Electronic Filters Explained-High Pass, Low Pass, Band Pass, Band Reject Filters

Electronic Filters Explained with Types and Application

I have noticed that many students face problems when they read 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 explanations and many equations. So if you are an electronic engineering student then you must read all about the electronic filter but if an electrical engineering student or have another 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 that allows the passing of the signals of a predetermined frequency and rejects all other frequency signals.

We know that any electronic signals have some frequency. In most of the cases, we used only the signal having a particular frequency.
In the signal transmission purposes, 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 are 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 that allows up to 10 MHz signals and rejects beyond the 10MHz signals.

Types of Electronic Filters:

According to the circuit element types, the electronic filter is classified into two types,

1. Active Filter
2. Passive Filter

Active Filter:

The filter circuit which is made of active elements like Transistors, opamps, 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 of passive elements like resistors, capacitors, and 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 by taking active elements or passive elements. These basic four types of filters we will discuss.

Low Pass Filter :

The filter circuit that allows passing the signals having the low frequency (starting from 0 to a predetermined cut-off frequency) and rejects the signal having a 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.

The frequency Spectrum of the 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 that is designed by the Resistor and inductor.
3. RLC Low Pass Filter: This is the filter circuit that is designed by a resistor, capacitor, and inductor.

To better understand the working of a low pass filter let's discuss the 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 the 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 completes 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 is used for Audio Equalization purposes.
3. It is used to reduce high-frequency noise in Audio.
4. It is used as an anti-aliasing filter in Analog to Digital Conversion.

High Pass Filter:

The filter circuit that allows passing the signals having a high frequency starting from a predetermined cut-off frequency and rejects the signals having a 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 a frequency below 25MHz and allow the beyond 25MHz signals.

The frequency Spectrum of the 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 that allows passing the signals having a frequency that is under a certain band and rejects the signals having a frequency that 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 that will pass the 20-30MHz signals and will reject below the 20MHz and beyond the 30MHz signals.

The frequency Spectrum of the 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 that 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 that 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.


The frequency Spectrum of the 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 the Telecommunication system, Band Reject Filters are used.

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