Advantages of Active Low Pass vs Passive Low Pass

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Discussion Overview

The discussion centers on the advantages of active low pass filters compared to passive low pass filters, exploring theoretical and practical implications in filter design.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the advantages of active low pass filters over chaining passive low passes, noting concerns about DC drift.
  • Another participant states that active filters have gain, which may be a significant advantage.
  • It is mentioned that active filters can be comparatively inexpensive, especially when a certain number of poles necessitate a stage of gain.
  • A participant points out that calculating component values for passive filters at lower cut-off frequencies can reveal advantages of active filters.
  • Concerns are raised about the drawbacks of active filters, particularly the potential introduction of additional noise due to components like op-amps.
  • One participant highlights that active filters do not require coils, which can be cumbersome to work with and expensive, while also noting that active filters typically consist of resistors and capacitors along with op-amps.

Areas of Agreement / Disagreement

Participants express various advantages of active filters, but there is no consensus on the overall superiority of active versus passive filters, as drawbacks are also acknowledged.

Contextual Notes

Limitations include the potential for noise in active filters and the practical challenges associated with passive filters, such as the complexity of using coils.

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What advantage does an active low pass have over chaining passive low passes?

Everyone seems to warn me about dc drift and such things, but there must be some reason why active low passes were invented.
 
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Active filters have gain.
 
They are comparatively inexpensive. After a certain number of poles a stage of gain may be required so why not go active to start with?
 
Also, if you caluclate the component values for a passive low-pass filter with a cut-off of about 40 Hz or so you will realize that active filters have another advantage.

Note that active filters have their drawbacks as well. The most obvious being that there is always a risk that you introduce additional noise in the circuit due to the component in the filter (e.g. op-amps can be quite noisy).
 
The main advantage of active filters is that they don't need COILS.

COILS are a pain to wind and measure, or expensive to buy, and they are also quite bulky for the air wound ones.

Active filters have to have a power supply, but they only have resistors and capacitors apart from the op amps. And, they can have gain or at least not lose any signal amplitude for the wanted part of the spectrum..
 

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