12DB/OCT low-pass filter composed of OP amplifier

Function of the circuit

This circuit has been used as a basic circuit for active filters. It can be used as a carrier filter for various demodulation circuits, a low-pass filter for noise removal in sensor amplifiers, and a simple anti-folding filter. It is equivalent to the A part of the buffer amplifier. In the past, voltage output circuits such as emitter followers were used. Here, an OP amplifier is used to reduce DC drift.

How the Circuit Works

The Q value of the 12DB/OCT Butterworth filter circuit must be 0.707 to obtain the flattest characteristics, and for C2, the feedback capacitor C1 should be large to ensure that the specified Q value is achieved. The Q value is calculated by the formula Q=√C1/C2/3. In order to make Q=0.707, C1 and C2 must establish the relationship of C1=4Q2. C2=2C2. When calculating the actual parameters, calculate according to C1=2QXFL.RO, C2=1/2Q*2πFLRO, so that C1 and C2 are unbalanced, so the Q value can be verified in reverse.

Although the selection of the cut-off frequency has a certain degree of freedom, if the resistance of RO is too large, it will be affected by the input bias current of the OP amplifier and produce imbalance. I.RO should be calculated to analyze whether there is a problem. Here, R2 is added to the feedback loop to offset the input bias current. If a FET input OP amplifier is selected, the input bias current is very small, only a few hundred picoamperes, and R3 can be removed. The role of C2 is to suppress pulse spikes, and the base capacitance range is 22~51PF.