Basic knowledge of filter在线视频

Hello, everyone!

In this lecture, I will focus on active filters.

Above all, let’s go to some basic concepts of filters.

The first is about its function.

The filter allows a certain frequency component of the signal to pass,

while other frequency components are greatly suppressed.

It is essentially a frequency selection circuit.

A filter is also called frequency selection device.

Here are several specific terms.

Passband—the frequency range that signals can pass.

Stopband—the frequency range that signals cannot pass.

Cutoff frequency—the boundary frequency between passband and stopband.

According to different operating frequency,

filters can be classified into low-pass filter, high-pass filter, band-pass filter, band-stop filter and all-pass filter.

The picture is amplitude-frequency characteristic of low-pass filter.

Its passband is the part where f is less than or equal to fc.

The signals ranging from 0 to fc are allowed to pass.

Next let’s go to the amplitude-frequency characteristic of high-pass filter.

Its passband is the part where f is more than or equal to fc.

The amplitude-frequency characteristic of band-pass filter is that

the signal with frequency ranging between f2 and f1 are allowed to pass.

This is band-stop filter

which does not allow signals with frequency between f1 and f2 to pass.

In these filters, passband gain is a constant and the stopband gain is 0.

This type of filter is called an ideal filter,

which actually does not exist at all.

Next I will explain the amplitude-frequency characteristic of actual filter by the example of a low-pass filter.

For actual filter, the passband gain has been attenuated near the cutoff frequency.

The stopband gain decay 0 at a certain rate but not decay to 0 directly.

By using different components, filters can be classified into passive filter and active filter.

The first is passive filter

which is composed of passive components like resistor, capacitor, and inductor.

This is a passive high-pass filter composed of resistor and capacitor.

Here are the advantages of it.

First, it is simple in circuit.

Second, it is good in high-frequency performance.

Third, it is reliable in work.

Its disadvantages include:

first, energy loss for passband signal

Because the output is always smaller than the input;

Different load resistances have different cut-off frequencies;

third, larger volume and weight.

Inductor can also cause electromagnetic interference.

An active filter is a filter composed of resistor, capacitor and active devices.

Its advantages include

small circuit size, light weight, amplifiable passband signal, high precision, stable performance and easier debugging.

The active filters can be connected in multiple stages without load effects.

Its shortcomings are the small passband range and the need for DC power.

So active filters are suitable for low frequency, low voltage and low power environments.