# Active filtering circuit design

#### circuitt

Joined Jun 24, 2021
4
Hello everyone,

I need to design a circuit on breadboard that give filtered outputs as "0.5-4 Hz; 4-7 Hz; 7-12 Hz; 12-30 Hz; 30-100 Hz; 100-1000 Hz" using active filter principles with introducing input signal.
Can anyone help me? Thanks in advance.

#### Papabravo

Joined Feb 24, 2006
20,369
Hello everyone,

I need to design a circuit on breadboard that give filtered outputs as "0.5-4 Hz; 4-7 Hz; 7-12 Hz; 12-30 Hz; 30-100 Hz; 100-1000 Hz" using active filter principles with introducing input signal.
Can anyone help me? Thanks in advance.
Sure. You need to understand first what kinds of results are possible and what it takes to achieve them. The kind of filters you are talking about are called bandpass filters. They have a passband where signals inside a certain range are passed with minimal attenuation. They have two stopbands, one above the passband and one below the passband where very little signal energy gets through. A typical passsband attenuation might be 60 dB. Do you know what measuring signals in dB is all about? In between the passband and either stopband is a transition band where the attenuation increases as the frequency moves away from the passband. These transition bands are typically not sharp or abrupt. they slope gently away from the passband at so many dB per decade.

To your problem. You need to think of what you want the attenuation to look like as the frequency goes from the passband to the stopband for each of your passbands. In particular there is no room for any stopbands between your contiguous passbands. This means that by looking at your filter outputs the band edges will be "fuzzy" and ill-defined.

Is that what you imagine would happen or did you have something else in mind?

#### LowQCab

Joined Nov 6, 2012
3,448
What are you trying to accomplish with these Filters ?
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#### Ramussons

Joined May 3, 2013
1,354
Hello everyone,

I need to design a circuit on breadboard that give filtered outputs as "0.5-4 Hz; 4-7 Hz; 7-12 Hz; 12-30 Hz; 30-100 Hz; 100-1000 Hz" using active filter principles with introducing input signal.
Can anyone help me? Thanks in advance.
Can you give a plot of the Frequency Response of the Filter(s) that you want?

#### Ian0

Joined Aug 7, 2020
8,373
See
http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm
The top section calculates frequency response from known R and C values
Scroll down to the section which calculates R and C values from the required frequency response.
You will need the centre frequency, which is the geometric mean of the upper and lower values you have quoted (multiply them together and take the square root).
Is this homework?

#### circuitt

Joined Jun 24, 2021
4
Thanks for your answers. Yes, this is homework and the only information given is what I wrote. How I can design a circuit with these information and which components should I use, can you help me?

#### circuitt

Joined Jun 24, 2021
4
Also how can I use choose which opamps should I use and the values I need for capacitors and resistors? Also to create the circuit on breadboard, what should I use to give signal and measure output signal. I don't have lab environment. Can I use phone or something else?

#### LowQCab

Joined Nov 6, 2012
3,448
Use a "Theoretical Op-Amp" for your calculations.
A "Theoretical Op-Amp" is basically nothing more a Math-Equation,
or maybe a perfect "Analog-Calculator".

And then there are all the compromises that
come along with nothing being perfect in the real World.

Every filter has a "slope" defining the rate of Attenuation.
This may be with increasing OR decreasing Frequency,
or in the case of Notch, or Peak Filters, it would be referred to as the Filter's "Q".

You assignment does not specify the "Slopes" or "Q" of the Filters,
so I suppose You get to pick.
It also doesn't specify that you have to use any kind of Amplifier.

All of the specified Filters can be assembled using simple R-C-Filters.

Just search for RC-Filter online, you'll get at least a million hits.
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#### Papabravo

Joined Feb 24, 2006
20,369
Thanks for your answers. Yes, this is homework and the only information given is what I wrote. How I can design a circuit with these information and which components should I use, can you help me?
You can't design what you have described, at least the way I read it, because it is divorced from reality. If you are unable to specify an objective reality, then picking components is a pointless exercise IMHO. What you "seem" to have described is a series of bandpass filters with transition bands having zero width. This is not possible using real components. That is what I mean by divorced from reality. Entertaining such a concept does not appear to serve a useful purpose.

If you want to learn something, you should learn how to specify a filter response from a reputable source.

