RLC parallel resonance circuit

Gary B said:

Hi, I think that you may be getting confused by terminology. Let me attempt to consolidate some of what has already been said in a way that you might find easier to understand.
1) LC circuit will resonate at a specific center frequency. RLC is the same thing; it just does not ignore the resistance that will be in the wire and, in most cases, intentionally added to broaden the response.
2) Q defines the bandwidth and yes, it is perfectly permissible to look at it the other way around and say bandwidth defines Q. The more resistance in the RLC filter circuit, the lower the Q, the wider the bandwidth and the lower the output voltage and the more uniform over the entire bandwidth until Q= 1. At that point we have what is called a maximally flat response or Butterworth filter. To widen the bandwidth past that point, you have to add additional sections to the filter tuned above and/or below the original center frequency. The previous sentence is only practically correct, you can lower below 1 for wider bandwidth but, gain falls of so the circuit looks more and more like just a resister and you lose frequency rejection outside the desired range.

Click to expand...

At the sentence I highlighted you must mean the resistance in the inductor and capacitor (though capacitor resistance will be low). If you DO add 1 physical resister to the LC circuit in parallel like this **broken link removed** you will have a lower quality factor the lower that physical resistor is. Am I right?

Also I see that the circuit I link to have a stable current generator. I guess that is needed for this kinda circuit if you want the amplitude to change?

SimonTHK,

I think you will find the thread below very helpful. Be sure to go the the link referenced in post #5 of that thread.
https://www.electro-tech-online.com/threads/rlc-question-help.107090/

Now let me say a few things.

1) If you have a two branch circuit, consisting of a inductor and a resistance in series for one branch, and a capacitor and a resistor in series for the other branch, you can adjust the component values so that the circuit is resonant at all frequencies. So you can control the frequency of resonance with L, C, Rl, and Rc.

2) No one has given you a very definitive description of Q so far, but you should know from your studies what it is. Q is the quality factor of coils, capacitors, circuits, and is defined as Q = 2*pi*(maximum stored energy)/(energy dissipated per cycle) = reactive power/real power. The Q of a coil in series with a resistor is Ql = reactive power/real power = Il^2*Xls/Ir^2*Rs . Since Il and Ir are equal in a series circuit, we get Q = Xls/Rs . For a coil in parallel with a resistor, we have Ql = reactive power/real power = (El^2/Xlp)/(Er^2/Rp) . Since El and Er and equal in a parallel circuit, we get Q = Rp/Xlp . Note that Q is not the same for series and parallel resistance.

Ratch