4 order filter with a single IC

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Filters with high orders are designed usually with 2 or more cascaded sections. This order 4 filter need only one OA IC , so we can achieve lower distorsions, lower intermodulation …
Resistors values represent the load on the OA output, the maximum TL081 load is 2kΩ . R1-R4 build a 2.5kΩ impedance and so the external load cannot be lower than 10kΩ .

The filter characteristic is a Bessel polinome. With the actual components value, at -3dB the frequency is 1kHz. You can obtain different frequencies by changing the components value.

4-th order filter circuit schematic

4 order filter schematic