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What are electronic filters?

In electrical circuits, filters are used to process signals. The main purposes of a filter are signal separation and signal restoration. Filters are circuits that can pass signals within a band of frequencies while rejecting or blocking signals of frequencies outside this band. This property of filters is known as the “frequency selectivity” of the filters. Filters are categorized in several different ways.

Classification of electronic filters

According to the component usedAccording to the input signal
Active filters
Passive filters
Digital filters
Analog filters
Table 01: Classification of filters

Active and passive filters

  • Active filters are constructed using active devices with the employ of passive devices.
  • Passive filters are constructed only using passive components. 

Active devices are devices that can amplify, producing an output signal with more power in it than the input signal. The additional power comes from an external source (Power supply). The devices with power gain are distinguishable by their ability to make oscillators by feeding some output power back into the input.

Eg: Transistors, circuits containing transistors, ICs.

If a device does not have a built-in power source, it is called a passive device.

Eg: Resistors, capacitors, inductors, diodes, transformers, switches, relays, etc.

Anagog and digital filters

  • Analog filters are made out of analog components such as resistors, capacitors, inductors, and op-amps.
  • Digital filters are made out of digital signal processors (DSP), Microcontrollers (MCU), etc.

The main purpose of both analog and digital filters is to remove unwanted signals and pass signals in a specific region. But analog filters remove all the signals above or below a certain frequency whereas digital filters can precisely be programmed.

Basic filter responses

Basic filter responses
Figure 01: Basic filter responses

If a filter (any type of filter) is supplied voltage, Vin and the output voltage will be Vo. The gain of voltage (A) of the filter is given by,

Introduction to Digital Filters eq 01

Voltage gain vs Frequency response of low pass filters
Figure 02: Voltage gain vs Frequency response - low pass filters

The stop band is the range of frequencies that have the most attenuation.

Critical frequency, fc (also called the cutoff frequency) defines the end of the pass band and is normally specified at the point where the response drops –3 dB (70.7%) from the pass band response. 

Introduction to Digital Filters eq 02


  • fc = critical frequency
  • R = Resistance of the resistor.
  • C = capacitance of the capacitor
  • ωc = 3dB cut off frequency

The pass band of a filter is the range of frequencies that are allowed to pass through the filter with minimum attenuation (usually defined as less than -3 dB of attenuation).

The area where the fall-off occurs is the transition region.

Types of filters according to the components used

Active filters are constructed by using an op-amp as the active element combined with RC (resistor-capacitor circuit), RL (resistor–inductor circuit), or RLC (resistor–inductor-capacitor circuit) circuit as the passive elements. Passive filters are constructed using capacitors, resistors, inductors, etc. There are no amplifying components like op-amps in passive filters. There are four main types of filters. 

  1. Low pass filters
  2. High pass filters
  3. Bandpass filters
  4. Band reject filters

Each of these filters will be described in detail in separate articles.

Advantages of active filters over passive filters

  • Active filters can be programmed to provide the required gain. So no attenuation (loss of strength of a signal) like passive filters.
  • No loading problem, because of high input impedance and low output Impedance of op-amp.
  • Active Filters are cost-effective because there is a wide variety of economical op-amps available in the market.

Applications of filters

Active filters are mainly used in communication, Analog Instruments, and signal processing circuits.

Filters are also used in a huge range of applications.

Eg: entertainment, medical electronics, modern physics applications, etc.

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References and Attributes


The cover image was created using an Image by Tide He from Pixabay

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