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# Simple Filter and 2 Differente Sources

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#### dshoter13

##### Member level 4
Hi,

I have a simple doubt about how filters act when driven by current sources or voltage sources.
Let's imagine a simple case like a Resistor in parallel with a LC tank. Now, let's imagine that in the first scenario the filter is being driven by a voltage source, and the second case it is being driven by a current source. What is the basic differente, and how can we analytically describe the basic differences?

P.S.- If you know some book that treats this subject, I would be appreciated.

With best regards.

You need to define the output too. Because in the configuration you mentioned, if you take voltage output, voltage source wouldn't interact with the filter at all.

But in general to be able to understand it intuitively I usually think of parallel LC as a resistor that is short for every frequency except the tuning frequency, and series LC as a resistor that is open for every frequency except the tuning frequency.

So if you drive with a current source and take "voltage across LC" as output, you'd see a peak at tuning frequency, but since you have that resistor over there that peak is not as sharp as or not as infinite as (stupid I know) it would be had there been no resistors anywhere. Even if you don't put that resistor, the limited Q values of LC shows up as resistor, reducing your Q (recursive answer because that's the definition of Q).

To explain other scenarios you have to define them.

dshoter13

### dshoter13

Points: 2
If you connect a voltage source to a R and a L and a C all in parallel, then each one will take a current depending on its resistance or impedance. At some frequency when the impedances of the C and L are equal they will take a much smaller current as their part of the circuit is at resonance and will have an impedance of L/C r where r is the loss resistance of the L.
If you use a constant current generator, The voltage across the circuit will peak when C and L resonate. As above the impedance of the C and L at resonance is L/C r and this in parallel with the R times the current will give a high voltage.
Frank

dshoter13

### dshoter13

Points: 2
Thank you kemiyun and chuckey. It helped. It was just to understand the basic differences.
Thank you again.

With best regards.

Parallel resonant LC circuits are generally driven by current sources ( e.g. open collector via coil to V+) such that Q= Rload/X(f) where impedance X(f) is for either C or L at resonance. and preferred for low f.

Series resonant LC circuits are driven by voltage sources so ESR or total series R determines the Q = X(f)/ESR and preferred for high f

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