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Active filters

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Zener_Diode

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Hey all,

What are the advantages of active filters vs passive one?

I read this document:
https://www.electro-tech-online.com/custompdfs/2010/11/sloa088.pdf

I read in the first time about Q-factor for LPF/HPF. In the document on page 12 (16-11) is written "The higher the Q value, the more a filter inclines to instability."

in section 16.3.2.2 there is an explanation about Multiple Feedback (MFB) topology. the first sentance is "The MFB topology is commonly used in filters that have high Qs and require a high gain".

As I undestand we don't want filters to oscillate, why should I want high Q for filter?



on page 4 I see alfa1=alfa2=alfa3=...=alfa=sqrt(2,sqrt(n,2)-1) . Where is it come from?


Thanks ahead
Zener_Diode`
 
A high-Q filter has a peak with a narrow bandwidth. If it has a short transient input then the filter will "ring" at its peaked frequency. The ringing indicates that it is close to feeding back.
 
What are the advantages of active filters vs passive one?
An active filter can generate high-order filters will good rolloff in the stop band,using only resistors and capacitors. Passive filters require capacitors and inductors for the same roll-off characteristics.
 
An active filter can generate high-order filters will good rolloff in the stop band,using only resistors and capacitors. Passive filters require capacitors and inductors for the same roll-off characteristics.


As I understand 1st-order filter Active or passive have one R and one C and have 20dB/dec.
2nd order filter I can do
passive: RC - RC filter or LRC filter in both cases I get 40dB/dec
Active filter has 2 RC filters.

So what is the diff?

Thanks ahead
 
So why would I want high Q in low-pass/high-pass filter?
I made very high quality boardroom telephone conference systems for the head offices of major banks. But the telephone lines had terrible high frequency response that made voices sound muffled and difficult to understand what was being said. So I added treble boost with a peak of about 10dB at 3.5kHz and it made voices sound very crisp and clear. Whenever I demonstrated it then I made a sale.

I used a second-order Sallen and Key lowpass filter with a high Q to make it peak where it normally would cut-off.
 
As I understand 1st-order filter Active or passive have one R and one C and have 20dB/dec.
2nd order filter I can do
passive: RC - RC filter or LRC filter in both cases I get 40dB/dec
Active filter has 2 RC filters.

So what is the diff?
The "diff" is that the passive RC 2nd order filter has a rather slow rolloff at the corner frequency. For a sharper cut-off at the corner frequency you need a higher Q which can be generated by an active RC filter or a passive LC filter. Simulate them with LTSpice (free download) to see the difference.
 
A passive 2nd-order RC filter has a very gradual dropoff. Then it also drops frequencies that are 1/10th or 10x the cutoff frequency. Each order reduces the output by 3dB so two orders reduce the cutoff frequency to -6dB.
You need inductors and capacitors to make a second-order or more passive Butterworth filter. The inductors must be huge and expensive for low audio frequencies.
 
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Concerning this, what applications are passive filters used in? Are they used in the same applications as active filters?
 
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Radio circuits use many passive filters.
A hi-fi speaker with a woofer and tweeter uses passive filters to keep the high frequencies away from the woofer and keep the low frequencies away from the tweeter. But active filters and separate amplifiers work much better.
 
Yes I did. But the results weren't specific enough. I typed in passive filters and its applications and the links didn't specify its applications, is it used for radios, appliances, etc those kind of stuff.
 
Theyr also used in tesla coils to prevent the high frequency high voltage air core transformers feedback reaching the low frequency low voltage LC tank circuit :) usefull for not cooking things.... there may also be one in microwave circuits from memory... doing a similar job...although my memory is hazy on that one.
 
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