1
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 100
Logarithmic scale
Linear scale
A logarithmic scale compresses large values and allows a large range to be covered
without losing resolution for the smaller values.
40
Av db Linear scale
Frequency in Hz (log scale)10 100 1k 10k 100k 1Meg 10Meg
Cutoff Frequencies -3dB30
20
10
0
-10
-20
-30
-40
AvMid
20 db per decade rolloff
4
10 Hz 100 Hz 1 kHz 10 kHz 100 kHz
50 dB
40 dB
30 dB
20 dB
10 dB
0 dB
Ideal Bode plot of a dc amplifier with two break frequencies.
20 dB/decade
40 dB/decade
fb1 fb2
5
R
C
Amplitude responseof RC lag circuit
0 dB
-20 dB
-40 dB
-60 dB
10f2f2 100f2 1000f2
f2 = RC1
f2
f( )
1+
1AV =
6
0o
0.1f2 f2 10f2
Angular responseof RC lag circuit
-90o
-45o
R
C
f2
f = -arctan
Phase angle is between 0 and 90 degrees
Angle(Av) degrees
Frequency in Hz (log scale)10 100 1k 10k 100k 1Meg 10Meg
Cutoff Frequencies -3dB(f2)90
45
0
-45
-90Change at -45degrees/decade
