Second order RLC filter can be realized in series and parallel forms.
Suppose we need a high Q factor, if we choose a series form, we need the inductor to have a larger value than the capacitor and the resistor.
Conversely, if we choose a parallel form, we need the inductor to have a much lower value than the capacitor and resistor.
So in real application, how can we decide which form to use?
Thanks.
Best Answer
One wishes to choose a resonating inductance that is as small as possible, since the iron or ferrite required to achieve high-inductance often has undesirable characteristics. A large inductance may also require many windings of copper wire.
A resonator sometimes is mechanical, such as a piezoelectric crystal, or ceramic resonator. In these cases, inductance is often quite high, but cannot be easily chosen by the designer.
Both source resistance and load resistance must be considered, one or both are usually out of control of the designer. When both are low, the series RLC resonator yields a high-Q resonator with least inductance:
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When both source and load resistance are high, parallel RLC resonator yields a high-Q resonator with least inductance (below). Both these series and parallel example circuits have similar "loaded" Q.
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Where you have a crystal resonator, the equivalent inductive reactance is often very, very high. For reasonable source and load resistances found in electronic circuits, the series form is more reasonable than the parallel form:
simulate this circuit