Filter circuit diagram for obtaining four characteristics simultaneously

As shown in the figure, this is a filter circuit that can obtain four characteristics at the same time . This circuit is a filter circuit that can obtain four characteristics of bandpass (BPF), lowpass (LPF), highpass (HPF) and bandstop (BEF) at the same time. The circuit is composed of 4 operational amplifiers. Among them, RF and CF determine its center frequency f0, f0=1/2πRFCF. The Q value in the circuit is determined by R5/R2, and the gain is determined by R2/Rl. R5=10kΩ, so Q=1, then for BPF, LPF and HPF. The output is 0dB. When the input is a sine wave, the BEF output is 0, and the other filters output the same voltage. There is a 90o phase difference between LPF and HPF. When LPF and HPF are used to separate low frequency and high frequency, the loss of f0 is -3dB, so Q=0.7, but at this time R5=7kΩ. The frequency is adjusted by RF and CF in the circuit, but it is generally limited to tens of kilohertz, especially A3 and A4 should use operational amplifiers with good high-frequency characteristics.