ECE 3200
Sallen-Key
S13
Part 2
With the process for solving a 2 order Butterworth filter using a single stage SallenKey, we will try to construct a 4th order Low Pass Butterworth filter with unity gain and a
corner frequency at 1 kHz. A 4th order Butterworth filter has a normalized denominator
of:
nd
( )
(
) (
)
A two stage Sallen-Key will be used to implement the filter, meaning the stages will be
one of the 2nd order equations defined above. Again we should try to make C1 a
standard capacitor value. We will start by letting C 1 be 4.7nF.
Stage 1
(
)
(
)
Stage 2
The two stages act together to form the 4th order filter. Cascading two 2nd order
Butterworth filters does not make a 4th order filter.
1
ECE 3200
Sallen-Key
S13
The 4th order Butterworth filter breaks at 1 kHz, descends at a slope of 80dB/dec, and
has unity gain.
2