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1050 ⏐⏐⏐ DECIBELS, FILTERS, AND BODE PLOTS<br />

90°<br />

45°<br />

0°<br />

θ θ (Vo leads Vi )<br />

10 Hz<br />

+<br />

V i<br />

–<br />

R<br />

C<br />

FIG. 23.51<br />

Low-pass filter.<br />

100 Hz fc = 159.15 Hz<br />

10<br />

+<br />

V o<br />

–<br />

b. Eq. (23.33):<br />

1<br />

|AvdB| � 20 log10 �� � 20 log10 �1� � � �� �f �<br />

c ��� f<br />

2<br />

1<br />

���<br />

� 1 � � � �� �1<br />

� 5<br />

� 9<br />

�1 1<br />

� 0<br />

.5<br />

0� 5<br />

0� H<br />

� z<br />

dB<br />

� �� 2<br />

1<br />

� 20 log10�� � 20 log10 0.5318 � �5.49 dB<br />

2 �1� �� (�1�.5�9�2�) �<br />

as verified by Fig. 23.49.<br />

c. See Fig. 23.50. Note that v � 45° at f � fc � 1591.55 Hz, and the<br />

difference between the straight-line segment and the actual response<br />

is 5.7° at f � fc/10 � 159.2 Hz and f � 10fc � 15,923.6 Hz.<br />

Difference = 5.7°<br />

1 kHz<br />

f c = 1591.55 Hz<br />

Low-Pass R-C Filter<br />

For the low-pass filter of Fig. 23.51,<br />

V o<br />

45°<br />

FIG. 23.50<br />

Phase plot for the high-pass R-C filter.<br />

�jX C<br />

10 kHz<br />

1<br />

Av � � � � �–– � 1<br />

Vi R � jXC R<br />

�� �j<br />

1 1 1<br />

� –––––––– � –––––––––– � –––––––––––<br />

f<br />

1 � j 1 � j 1 � j ––<br />

1<br />

�<br />

2pRC<br />

�<br />

R<br />

R<br />

� —<br />

XC<br />

1<br />

�� 2pfC<br />

1<br />

and Av � ��<br />

(23.37)<br />

1 � j ( f/fc)<br />

1<br />

with fc � � (23.38)<br />

2pRC<br />

�<br />

as defined earlier.<br />

Note that now the sign of the imaginary component in the denominator<br />

is positive and f c appears in the denominator of the frequency<br />

ratio rather than in the numerator, as in the case of f c for the high-pass<br />

filter.<br />

X C<br />

�<br />

Difference = 5.7°<br />

f (log scale)<br />

10 f c = 15,915.5 Hz<br />

100 kHz

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