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