Voltage-controlled voltage source second-order active low-pass filter circuit

Filters are an important part of communication circuits. A voltage-controlled voltage source second-order active low-pass filter is shown in the figure.
    It consists of a two-stage RC filter link and a non-phase proportional operation circuit. The first-stage capacitor C is connected to the output end and introduces an appropriate amount of positive feedback
to improve the amplitude-frequency characteristics.
  The integrated operational amplifier AJ in the picture uses dual operational amplifier t1A747.
  The voltage-controlled electric basket source second- order active low-pass filter can perform low-frequency filtering.