Modified Sallen-Key

Hello you all!

Despite the unusual layout, you may recognize the common Sallen-Key biquadratic cell, here as a low-pass:

s is j*2pi*F and A is the attenuation Vin/Vout or 1/H.

The low-pass Sallen-Key worsens if the resistors differ, so the Q factor resides in the capacitor ratio only, but precise capacitors are often limited to the E3 series (10-22-47).

The modification I used for >20 years improves that by adding one buffer:

Now the resistors can usefully differ. Most often, a single capacitor value fits the biquad cell and even the complete filter. Big Q-factors can result from the combined resistor and capacitor ratios, the latter being simple like 1 or 10. Store fewer expensive components.

Additionally, the Q-factor accepts component ratios four times lower. This as well helps to keep reasonable capacitances. The main drawback of the Sallen-Key-Schaefer is the 14-lead chip for two biquadratic cells instead of an 8-lead chip.

A high-pass can be made this way, but the original Sallen-Key already takes identical capacitors as a high-pass.

No band-pass is possible, neither with the modification, and a low-pass with notch neither.

The Multiple-Feedback lowpass biquad as well can improve with the added buffer - on the paper, as I haven't tried that one.

Marc Schaefer, aka Enthalpy

Edited October 7, 201213 yr by Enthalpy