# series RLC circuit

Discussion in 'Homework Help' started by Steven5757, Jul 20, 2012.

1. ### Steven5757 Thread Starter New Member

Jul 2, 2012
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0
I have a homework question, that I could use some assistance with: it is desirable to increase the selectivity of a series RLC circuit without affecting the resonant frequencies. Which component would you change and how (larger or smaller) and why?
Thank you, Steven

2. ### panic mode Senior Member

Oct 10, 2011
1,319
304
LC affect resonant frequency so do not change those.
by lowering R, you will increase Q factor of the circuit and increase selectivity.

3. ### WBahn Moderator

Mar 31, 2012
17,716
4,788
As panic mode says, you want to increase the Q without changing the resonant frequency, but do you understand why there is a relationship between Q and selectivity?

So start with the equations for the resonant frequency and for the Q of a series RLC circuit (hopefully you were required to either derive these equations yourself or at least had the derivations presented in class).

If you can only change one component, then can you change either the L or the C? because changing one and not the other will change the resonant frequency. You are therefore left with changing R. So now look at the equation for Q and see whether larger or smaller R will result in an increase in Q.

Now let's assume that you can't change R but that you can change both L and C. Would could you do then?