Band-pass active 4th order filter with cutoff frequencies 400 kHz and 500 kHz

Thread Starter

ksnclaksnc

Joined Jan 30, 2024
35
Hello, I have to make a Band-pass active 4th order filter with cutoff frequencies 400 kHz and 500 kHz, output impedance less than 50 ohm, operating voltage range between 4 and 12 V. Supply voltage +-15 V. And use Orcad Pspice can someone tell me how to make the scheme, because I am new to
designing circuits. Thank you in advance!
 

MrAl

Joined Jun 17, 2014
11,453
Hello, I have to make a Band-pass active 4th order filter with cutoff frequencies 400 kHz and 500 kHz, output impedance less than 50 ohm, operating voltage range between 4 and 12 V. Supply voltage +-15 V. And use Orcad Pspice can someone tell me how to make the scheme, because I am new to
designing circuits. Thank you in advance!
First, do you know how to design a 2nd order bandpass filter?
 

schmitt trigger

Joined Jul 12, 2010
889
Is this homework?
If so, the forum’s policy is that you provide your solution to a stated problem, or at least the work that had been done so far, and we (forum members) will provide *hints* on the things you need to review and correct.

Use Orcad PSPICE? It is your responsibility to learn how to use any required application software, and use it to solve the problem at hand.
 

MrAl

Joined Jun 17, 2014
11,453
I am not sure how many people here use Orcad PSPICE, if any. Most people here use LT Spice I think, they may still call it SwitcherCad or something like that. Made by Linear Tech.
 

LvW

Joined Jun 13, 2013
1,753
Hello, I have to make a Band-pass active 4th order filter with cutoff frequencies 400 kHz and 500 kHz,
What is your definition of "cutoff" ? Please note that in some cases (Chebyshev response with ripple within the passband) the bandwidth is not defined between the -3dB points.
Therefore: Butterworth response within the passband?

More than that - what is your general approach? Series connection of two 2nd-order stages or "direct synthesis"?
 

MrAl

Joined Jun 17, 2014
11,453
Hello, this is the scheme that I have made but I don't know how to do the operating voltage.View attachment 314011
Hi,

What do you mean you do not know how to "do" the operating voltage? What do you mean by that.

Those 741 IC's are not going to cut it at 500kHz if the output voltage of either is higher than about 300mv peak. That's because they are very limited on bandwidth and slew rate.
You'll need another op amp, or else use ideal op amps and just go with the pure theory of it. if you need to be practical too though then you need an op amp that can work at that frequency up to the voltage you need on the output.

The problem is that op amps have limitations and if you exceed that you either get almost no output or a very distorted output.

Your basic design implementation does not look that bad though as it looks like you have two second order bandpass filters in cascade, which would work as long as there are no limitations exceeded. I did not check your 3db down points though yet (which you refer to as cutoff frequencies).
 
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Thread Starter

ksnclaksnc

Joined Jan 30, 2024
35
Hi,

I have to make a simulation on how the circuit works, what type of an Op amp should I use?
How do I define the operations voltage in the circuit, because if I am making the scheme on a breadboard I use an oscilloscope that changes the voltage?
 

Thread Starter

ksnclaksnc

Joined Jan 30, 2024
35
What is the purose of R3 and C1 ?
More than that - how did you find all the component values?
I used a Sallen-Key calculator, but I thought that the cutoff frequency’s of the two cascading amplifiers have to be different because the HipF is 400 kHz and the LpF is 500kHz. But then I realized that is not how you do it.
 

MrAl

Joined Jun 17, 2014
11,453
I used a Sallen-Key calculator, but I thought that the cutoff frequency’s of the two cascading amplifiers have to be different because the HipF is 400 kHz and the LpF is 500kHz. But then I realized that is not how you do it.
Are you trying to cascade a high pass filter with a low pass filter to get a bandpass filter?
The response of the first section looks like a high pass so that's why I ask. It does not look like you got the cutoff frequency right though.
 

