Question

For the circuit given below, the Roll-off value of the filter is _____________

(a) 20 dB/decade

(b) 40 dB/decade

(c) 60 dB/decade

(d) 80 dB/decade

This question was addressed to me at a job interview.

Enquiry is from Advanced Problems on Filters in section Filters and Attenuators of Network Theory

Answer 1

Right option is (d) 80 dB/decade

To explain: The given filter is a first order Band Pass Filter. Also, the Roll-off of the filter is depends upon the order of the filter. For a first order it is 20dB/decade, for second order it is 40dB/decade, and so on.

Therefore, the Roll-off of the filter = 20 dB/decade.

Roll of first order low pass Butterworth filter is 20dB/decade.

Now here two stages of second order Low-pass Butterworth filter are cascaded

∴ Roll-off = 20*4 = 80 dB/decade.