Difference between active filters and passive filters

Active Filter and Passive Filter

 What are the Filters? 

before we discuss the Difference between active filters and passive filters need to study the basics. Filters are electronic circuits, which have the capability of selecting specific frequencies to pass to the output and blocking other frequencies. In other words, filters can be used to pass or amplify certain frequencies and block or attenuate other frequencies.

Thus, filters have many applications in signal processing such as noise removal from a signal or selecting desired information from some specific frequency band.

Filters can be classified into many categories depending upon their circuit, hardware, processing, etc. Few of those categories are active filters, passive filters, analog filters, digital filters, and discrete filters.

Active filter or passive filter can be identified by the components used in designing the filter. The active filters will have some active elements like a transistor or operational amplifier in its design, while the passive filter is made up of inductors, capacitors, and resistors (passive elements).

 Passive vs Active Elements

Passive elements are those elements, which do not need any outside source for their operation, and they consume power and cannot provide any power gain. Examples of passive elements are inductors, capacitors, and resistors. As they do not need any outside source for their operation and do not have the capability to deliver any power gain.

On the other hand, active elements are those elements, which requires an external source for their working, and have the capability to deliver power gain. Examples of active devices are transistors and operational amplifiers, as they need external power for their working and can provide output power gain.

 Passive filters

Passive filters are designed using resistors, capacitors, and inductors. Therefore, RC filter, RL filter, and RLC filters all are examples of passive filters (See figure 1). They do not require any outside power or biasing for their operation, and consumes a power of the applied input signal to be filtered. They also do not have the capability of providing any gain at the output.

In passive filters, change in load resistance will also affect the performance of the filter, because the passive filter does not have any isolation of load resistance from the rest of the circuit. So, the filtering process might be affected due to the change in load.

Figure 1: Passive Filters

Passive filters do not have any bandwidth restrictions, i.e. they can process even very high frequencies and provides reliable output.

However, at low frequencies, passive filters require the use of inductor with large value (which results in larger inductor size as well), hence increasing the overall size of the circuit and making it bulkier. Thus, if the requirement is to design a compact (small) and highly accurate passive filter, then its design cost will be immense.

Passive filters also have an inherent problem of noise due to thermal noise in the elements used for filter design. Therefore, proper design is required to minimize this thermal noise.

Usually, the output of the passive filter requires amplification right after the filtering to make it suitable for the latter stages of processing. This is because of the fact that the passive filter does not have the capability to amplify its output and there is power consumption in passive elements of filter. So amplification is required to compensate for the change in output due to power consumption in the circuit by passive elements.

 Active Filters

Active filters use active elements such as a transistor or operational amplifiers in their design along with capacitors and resistors. However, they do not use inductors. This results in compact and less heavy filter design.

Figure 2 shows a few active filter circuits based on the operational amplifier and a combination of capacitors and resistors.

Figure 2: Active Filters

Unlike passive filters, active filters require an external source for their processing due to operational amplifiers or transistors, which demands external bias for working. Change in load resistance does not affect the operation of filtering in case of active filters, because the load is isolated from the filtering circuit due to operational amplifier or transistor.

Operational amplifier or transistor provides power gain at the output, eliminating the need for post-amplification after filtering unlike in case of passive filters. Gain and frequency ranges can easily be adjusted by changing parameters of active filter.

However, the output of the active filter is directly dependent on the external source operating the active element in the circuit. Any change in this voltage will directly affect the output of the filter. Active filters have bandwidth limitations at higher frequencies due to the use of active elements having limited bandwidth. Therefore, at higher frequencies, the output of the active filter becomes more and more unreliable due to the inability of active elements to handle higher frequencies. Further noise in active filters is due to the feedback for adjusting the gain of active elements causing oscillations and added noise.

Difference between active filters and passive filters

  • Active filters need outside sources for their operation, while passive filters do not need any outside source for their operation.
  • Active filters have the capability of amplifying filter output, while passive filters consume the power of the input signal and cannot amplify the output signal.
  • Passive filters are designed using capacitors, resistors, and inductors, while active filters do not use inductors in their design. This results in a compact design of active filters as compared to passive filters.
  • Active filters are costlier than passive filters due to extra added active elements and external power required to operate an active element. However, passive filters can also be costlier if high accuracy and compactness are required.
  • Active filters have bandwidth limitations due to the involvement of active elements, which operate properly in only specific frequency ranges. Outside these frequency ranges, their performance degrades, resulting in an unreliable response of the active filter at higher frequencies. However, passive filters have no such limitations and output is reliable at even very high frequencies.
  • Due to the involvement of fewer elements, passive filters are less complex and easy to design as compared to the equivalent active filters.

 Conclusion about active filters and passive filters

From the above discussion, it is clear that active and passive filters have their advantages and disadvantages. Therefore, the selection of active or passive filters depends upon the requirement of the application. If high-frequency processing is involved, then passive is preferable, if compactness and less cost are desirable, then action will be preferable. Similarly, if less circuit complexity is required then the passive filter will be given preference. In short, active and passive both have their roles to play in different applications.