both formulas give the same result.

3. What is the significance of Q in an RLC circuit?

The Q value of an RLC circuit is important as it determines the bandwidth and selectivity of the circuit. A higher Q value means a narrower bandwidth and better selectivity, while a lower Q value means a wider bandwidth and less selectivity. It also affects the resonance frequency and the rate of energy dissipation in the circuit.

4. How does phasor analysis help in understanding Q of an RLC circuit?

Phasor analysis is a method used to analyze the behavior of an RLC circuit in the frequency domain. It represents the voltage and current in the circuit as complex numbers, making it easier to visualize and analyze the circuit. By using phasor analysis, the Q value can be calculated and the effects of frequency on the circuit can be studied.

5. Can the Q of an RLC circuit be changed?

Yes, the Q value of an RLC circuit can be changed by altering the values of the components in the circuit. For example, increasing the inductance or decreasing the resistance will result in a higher Q value. Additionally, the Q value can also be affected by external factors such as temperature and humidity.

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