A Sallen and Key filter is a type of active filter, particularly valued for its simplicity. The circuit produces a 2-pole (12dB/octave) lowpass or highpass response using two resistors, two capacitors and a unity-gain buffer amplifier. Higher-order filters can be obtained by cascading two or more stages.
All Sallen and Key filters have a gain of 1 in the passband. They are relatively resilient to component tolerance, although obtaining high Q factor may require extreme component values.
Low-pass configuration
An example of the low-pass configuration is shown below:
An operational amplifier is used as the buffer here, although an emitter follower is also effective. The cutoff frequency and Q factor are given by the following equations:
So, for example, the above circuit has an Fc of 15.9Khz and a Q of 0.5.
High-pass configuration
An example high-pass filter with an Fc of 72Hz and Q of 0.5 is shown below.
The relevant equations are:
(as before), and
where
See Also
External link
