Electronic – Gain in Sallen-Key filter


How can we increase the passband gain in a Sallen-Key type low pass / high pass filter, preferably without using additional op-amps.

Best Answer

The Sallen-Key filter can be realized with gain but component value adjustments are necessary to reduce Q. Q rises as gain increases from unity and the feedback capacitor is usually the component to lower. Another way of looking at this is by thinking about the feedback capacitor - it's the only component that is affected by gain - if the gain were (say) 10 and the feedback signal to the cap were reduced by 10 the circuit would perform the same theoretically. Reducing the cap or potting down the input to the cap has the same effect.

Here is a diagram from TI source: -

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As you can see, the frequency cut-off is exactly the same as the unity gain circuit but Q is now dependent of gain 1+R4/R3.

The source (TI) linked above should allow you to derive the high pass version