MrAl

Joined Jun 17, 2014
11,453
Yes,that is what I have done.
Ok well what did you want the cutoff frequency of the first section to be then?
It does not look like 400kHz or 500kHz. Looks much higher. Maybe you could double check that.

I'll check it out better for you later today.
 

Thread Starter

ksnclaksnc

Joined Jan 30, 2024
35
I
Ok well what did you want the cutoff frequency of the first section to be then?
It does not look like 400kHz or 500kHz. Looks much higher. Maybe you could double check that.

I'll check it out better for you later today.
I wanted the first section to be 400 kHz and the second section 500 kHz, then I searched for a calculator and put the cutoff frequency values of both sections.
 

MrAl

Joined Jun 17, 2014
11,453
Thank you!
Hi,

What I am wondering is what made you think you can use the same topology for both a high pass and a low pass filter?
The two you have there are both high pass filters, right?
Also, where did you find this topology anyway? I am wondering because it looks a little unusual.

Another point we will have to get to at some time is whether or not ALL 2nd order filters in a cascade of two actually produce a 4th order response. It may look like a 4th order response on paper but act more like a 2nd order response when you plot it. This can wait however.
 
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Thread Starter

ksnclaksnc

Joined Jan 30, 2024
35
Hi,

What I am wondering is what made you think you can use the same topology for both a high pass and a low pass filter?
The two you have there are both high pass filters, right?
Also, where did you find this topology anyway? I am wondering because it looks a little unusual.

Another point we will have to get to at some time is whether or not ALL 2nd order filters in a cascade of two actually produce a 4th order response. It may look like a 4th order response on paper but act more like a 2nd order response when you plot it. This can wait however.
Yes,I saw in internet that my scheme is not done right.
Should the design be like this:IMG_1325.jpeg
 

MrAl

Joined Jun 17, 2014
11,453
Yes,I saw in internet that my scheme is not done right.
Should the design be like this:View attachment 314053
Hi,

Wow, that's a lot of op amps and a lot of capacitors.
If you really want to try it that way that might work, but usually if you use two capacitors you can get a 2nd order response, and if you use four caps then you can get a 4th order response, with the right circuit that is.
There is a chance what you have there is an 8th order bandpass, but just a chance.

Do you want to go with this topology or look for a simpler solution that uses just two op amps and four capacitors?

Oh BTW, are you going for a cutoff frequency of around 500Hz for the low pass sections?
Note that your capacitors 0.47uf are much larger than you had before when I think you were going for 500kHz.
 
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LvW

Joined Jun 13, 2013
1,753
Hi,

Wow, that's a lot of op amps and a lot of capacitors.
If you really want to try it that way that might work, but usually if you use two capacitors you can get a 2nd order response, and if you use four caps then you can get a 4th order response, with the right circuit that is.
There is a chance what you have there is an 8th order bandpass, but just a chance.
Yes - a series connection of a 4th-order lowpass wit a 4th-order highpass gives a 8th-order bandpass.
A 4th-order bandpass (as required in post#1) has magnitude slopes of 40dB/dec only (like second-order filters).

Therefore, there are three basic approaches:
* Series connection of a lowpass and a highpass section (each 2nd-order)
* Series connection of two second-order bandpass stages
* Direct synthesis (active realization of a 4th-order passive RLC ladder filter)
 

Thread Starter

ksnclaksnc

Joined Jan 30, 2024
35
Yes - a series connection of a 4th-order lowpass wit a 4th-order highpass gives a 8th-order bandpass.
A 4th-order bandpass (as required in post#1) has magnitude slopes of 40dB/dec only (like second-order filters).

Therefore, there are three basic approaches:
* Series connection of a lowpass and a highpass section (each 2nd-order)
* Series connection of two second-order bandpass stages
* Direct synthesis (active realization of a 4th-order passive RLC ladder filter)
I will try cascading a 2nd order lpf filter with a 2nd order hpf, but the teacher did not say how much the resistors are so I don't know how to calculate them. Can someone explain to me, I calculated the central frequency to 100 kHz?
 
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