# Can passive filter magnify signals?

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#### xiongyw

##### New Member
Hi,

I am studying basic electronics in spare time.

Enclosed is a simple RLC circuit I made by MultiSim (the capacitance and inductance values are chosen intentively small and big). It seems the amplitudes (regardless the phase shift here) of the voltage drops on C and L can be very large, as compared to that of the input voltage. That means, if this circuit is used as a filter network, then we can get magnified signal from C or L. That means we get both filtering and amplification functionality from the passive network. My question is, is this correct, or if this kind of configratuion often used in pracitical circuits? If not, why?

Btw, yes, I am aware of that, if only C or L is used with R, the amplitude of the voltage drop on either of them (R and C or L) can not be bigger than the amplitude of the input voltage (i.e., according to side length relationship of right-angled triangles).

Thanks,
/bruin

#### MikeMl

##### Well-Known Member
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.

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#### xiongyw

##### New Member
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

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#### Tesla23

##### Member
I had this question is because I have a (wrong) impression that the gain in amplitude of a filter is always less than 0dB.

The power gain of a passive circuit is never greater than 0dB.

The voltage or current gain may be greater than 0dB. Think of a simple transformer as an example.

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