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#### Papabravo

Joined Feb 24, 2006
20,369
Also how can I use choose which opamps should I use and the values I need for capacitors and resistors? Also to create the circuit on breadboard, what should I use to give signal and measure output signal. I don't have lab environment. Can I use phone or something else?
You need a signal generator with a sinewave output and an oscilloscope.
You could also do this with a simulator.

#### circuitt

Joined Jun 24, 2021
4
I'm searching for studies about band pass filtering, but couldn't find an application including multiple separate filtered outputs as in my homework. How can I apply it in a circuit, should I connect opamp circuits sequentially to each other's outputs?

#### Papabravo

Joined Feb 24, 2006
20,369
... should I connect opamp circuits sequentially to each other's outputs?
That would serve no useful purpose. I think that you want each of the outputs to be separate.

#### Ian0

Joined Aug 7, 2020
8,373
I'm searching for studies about band pass filtering, but couldn't find an application including multiple separate filtered outputs as in my homework.
https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter8.pdf
How can I apply it in a circuit, should I connect opamp circuits sequentially to each other's outputs?
Connect your single input to the inputs of several filters.
If your input is not low impedance, employ a unity-gain buffer first.
There's a nice signal generator app for iPad called "Function Generator Pro", and as it drives what is normally the headphone output, it is a good low impedance.

#### BobaMosfet

Joined Jul 1, 2009
2,081
Hello everyone,

I need to design a circuit on breadboard that give filtered outputs as "0.5-4 Hz; 4-7 Hz; 7-12 Hz; 12-30 Hz; 30-100 Hz; 100-1000 Hz" using active filter principles with introducing input signal.
Can anyone help me? Thanks in advance.

Active Filter Cookbook; 2nd Ed.
Author: Don Lancaster
ISBN-13: 978-0750629867
ISBN-10: 075062986X

#### Ian0

Joined Aug 7, 2020
8,373
Active Filter Cookbook; 2nd Ed.
Author: Don Lancaster
ISBN-13: 978-0750629867
ISBN-10: 075062986X
I’d recommend that one as well.

#### bertus

Joined Apr 5, 2008
22,215
Hello,

Bertus

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#### MrAl

Joined Jun 17, 2014
10,587
Hello everyone,

I need to design a circuit on breadboard that give filtered outputs as "0.5-4 Hz; 4-7 Hz; 7-12 Hz; 12-30 Hz; 30-100 Hz; 100-1000 Hz" using active filter principles with introducing input signal.
Can anyone help me? Thanks in advance.
From your other posts it appears that you are designing a filter circuit with more than one output where each output has to have a certain band of frequencies it passes. If i am wrong please correct me.

With that in mind, you first have to know how to design a general filter section that will act as a bandpass filter. This might require some study on your part, but once you get that concept you will be able to design this filter yourself because each output will have the same circuit just designed for a different frequency range.

There are several ways to design a filter like a bandpass. Since you have not been given a Q for each filter this should be easier because you can assume that the Q does not have to be high and that means you can even use two passive networks to get the response again if you dont mind a low Q.
Since this is an op amp circuit though you may want to go for a higher Q. That means you have to study how to design a single op amp bandpass filter. There are several ways to go about hat too so i wonder if you have been taught anything about how to do this already and then you would use that method.
If you havent been taught yet then we could go over a few different design techniques.

#### Ian0

Joined Aug 7, 2020
8,373
we can work out the centre frequency from
√(Fmax.Fmin)
and if we suppose that the frequency ranges specified are the -3dB points, then Q = fcentre/(fmax-fmin).

Is it just a coincidence or do four out of six bands correspond to the α, β, γ and δ brainwave frequencies?

#### MrAl

Joined Jun 17, 2014
10,587
we can work out the centre frequency from
√(Fmax.Fmin)
and if we suppose that the frequency ranges specified are the -3dB points, then Q = fcentre/(fmax-fmin).

Is it just a coincidence or do four out of six bands correspond to the α, β, γ and δ brainwave frequencies?
Just quickly, i guess the dot in there is multiplication.
Fmax*Fmin

#### Ian0

Joined Aug 7, 2020
8,373
Just quickly, i guess the dot in there is multiplication.
Fmax*Fmin
indeed. It’s the geometric mean.