What importance is Q in practical circuits

Wednesday, November 06, 2013

Q. I have studied lessons 2315 and 2316 , and I have a question, I know that Q affects the impedance of a circuit, and that it makes a big difference in the currents and voltages in series and parallel circuits. But of what importance is Q in practical circuits?

A. The main use for RLC circuits is in building tuned amplifiers for radio circuits. In a receiver, a resonant circuit is tuned to the frequency of the station you wish to receive. The circuit basically selects the frequency to which it is tuned and rejects the signals of all the other stations on the band.

But a broadcast signal does not consist of a single frequency. When we put sound on the signal, it spreads the signal out over a range of frequencies. The receiver must tune in all these frequencies, yet tune out the signals from other stations on nearby frequencies. The Q of the tuned circuits in the receiver determines how "broad" the tuning is. The higher the Q, the narrower the range of frequencies that will get through the tuned circuit. The designation "Q" stands for "quality factor". So a higher-Q, more narrowly-tuned circuit has a higher "quality factor".

The Q of a tuned circuit is mainly determined by the resistance of the inductor. While the number of turns determines the inductance, the gauge of the wire determines its resistance. The thinner the wire used to make the coil, the higher the resistance, and the lower the Q. So the inductor's designer selects wire of the right size to obtain the desired Q. The Q also depends in part on the frequency of the signal. Radio-frequency currents tend to travel on the surface of conductors, a phenomenon known as "skin effect". Skin effect becomes more and more important at higher and higher frequencies. So in some cases, the wire used in the inductors may be plated with silver to minimize the resistance, and in turn raise the Q of the inductor.

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