Useful information | This article explains low-pass, high-pass, band-pass, band-stop, and state-adjustable filters

Latest update time：2019-10-22

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Second-order voltage-controlled low-pass filter

The second-order voltage-controlled low-pass filter circuit is shown in the figure. R1, C1 and R2, C2 respectively form two first-order low-pass filters, but C1 is connected to the output end to introduce voltage positive feedback to form a voltage-controlled filter.

(1) Transfer function

(2) Frequency characteristics

It can be seen that the characteristic of this low-pass filter is that the damping coefficient ζ is determined by the ratio of resistors R1, R2, C1, C2; and the natural frequency ω0 is related to the specific values of R1, R2, C1, C2, that is, ω0 and ζ are independently adjustable and do not affect each other.

(3) Parameter selection

For the convenience of parameter matching, considering the small number of nominal capacitors, C1=C2=C is generally selected. By selecting different R1 and R2, specific natural frequencies ω0 and ζ can be met.

Unity-gain second-order voltage-controlled low-pass filter

For a second-order voltage-controlled low-pass filter, when the passband gain Aup = 1 (unit gain), the circuit shown in the figure becomes as shown in the figure, where RF = R1 + R2.

(1) Basic Relationship

(4) Parameter selection

When the natural frequency ω0 and damping coefficient ζ are known, the design steps are as follows:

Second-order low-pass filter

(1) Transfer function

(2) Frequency characteristics

4 Infinite Gain Multi-Feedback Low Pass Filter

(1) Transfer function

(2) Frequency characteristics

Source: Internet

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