Amplifier Circuit Design - Narrowing bandwidth for an active band pass filter

Thread Starter

Elevon

Joined Jun 7, 2020
16
How would you increase the order of an active band pass filter? And would this help to close the gap between the range of frequencies where gain is level for the amplifier circuit and the edge of the bandwidth (the frequencies at which their gain is 0.7071 x the level gain value)?
 

OBW0549

Joined Mar 2, 2015
3,509
How would you increase the order of an active band pass filter? And would this help to close the gap between the range of frequencies where gain is level for the amplifier circuit and the edge of the bandwidth (the frequencies at which their gain is 0.7071 x the level gain value)?
The attached application note from TI should help. In general, you increase the order of a filter (highpass, lowpass or bandpass) by adding more sections to it.
 

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Papabravo

Joined Feb 24, 2006
14,244
Depending on your requirements, the topology of the filter may allow you to satisfy your requirements with fewer parts than the shotgun approach. When it comes to filters, solutions are seldom unique. In other words there are many realizations capable of meeting the requirements.

Q: Do you have a suitable textbook that you can refer to, or are you picking up bits and pieces from the interwebs?
 

MrAl

Joined Jun 17, 2014
7,758
How would you increase the order of an active band pass filter? And would this help to close the gap between the range of frequencies where gain is level for the amplifier circuit and the edge of the bandwidth (the frequencies at which their gain is 0.7071 x the level gain value)?
Hi,

By increasing the number of poles in the transfer function.

The order is usually increased when the filter response has to be made more sharp, but it could also be needed when the response just has to be more complicated.
 

Thread Starter

Elevon

Joined Jun 7, 2020
16
Depending on your requirements, the topology of the filter may allow you to satisfy your requirements with fewer parts than the shotgun approach. When it comes to filters, solutions are seldom unique. In other words there are many realizations capable of meeting the requirements.

Q: Do you have a suitable textbook that you can refer to, or are you picking up bits and pieces from the interwebs?
Thanks for the response, I was using bits and pieces and have now gotten to where I need to be, I appreciate all the help!
 

Papabravo

Joined Feb 24, 2006
14,244
Thanks for the response, I was using bits and pieces and have now gotten to where I need to be, I appreciate all the help!
I did the same thing at the beginning. I have found it enormously helpful to have gone back to fill in the blanks with a good textbook. I wish I had taken the slow and steady path from the beginning, but I was always in too big a hurry. Not so much any more.
 

LvW

Joined Jun 13, 2013
1,078
Finally, I like to add some more general information.
As outlined in the mentioned pdf-papers, you can realize 2nd-order bandpass filters - similar to lowpass and highpass stages - in Sallen-Key or multi-feedback topology.
This may work also for a 4th-order bandpass (staggered tuning).
However, if you need higher-order bandpass filters you should use another approach.

As you probably know, there are passive R-L-C ladder structures which can be designed using normalized filter tables (for filter orders up to 6th or 8th order) readily available. And there are certain techniques for realizing these filters with opamps which replace the function of the inductors: Direct active realization of passive reference filter structures.

Key words: Leapfrog structure, Follow-the-Leader structure, Primary-Resonator-Block topology, FDNR-technique,
 

Audioguru again

Joined Oct 21, 2019
1,767
Usually when a bandpass filter is mentioned its frequencies are also mentioned.
Since you did not say the frequencies then I am showing an audio filter that makes sounds tinny and muffled like an old telephone.
It is a simulation of a second-order Sallen-Key Butterworth highpass filter and a second-order Sallen-Key Butterworth lowpass filter with a total gain of 8dB so that the R and C values in each filter are equal.
 

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LvW

Joined Jun 13, 2013
1,078
How would you increase the order of an active band pass filter? And would this help to close the gap between the range of frequencies where gain is level for the amplifier circuit and the edge of the bandwidth (the frequencies at which their gain is 0.7071 x the level gain value)?
By the way.....when bandwidth is your only concern, you can have a very small bandwidth even for a 2nd-order high-Q bandpass.
It is only the out-of-band attenuation which makes higher-order filters necessary.
 
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