Sallen Key filter
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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.
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Low-pass configuration
An example of the low-pass configuration is shown below:
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Sallen-Key.png
Image:Sallen-Key.png
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:
<math> F_c = \frac{1}{2\pi\sqrt{R_1R_2C_1C_2}} <math>
<math> Q = \frac{\sqrt{R_1R_2C_1C_2}}{C_2(R_1+R_2)} <math>
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.
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Sallen-Key-hp.png
Image:Sallen-Key-hp.png
The relevant equations are:
<math> F_c = \frac{1}{2\pi\sqrt{R_1R_2C_1C_2}} <math>
(as before), and
<math> Q = \frac{R_2C_x}{\sqrt{R_1R_2C_1C_2}} <math>
where
<math> C_x = \frac{C_1C_2}{C_1+C_2} <math>
See also
External links
- TI filter design source FAQ (http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/sk.htm)