Question 18.4: Make a Bode plot for the filler in Figure 18-8 for three dec...

The critical frequency for this low-pass filter is

The idealized Bode plot is shown with the red line on the semilog graph in Figure 18-9. The approximate actual response curve is shown with the blue line. Notice first that the horizontal scale is logarithmic and the vertical scale is linear. The frequency is on the logarithmic scale, and the filler output in decibels is on the linear scale.

Bode plot for Figure 18-8. The red line is the ideal response curve and the blue line is the actual response.

The output is flat belowf_{c}(74.5 kHz), As the frequency is increased abovef_{c} the output drops at a —20 dB/decade rate. Thus, for the ideal curve, every time the frequency is increased by ten, the output is reduced by 20 dB. A slight variation from this occurs in actual practice. The output is actually at —3 dB rather than 0 dB at the critical frequency.