Filters are very common in application circuits. Filters are electronic devices that allow useful frequency signals to pass through while suppressing unwanted frequency signals, and are commonly used in signal processing, data transmission, and interference suppression. Simply put, it is the equivalent of a filter, filtering out what you don’t need and leaving what is useful to you.
With the widespread use of integrated op amps, active filters are more widely used. An active filter is actually an amplifier with a specific frequency response. It is composed by adding some passive components such as R and C on the basis of operational amplifier. Usually active filters are divided into: (classified by passband) low-pass filter (LPF); high-pass filter (HPF); band-pass filter (BPF); band-resistance filter (BEF), we mainly discuss low-pass filter.
Among them, the active low-pass filter circuit consists of an integrated op-amp and passive components resistors and capacitors. Its function is to allow the signal from zero to a certain cutoff frequency to pass without attenuation, while the signal at other frequencies has a suppression effect, the active low-pass filter circuit can be used to filter out high-frequency interference signals. As shown in the figure below, the active low-pass filter circuit is actually below a certain critical frequency point can be passed, while the critical frequency point above the filtered out.
1、 active low-pass filter (lpf) working principle and role – the classification of the filter
According to the classification of components, filters can be divided into: active filters, passive filters, ceramic filters, crystal filters, mechanical filters, phase-locked loop filters, switched capacitor filters, etc.
According to the classification of signal processing methods, filters can be divided into: analog filters, digital filters.
According to the classification of passband, filters can be divided into: low-pass filters, high-pass filters, band-pass filters, band-reject filters, etc.
In addition, there are some special filters, such as special filters to meet certain frequency response characteristics, phase shift characteristics, for example, linear phase shift filters, time delay filters, audio fork network filters, television in the amplification of the sound surface wave filter, etc..
At the same time, active filters can be divided into many sub-categories.
According to the passband classification, active filters can be divided into: low-pass filters (LPF), high-pass filters (HPF), band-pass filters (BPF), band-resistance filters (BEF), etc.
According to the classification of passband filter characteristics, active filters can be divided into: maximum flat type (Butterworth type) filters, equal ripple type (Chebyshev type) filters, linear phase shift type (Bessel type) filters, etc.
According to the composition of the op amp circuit, active filters can be divided into: infinite gain single feedback loop filter, infinite gain multi-feedback loop filter, voltage controlled power supply filter, negative resistance converter filter, slew type filter, etc.
This article is mainly to explain the active low-pass filter (lpf) in detail.
2、 active low-pass filter (lpf) working principle
Active filter is to use current transformer to collect the current on the DC line, after sampling, the resulting current signal for harmonic separation algorithm processing, to get the harmonic reference signal, as the modulation signal, compared with the triangular wave, so as to get the switching signal, with this switching signal to control the single-phase bridge, according to the principle of technology, the upper and lower bridge arms of the switching signal reversed, you can get and the line harmonic signal of equal size, the The harmonic current of equal size and opposite direction will be obtained, and the harmonic current on the line will be cancelled out. This is the feedforward control part. Then the harmonic component of the line current after the access point of the active filter is fed back as the input of the regulator to adjust the error of the feedforward control.
In a nutshell, it is the principle of using capacitors with high frequency to block low frequency and inductors to pass low frequency to block high frequency. For the need to cut off the high frequency, the use of capacitance absorption inductance, blocking the method does not make it through, for the need of low frequency, the use of capacitance high resistance, inductance low resistance characteristics to make it through.
An ideal filter should have a uniform and stable gain at the required frequency (within the pass, and an infinite attenuation outside the passband. However, the actual filter is a certain difference from this, for this reason people use a variety of functions to approximate the frequency characteristics of the ideal filter.
3、the role of active low-pass filter (lpf)
A filter is a frequency-selective device that allows specific frequency components of a signal to pass through, while greatly attenuating other frequency components. Using this frequency selection role of the filter, you can filter out interference noise or spectrum analysis.
1、Separate the useful signal from the noise to improve the signal’s anti-interference and signal-to-noise ratio.
