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ƒ r<br />

reactive power, the larger the Q s factor and the more concentrated and<br />

intense the region of resonance.<br />

Substituting for an inductive reactance in Eq. (20.8) at resonance<br />

gives us<br />

Q s �<br />

and Qs � � (20.9)<br />

XL<br />

� � �<br />

R<br />

qsL<br />

�<br />

R<br />

If the resistance R is just the resistance of the coil (R l), we can speak<br />

of the Q of the coil, where<br />

R � R l<br />

(20.10)<br />

Since the quality factor of a coil is typically the information provided<br />

by manufacturers of inductors, it is often given the symbol Q without an<br />

associated subscript. It would appear from Eq. (20.10) that Ql will<br />

increase linearly with frequency since XL � 2pfL. That is, if the frequency<br />

doubles, then Ql will also increase by a factor of 2. This is<br />

approximately true for the low range to the midrange of frequencies such<br />

as shown for the coils of Fig. 20.6. Unfortunately, however, as the frequency<br />

increases, the effective resistance of the coil will also increase,<br />

due primarily to skin effect phenomena, and the resulting Ql will<br />

decrease. In addition, the capacitive effects between the windings will<br />

increase, further reducing the Ql of the coil. For this reason, Ql must be<br />

specified for a particular frequency or frequency range. For wide frequency<br />

applications, a plot of Ql versus frequency is often provided. The<br />

maximum Ql for most commercially available coils is less than 200, with<br />

most having a maximum near 100. Note in Fig. 20.6 that for coils of the<br />

same type, Ql drops off more quickly for higher levels of inductance.<br />

If we substitute<br />

qs � 2pfs and then f s �<br />

into Eq. (20.9), we have<br />

Qcoil � Ql � � XL<br />

�<br />

R<br />

q<br />

�<br />

sL<br />

R<br />

Q s � � � � � L<br />

�L�<br />

�<br />

�L�<br />

L 1<br />

� � ��� �<br />

� �L�C�<br />

�<br />

1 L<br />

and Qs � ����� R C<br />

� R<br />

2pf<br />

�<br />

sL<br />

R<br />

I 2 X L<br />

� I 2 R<br />

1<br />

� 2p�L�C�<br />

2p 1<br />

� �<br />

R 2p�L�C�<br />

L<br />

� R�L�C�<br />

(20.11)<br />

providing Q s in terms of the circuit parameters.<br />

For series resonant circuits used in communication systems, Q s is<br />

usually greater than 1. By applying the voltage divider rule to the circuit<br />

of Fig. 20.2, we obtain<br />

l<br />

Q l<br />

THE QUALITY FACTOR (Q) ⏐⏐⏐ 891<br />

100<br />

80<br />

60<br />

1 mH<br />

40<br />

20<br />

1 H 100<br />

mH<br />

10 mH<br />

0<br />

5 10 25 50 100 250 500<br />

Frequency (kHz) (log scale)<br />

FIG. 20.6<br />

Q l versus frequency for a series of inductors<br />

of similar construction.

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