The second order filter shown in the figure is suggested by Texas Instruments, based on the TL081 operational amplifier (LinMOS), with a gain of 1,586 or 4dB. From the cutoff frequency, the attenuation is 12dB per octave. The gain of the circuit can be changed, as it is given by the change in RB and RA, however, these resistors must have values that maintain the ratio indicated in the circuit. The power supply for this circuit must also be symmetrical and operational amplifiers from the same family can be used without problems.
