Electronic Filters Explained with Types and Application

I have noticed that many students face problems when they reading Electronic Filter. Most of the students want to know about the Electronic Filter in a simple way. If you go to read the Electronic Filter from books then you may face many problems or you may not properly understand the electronic filter. Because most of the books have detailed explanation and many equations. So if you an electronic engineering student then you must read all about the electronic filter but if an electrical engineering student or have other stream and want to get the overall concept of the Electronic Filter then this article will be helpful for you.

In this post, I have tried to give you the overall concept of the electronic filter. So Let's go.

What is an Electronic Filter?

The Electronic Filter is an electrical or electronic circuit which allows to the passing of the signals of a predetermined frequency and rejects the all other frequency signals.

We know that any electronic signals have some frequency. Most of the cases we used only the signal having a particular frequency.
In the signal transmission purpose, we send and receive high-frequency signals to get better bandwidth and to send or receive a large amount of data. But after receiving the signals we have required to filter those signals to get the signal of a particular required frequency.

For example, we received a signal of 20MHz but we need up to the 10MHz signal. So in this case, we use a filter circuit which allows up to 10 MHz signals and rejects beyond the 10MHz signals.

Types of Electronic Filter:

According to the circuit element types, the electronic filter classified into two types,
1. Active Filter
2. Passive Filter

Active Filter:

The filter circuit which is made by active elements like Transistors, Opamp, diodes etc is called Active Filter.

Types of Active Filter:

According to signal filtering characteristics, the Active Filter is classified into four types,
1. Active Low Pass Filter
2. Active High Pass Filter
3. Active Band Pass Filter
4. Active Band Reject Filter

Passive Filter:

The filter circuit which is made by passive elements like resistors, capacitors, inductors is called Passive Filter.

Types of Passive Filter:

1. Passive Low Pass Filter
2. Passive High Pass Filter
3. Passive Band Pass Filter
4. Passive Band Reject Filter
So from the above concept, we can say there are four types of Electronic Filter which are,
1. Low Pass Filter
2. High Pass Filter
3. Band Pass Filter
4. Band Reject Filter

These above filters can be designed either taking active elements or passive elements. These basic four types of filter we will discuss.

Low Pass Filter :

The filter circuit which allows passing the signals having the low frequency (starting from 0 to predetermined cut-off frequency) and rejects the signal having the frequency beyond that cut-off frequency is called Low Pass Filter.

For example, a 50MHz signal is present but we need the signal up to 25MHz. In this case, we have needed a low pass filter which will allow the passing of the signal having the frequency from zero(0) to 25MHz and will reject the signal having the frequency beyond 25 MHz.

Frequency Spectrum of low pass filter is shown below,

click on the image to enlarge

Examples of Low Pass Filter:

1. RC Low Pass Filter: This is the filter circuit which is designed by resistor and capacitor.
2. RL Low Pass Filter: This is the filter circuit which is designed by Resistor and inductor.
3. RLC Low Pass Filter: This is the filter circuit which is designed by resistor, capacitor, and inductor.
To better understand the working of low pass filter let's discuss RC Low pass filter,

 RC low pass filter circuit diagram

As you see in the above figure, the circuit has a capacitor is connected in parallel. The working of this circuit is very simple. According to the below equation, we know that the reactance of the capacitor is inversely proportional to the frequency.

So when a low-frequency signal is passing through the circuit the reactance of the capacitor will be high and we will get the output signal same as the input signal. But when a high-frequency signal is passing through the circuit the reactance of the capacitor will be low so the current( electronic signal is a flow of current) will flow through the capacitor or we can say the capacitor complete the circuit so we will not get any output.
You can also understand the principle by the below equation,

Applications of Low Pass Filter:

1. Low Pass Filter used in Digital Signal Processing
2. It used in Audio Equalization purpose.
3. It is used to reduce high-frequency noise in Audio.
4. It is used as anti-aliasing filters in Analog to Digital Conversion.

High Pass Filter:

The filter circuit which allows passing the signals having the high frequency starting from a predetermined cut-off frequency and rejects the signals having the frequency below that cut-off frequency is called High Pass Filter.

Suppose we have signals of 50MHz but we need the signal beyond 25MHz. In this case, we have needed a High Pass Filter which will reject the signals having the frequency below 25MHz and allow the beyond 25MHz signals.

Frequency Spectrum of High Pass Filter is shown below,

click on the image to enlarge

Examples of High Pass Filter:

1. RC High Pass Filter
2. RL High Pass Filter
3. RLC High Pass Filter
To better understand the working of the high pass filter let's discuss the RC high pass filter,

 RC high pass filter circuit diagram

As you see in the above figure a capacitor is connected in series. So when a high-frequency signal is flowing through the circuit, the reactance of the capacitor will be low and we will get the output signal but when the input signal has low frequency the reactance of the capacitor will be high and as it is connected in series it will oppose the flow of the current and we will not get output.

Applications of High Pass Filter:

1. High pass filter is used in digital image processing.
2. It used to reduce low-frequency noise
3. It is used as the treble booster circuit in the audio system.

Band Pass Filter:

The filter circuit which allows passing the signals having the frequency which is under a certain band and rejects the signals having the frequency which is outside of that band is called Band Pass Filter.

We have signals of 60MHz but we need the signals of 20-30MHz. So we have needed a band pass filter which will pass the 20-30MHz signals and will reject below the 20MHz and beyond the 30MHz signals.

Frequency Spectrum of Band Pass Filter is shown below,

click on the image to enlarge

Examples of Band Pass Filter:

1. RC Band Pass Filter
2. RL Band Pass Filter
3. RLC Band Pass Filter

We can easily make a Band Pass filter using an RC low pass filter and an RC high pass filter.

 RC Band pass filter circuit diagram

As you see in the above figure the circuit has a series capacitor and a parallel capacitor. The series capacitor can reject the low-frequency signal and the parallel capacitor can reject the high-frequency signal. By choosing the appropriate value of the capacitors we can make a frequency band at which the signals can pass through the circuit.

Applications of band Pass Filter:

1. In the telephone system, Band Pass filters are used.
2. It is used in SONAR system
3. It is used in Audio Technology

Band Reject Filter:

The filter circuit which rejects the signals having the frequency which is under a certain band and allows passing the signals having the frequency which is outside of that band is called Band Reject Filter.

We have signals of 100MHz but we need the signal of 0-30MHz and 60-100MHz. So we need a Band Reject filter which will reject the signals having the frequency of the band 31-59MHz and will allow the signals having the frequency of outside of the 31-59MHz band.

Frequency Spectrum of  Band Reject Filter is shown below,

click on the image to enlarge

Example of Band Reject Filter:

1. RLC Band Reject Filter
We can use an RLC parallel circuit as a Band Reject filter. In Parallel Resonance condition the circuit act as Band Reject filter circuit. The output can be obtained across the resistor.

 RLC Band reject filter circuit diagram

Applications of Band Reject Filter:

1. In Telecommunication system, Band Reject Filters are used.