What Does an Electronic Filter Do?

Electronic filters are a device for signal processing and filtering in the form of an electrical circuit. This circuit uses components and connects them into specific configurations for specific purposes. After they are interconnected, they are considered as a single device and can be analyzed.

Filter circuit is a very helpful AC circuit if we want to carefully be selective on filtering one range of frequencies mixed in our electrical circuit. A circuit that is capable of filtering a range of frequency based on our selection, is known as a filter circuit.

We will very depend on the filter circuit if we are designing an audio system where a noise can disturb our audio output. A certain range of frequencies need to be taken out from our circuit for our desired sound quality and better power consumption.

The very common example of filter circuits in audio systems is an equalizer. This device can help us amplify a specific range of frequencies to suit our audience’s ear.

Other examples aside from an equalizer are tweeter, crossover network, active and passive filter.

Tweeter (a high-frequency speaker) is great at producing high frequency signals. But we cannot use this directly to produce high frequency. Since it is excellent at producing high-frequency sound, it is terrible at producing low-frequency signals. This is why we use a crossover network between a tweeter and speaker’s output terminal.

With this configuration, our circuit is able to block low-frequency signals and only passes high-frequency signals.

A crossover network is able to block a specific range of frequencies to not pass to the output. With these, we can build an audio system with good efficiency and good performance at the same time.

We will learn about what an electronic filter is briefly in this lesson. There are several more detailed lessons here for you to learn separately since this will be quite long before we understand these circuit elements completely.

Electronic filters can either be built from digital circuits or analog circuits.

What is Electronic Filter

Just as stated before, an electronic filter is one type of circuit that is able to block a specific range of frequency and/or pass specific ranges of frequency. Not only block and pass, it is also able to amplify the passing range of frequency.

This type of circuit is very helpful for:

• DC power supplies: an electronic filter is able to block (or eliminate) specific ranges of frequency (mainly a high frequency which is considered as a noise), to act as a circuit to reduce ripple. If you build a DC power supply from an AC input, you might want to consider using a filter to smoothen your output.
• Audio system: crossover network is used to selectively pass a specific range of frequency to the corresponding audio devices. For example, pass the low-range frequency to woofer, mid-range frequency to speaker, and high-range frequency to tweeter.
• Communication system: a communication system such as radio can receive a specific frequency due to the electronic filter circuit. The desired frequency is captured while undesired frequencies are rejected.
• Analog to Digital Converter (ADC): electronic filter is a great help for Analog to Digital Converter to remove noise and interference from signal input and protect the circuit.

Types of Electrical Filter

There are four primary types of electrical filter, they are:

• Low pass filter
• High pass filter
• Band pass filter
• Band stop filter

The most basic electronic filter is a filter RC circuit. Filter RC circuits can act as a low pass filter or high pass filter.

The terms of low and high frequencies do not have specific values prior to designing a circuit. The real value of low and high frequencies are determined by our “how to design a filter circuit”. With a lot of circuit elements along with their values, we can control to pass or to block the desired and undesired frequency.

Our only job when designing an electrical filter is to determine the cut-off frequency first and move to the circuit design along with its circuit analysis to make sure everything is on the track.

Just like circuit elements where you can find active and passive elements, electronic filters also have active filters and passive filters.

Let us learn about four primary types of electronic filter briefly. We have provided the detailed explanation in other lessons.

Low Pass Filter

Low pass filter is an electronic filter which blocks frequencies higher than the cut-off frequency. This filter permits any signals that have lower than cut-off frequency. This circuit will help us to make sure high frequency noise is eliminated.

High Pass Filter

Opposite to the low pass filter, a high pass filter is an electronic filter that eliminates lower frequencies than the cut-off frequency and permits the higher frequency. It is commonly used to make sure the undesired DC voltage offset caused by low frequency components are not disturbing the output.

Band Pass Filter

Band pass filter, as its name implies, permits a certain range of frequency to pass while blocking the other. This electronic circuit has two cut-off frequencies, one for the lower range and one for the higher range.

Band Stop Filter

Opposite to the band pass filter, a band stop filter or notch filter is an electrical filter that blocks a certain range of frequency while passing the rest. Same as the band pass filter, this filter also has two cut-off frequencies at lower and higher range.

Differences of Active Filter and Passive Filter

Just as their name implies, an active filter is basically a filter built by active components such as an operational amplifier along with passive components such as resistor and capacitor to make it work properly.

On the other hand, a passive filter is built purely by passive components such as resistor, inductor and capacitor. Passive filter is very tricky to make since we need to calculate the values of resistors, inductors, and capacitors to make it filter the frequency nicely.

Passive filter will have a hard time on filtering very low frequency since we need a large inductance and capacitance, as it has limitations on lower end frequency. The very broad high frequency is caused by the parasitic inductances and capacitances.

This is why we need to be careful when designing a passive filter.

Not too much difficulty compared to the passive filter, an active filter utilizes active components such as an operational amplifier along with resistors and capacitors. This type of filter is very capable of processing low-range frequencies (near to 0 Hz) unlike passive filters.

Not only that, since it utilizes operational amplifiers, it is capable of amplifying such as voltage gain. This filter is very versatile to be used for high-order filters but less efficient on dealing with very high-range frequencies since every operational amplifier has their own limitations, their op-amp has bandwidth limitation.

How to Design a Filter Circuit

After learning the types of electronic filters and their components, what we should learn next is how to design a filter circuit. Just as stated above, the first step to design an electrical filter circuit is to calculate the components value used in our design.

We need to calculate the resistors, inductors, capacitor, operational amplifier configuration, desired cut-off frequency, bandwidth, and the order of our filter.

Next, to make it far easier, we can use an electrical circuit simulation to generate the estimated output without any cost for the trial stage. This simulation is easy for us to tweak our initial design to align with our desire.

The third step is to put the practical value into our design, for example, the tolerance of our components, the datasheets, availability in the market, cost, and many more.

The last step, and it requires more advanced analysis, is to determine our filter order.

Properties of Electrical Filter

• Elimination: Electrical filters have the property of elimination. It does not purely eliminate the undesired frequency from the circuit. More precisely, it diminishes the amplitude of the undesired frequencies.
• Phase shifter: Even with its strong property, electronic filters also come with a few problems. Since it is built with the help of capacitor (and inductor if it is a passive filter), both capacitor and inductor makes the output shifted in phase. We need to be careful using filters if our circuit is sensitive to phase shifting.

Frequently Asked Questions