Thread: Can passive filter magnify signals?

The magnitude of the voltage across the inductor and capacitor can be larger than the input voltage only because they have opposite phases. AC voltages are vectors, meaning they have both magitude and phase, so a series RLC circuit still satisfies Kirchoff's voltage law (sum of vector voltages around the loop = zero). Practical circuit, tuned stages in a radio utilize the voltage step-up across a high Q LC circuit.

Originally Posted by MikeMl

The magnitude of the voltage across the inductor and capacitor can be larger than the input voltage only because they have opposite phases. AC voltages are vectors, meaning they have both magitude and phase, so a series RLC circuit still satisfies Kirchoff's voltage law (sum of vector voltages around the loop = zero). Practical circuit, tuned stages in a radio utilize the voltage step-up across a high Q LC circuit.

Thanks for the information, Mike!

I had this question is because I have a (wrong) impression that the gain in amplitude of a filter is always less than 0dB. I will check out the tune stage circuits used in radio.

Actually this question emerged when I was trying to summarize what I learnt so far (as the attached picture). I was trying to answer a question raised during the process (item in red): why using L and C together in a series circuit when they voltage drops cancel each other? Can you also please give some hints or clarify a bit on that? It will also be very helpful if you can review the other items to see if there are faults or missed points.


Thanks again,
/bruin