Passive and Active Low Pass Filters

In summary, the conversation discusses the pros and cons of using reactive filters at lower frequencies and the potential issue of physical size with inductors. The question is raised whether an RC design circuit could be used instead, and the advantages and drawbacks of both passive and active filters are explored. The conversation concludes with a recommendation to use active filters for lower frequencies and a suggestion to use a gyrator to simulate a large inductor. A resource for active filter circuits is also provided.
  • #1
jendrix
122
4
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
 
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  • #2
jendrix said:
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.
 
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  • #3
tech99 said:
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
 
  • #4
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.
 
  • #5
jendrix said:
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").
 
  • #6
A good reference with active filter circuits can be downloaded here: https://archive.org/download/NationalSemiconductorLinearApplicationsHandbook1994/NationalSemiconductorLinearApplicationsHandbook1994.pdf
 

1. What is the purpose of a low pass filter?

A low pass filter is used to attenuate or reduce high frequency signals while allowing low frequency signals to pass through. This is useful in many electronic circuits to remove noise or unwanted signals.

2. What is the difference between a passive and active low pass filter?

A passive low pass filter uses only passive components such as resistors, capacitors, and inductors to achieve its filtering effect. An active low pass filter, on the other hand, uses active components such as transistors or op-amps in addition to passive components to achieve its filtering effect. Active filters are often preferred as they can provide better performance and can be designed to have a sharper cutoff frequency.

3. How does a low pass filter work?

A low pass filter works by allowing low frequency signals to pass through while attenuating high frequency signals. This is achieved by creating a voltage divider between the input signal and the filter's output. At low frequencies, the impedance of the filter is high, allowing most of the signal to pass through. At high frequencies, the impedance is low, causing most of the signal to be attenuated.

4. What is the cutoff frequency of a low pass filter?

The cutoff frequency of a low pass filter is the frequency at which the output signal is attenuated by 3dB (half the power) compared to the input signal. This is also known as the -3dB frequency. The lower the cutoff frequency, the more low frequency signals are allowed to pass through the filter.

5. How do I choose the right low pass filter for my application?

The right low pass filter for an application depends on the desired cutoff frequency, the type of input signal, and the required attenuation of high frequency signals. It is important to carefully select the appropriate filter design and components to ensure that the filter meets the desired specifications and does not introduce any unwanted effects in the circuit.

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