What is Q- factor? what defines its sharpness? what are the uses of it?
Resonant circuits are used to respond selectively to signals of a given frequency while discriminating against signals of different frequencies. If the response of the circuit is more narrowly peaked around the chosen frequency, we say that the circuit has higher "selectivity". A "quality factor" Q, is a measure of that selectivity, and we speak of a circuit having a "high Q" if it is more narrowly selective.
An example of the application of resonant circuits is the selection of AM radio stations by the radio receiver. The selectivity of the tuning must be high enough to discriminate strongly against stations above and below in carrier frequency, but not so high as to discriminate against the "sidebands" created by the imposition of the signal by amplitude modulation.
The selectivity of a circuit is dependent upon the amount of resistance in the circuit.
The Q is a commonly used parameter in electronics, with values usually in the range of Q=10 to 100 for circuit applications.