2、Filtering out the uninteresting frequency components to improve the analysis accuracy.
3、Separate the single frequency component from the complex frequency component.
For example: the figure below is an active low-pass filter, the main function is to low-pass filter the audio signal output from the audio decoder chip CS4360, and filter out the useless high-frequency signal.
4、active low-pass filter (lpf) working principle and role – the main difference between low-pass filter and high-pass filter
There are many kinds of low-pass filters, of which the most common are Butterworth and Chebyshev filters. Low-pass filters are electronic filtering devices that allow signals below the cutoff frequency to pass, but signals above the cutoff frequency cannot pass.
Low-pass filter (Low-pass filter) is a filtering method, the rule is that the low-frequency signal can pass normally, while the high-frequency signal above the set threshold is blocked and attenuated. However, the amplitude of the blocking and attenuation will change according to different frequencies and different filtering procedures (purposes). It is also sometimes called high-cut filter or treble-cut filter. Low-pass filtering is the antithesis of high-pass filtering.
Low-pass filtering can be simply considered: set a frequency point, when the signal frequency higher than this frequency can not pass, in the digital signal, this frequency point is also the cut-off frequency, when the frequency domain is higher than this cut-off frequency, then all assigned to 0. Because in this process, let the low frequency signal all through, so called low-pass filtering.
The meaning of ‘low’ and ‘high’ – for example, the cutoff frequency – depends on the characteristics of the filter. (The term “low-pass filter” simply refers to the shape of the filter response. A high-pass filter can be designed to have a lower cutoff frequency than any low-pass filter cutoff frequency. The different frequency response is what distinguishes them.) Electronic filters can be designed for any desired frequency range – up to microwave frequencies (over 1000 MHz) and beyond.
High and low pass filters are determined by the order number of the denominator.
Conversion function of the denominator order number is higher than the numerator order number for the low-pass filter, and the greater the difference between the better the filtering effect, the conversion function of the denominator order number is lower than or equal to the numerator order number for the high-pass filter.
Filter circuit is often used to filter out the ripple in the rectified output voltage, generally composed of reactance components, such as a capacitor C in parallel with the load resistor, or in series with the load inductor L, as well as a combination of capacitors, inductors and a variety of complex filter circuit. The simplest of these filter circuits are as follows.
5、 Active low-pass filter (lpf) working principle and function —- simple filter circuit
When the current flowing through the inductor changes, the inductive electromotive force generated in the inductor coil will prevent the current from changing. When the current through the inductor coil increases, the self-induced electromotive force generated by the inductor coil is opposite to the direction of the current, preventing the current from increasing, while converting part of the electrical energy into magnetic energy stored in the inductor; when the current through the inductor coil decreases, the self-induced electromotive force is in the same direction as the current, preventing the current from decreasing, while releasing the stored energy to compensate for the decrease in current.
Therefore, after inductor filtering, not only the pulsation of load current and voltage is reduced, the waveform becomes smooth, and the conduction angle of rectifier diode is increased.
In the case of constant inductor coil, the smaller the load resistance, the smaller the AC component of the output voltage. The larger the L, the better the filtering effect.
6、active low-pass filter (lpf) working principle and role — active low-pass filter circuit case study
To obtain good filtering characteristics, a higher order is generally required. The design calculation of the filter is very troublesome and can be done with the help of engineering calculation curves and relevant computer-aided design software when needed. The number of filter stages is determined mainly by the requirement for out-of-band attenuation specificity. Each order of low-pass or high-pass circuit can obtain -6dB per octave (-20dB per decade) of attenuation, and each second-order low-pass or high-pass circuit can obtain -12dB per octave (-40dB per decade) of attenuation. When the multi-stage filter is connected in series, the total number of orders of the transfer function is equal to the sum of the orders of all levels. When the required out-of-band attenuation characteristics of -mdB per octave (or mdB per decade), the number of steps n should meet n greater than or equal to m/6 (or n greater than or equal to m/20).
