AC2 Fundamentals Unit 5 – Low- and High-Pass Filters
The cutoff frequency (fc) is the point where the output voltage (Vo) drops to 70.7% of, or 3 dB
down from, the input voltage.
Frequency response data may be expressed in terms of output voltage but is usually expressed in
decibels (dB). Decibels are units that express or measure the gain or loss (attenuation) in a
circuit. The decibel can be based on the ratio of the output voltage (Vo) to the input voltage (Vi).
Vo
Attenuation (dB) = 20 log 10 ───
Vi
NOTE: In the type of filters studied in this volume, the output voltage (Vo) is always less than
the input voltage (Vi).
The rate of attenuation, or loss, beyond the cutoff frequency (fc) is highly predictable. This
attenuation is 6 dB per octave or 20 dB per decade. An attenuation rate of 6 dB per octave is
the same rate as 20 dB per decade.