Passive and Active Low Pass Filters

[jendrix]

Hello,

I have been doing some research into the above filters and ran into a question. I saw a video that covered the pros and cons of each type of filter and it was recommended that a reactive filter was used at lower frequencies, the reason given was that the inductor that would be required for a low cutoff point (sub 100Hz) would be physically large.

My question was, couldn't this be remedied by using an RC design circuit instead? And even if you increased the order you could still only use resistors and capacitors.

So this would leave a capacitor in the 4uF range assuming a resistor of 470Ω were to be used, these are physically small if they are required to be.

Thanks

 

[tech99]

Hello,

I have been doing some research into the above filters and ran into a question. I saw a video that covered the pros and cons of each type of filter and it was recommended that a reactive filter was used at lower frequencies, the reason given was that the inductor that would be required for a low cutoff point (sub 100Hz) would be physically large.

My question was, couldn't this be remedied by using an RC design circuit instead? And even if you increased the order you could still only use resistors and capacitors.

So this would leave a capacitor in the 4uF range assuming a resistor of 470Ω were to be used, these are physically small if they are required to be.

Thanks

By "passive" I presume you mean not active ie no amplifier.
By reactive I presume you mean containing only L and C.
An LC filter is able to provide sharper cut off and smaller losses in the passband. For the RC filter, the resistance is always there, and under matched conditions will create attenuation in the passband.

 

[jendrix]

By "passive" I presume you mean not active ie no amplifier.
By reactive I presume you mean containing only L and C.
An LC filter is able to provide sharper cut off and smaller losses in the passband. For the RC filter, the resistance is always there, and under matched conditions will create attenuation in the passband.

Thanks you, so there are advantages to both and it is about choosing the right configuration depending on the end goal? From what I have read, an active filter seems to be favoured for lower frequencies, with the RC in conjunction with an op-amp.

To clarify, I meant 'active' as in containing a combination of RLC along with an op-amp

And 'passive' to mean only using a combination of RL or C components.
Regards

 

[NascentOxygen]

Active filters almost always incorporate the OP-AMP in such a way that it isolates the anticipated loads from the frequency-setting components, so that the load can be changed without affecting the filter's characteristics. This means you can connect a low pass filter so it follows a high pass filter to give a bandpass response which is readily predictable.

Where the filter is restricted to all passive components, then the load becomes part of the filter design, and any change in the load is reflected as a change in the filter's response. This means it is, in general, not possible to connect multiple passive filters sequentially to produce a multi-stage filter having a response predicted by the filter responses of its individual stages. A multi-stage (higher order) passive filter must be designed in its entireity, and include the known impedances of the fixed source and the fixed load.

 

[Svein]

I have been doing some research into the above filters and ran into a question. I saw a video that covered the pros and cons of each type of filter and it was recommended that a reactive filter was used at lower frequencies, the reason given was that the inductor that would be required for a low cutoff point (sub 100Hz) would be physically large.

Yes, but with a little bit of electronics, you can simulate a large inductor using a transistor and some resistors and capacitors (Google "Gyrator").

 

[dlgoff]

A good reference with active filter circuits can be downloaded here: https://archive.org/download/NationalSemiconductorLinearApplicationsHandbook1994/NationalSemiconductorLinearApplicationsHandbook1994.pdf