# design amplifier band pass filter help please

#### lo0oty_dxb

Joined Nov 5, 2013
6
i was asked to design an operational amplifier band pass filter using " Crocodile Technology :

* Input signal , voltage 100mVp-p , Frequency varying between 100Hz and 7000Hz

*Voltage gain of 10

*Input resistance of 100 KΩ

*Filter should band pass between 300Hz and 3100Hz at 3dB

if someone can help me with my project it would mean alot to me and i will be so very thankful ,
if someone can show me how will the final drawing is and i will try to build it using Crocodile Technology

#### #12

Joined Nov 30, 2010
18,190
Do you need a bandpass amplifier or an example of using the Crocodile Technology software?

#### lo0oty_dxb

Joined Nov 5, 2013
6
Do you need a bandpass amplifier or an example of using the Crocodile Technology software?
i need the bandpass amplifier , and to do it i need to use the Crocodile Technology software
i have the software i just need to build the bandpass amplifier using the software ,

help me please if you can

#### #12

Joined Nov 30, 2010
18,190
Looks like about 3db per octave. Here's a start. Better people than me can polish the math if necessary.

#### Attachments

• 5.7 KB Views: 44

#### lo0oty_dxb

Joined Nov 5, 2013
6
Looks like about 3db per octave. Here's a start. Better people than me can polish the math if necessary.

did u draw that ? or it's an example of how it should look like ?

anyway I really appreciate you trying to help me thank you very much

#### LvW

Joined Jun 13, 2013
907
I must confess that I NEVER would use a bandpass filter as shown in post#4.
Loading the output will destroy the filter characteristic. Thus you need an output buffer.
Why not using any of the classical active filter topologies?
(I am not familiar with "crocodile" - is it a filter design program?).

#### #12

Joined Nov 30, 2010
18,190
If you know what the load is, design for it.
If it needs a buffer, add one.
If you want a classical topology, draw it.
It's that simple.

#### LvW

Joined Jun 13, 2013
907
I am afraid, its not so simple.
* To design for a certain load requires that the load is (a) resistive and (b) fixed (does not change). No reqirements easy to fulfill.
* Secondly, drawing a circuit diagram (topology) requires something more than just the band edges. What are the damping requirements outside the pass band? The OP states only "...band pass between....". Thats not enough.
More than that, what about "crocodile technology"???

#### LvW

Joined Jun 13, 2013
907
I must confess that I NEVER would use a bandpass filter as shown in post#4.
I think, I have misread the OPs description of the task.

Thus, we have a bandpass with two real poles (instead a complex pole pair) .
In this case I have to correct my post#6 - and the circuit as proposed by #12 in post#4 may be a suitable solution (with an output buffer).
Sorry for my mistake.
LvW

#### #12

Joined Nov 30, 2010
18,190
I only regret that you were not able to assist the original poster.
My offering was intended as a starting place.
Hopefully, the "Crocodile" software will be useful and the OP will find a reasonable solution to his requirements.

Then again, the whole thing might be a misprint and my offering will be found useless.
You never know how a thread will turn out

#### LvW

Joined Jun 13, 2013
907
I only regret that you were not able to assist the original poster.
Hi #12, yes - I also regret not being able to help the poster with a circuit diagram, but - as mentioned in my post#8 - an important input is still missing regarding the required out-of-band attenuation. As you know, this parameter determines the required filter order.
More than that, he didnt answer my question regarding "crocodile".

#### lo0oty_dxb

Joined Nov 5, 2013
6
i thank all for try'n to help

the problem is my teacher don't want to help me with anything
and to be honest i don't know anything in this subject .

every time i go ask him he said No i can't help it worth 20% of the overall mark you should do it by yourself .

i really don't know how explain the question better so i will attach what he asked us to do .

all i need is a hand draw of what the circuit will look look and i will try to draw in in crocodile Tech

thank you very much all

#### Attachments

• 91.4 KB Views: 31

#### lo0oty_dxb

Joined Nov 5, 2013
6
Hi #12, yes - I also regret not being able to help the poster with a circuit diagram, but - as mentioned in my post#8 - an important input is still missing regarding the required out-of-band attenuation. As you know, this parameter determines the required filter order.
More than that, he didnt answer my question regarding "crocodile".

yes crocodile Tech is a software to draw circuits and he want us to use it

i know a little in the programme

i just dont know how the circuit will look like
if someone can hand draw it so i know i know how it is and i will apply it on the programme

thank you very much

#### shteii01

Joined Feb 19, 2010
4,647

#### t06afre

Joined May 11, 2009
5,934
Hi #12, yes - I also regret not being able to help the poster with a circuit diagram, but - as mentioned in my post#8 - an important input is still missing regarding the required out-of-band attenuation. As you know, this parameter determines the required filter order.
More than that, he didnt answer my question regarding "crocodile".
As the task is described(and to be a nitpick/stickler ). Any filter regardless order, with the correct spec. Will be a correct answer

#### LvW

Joined Jun 13, 2013
907
every time i go ask him he said No i can't help it worth 20% of the overall mark you should do it by yourself .
i really don't know how explain the question better so i will attach what he asked us to do .
OK - it is your task and I can understand that he "cant help".
However, the task description must contain a clear specification. This is no help - but a necessity to solve the task.
And - I repeat: Any band pass specification consists of two parts:
* path band properties (is partly given),
* stop band properties (not given at all).

Thus, if you dont like to ask him again for clarification. you should select the lowest possible order (n=2) for the band pass.
This would be in accordance with the circuit as proposed by #12 (see his post#4) - however, as you are required to design an "opamp band pass filter ", I think your teacher wants a "true" opamp stage (instead of an opamp buffer only) beween two passive RC stages.
Thus, you must find a suitable topology based on an opamp with frequency dependent feedback (MFB, or Sallen-Key or integrator based).
In this context, I would recommend one of the various filter design programs.

#### shteii01

Joined Feb 19, 2010
4,647
OP. No offense to you, but I just opened up my textbook from my second year of college classes in electrical engineering, and I have a WHOLE CHAPTER on designing Op-Amp filters. Including bandpass. It actually has two sections in that chapter on op-amp bandpass filters, the Cascade Filter and Butterworth Filter.

Anyone who has been given a project like the one you are working on, MUST at least have a textbook with this material. I am not going to ask if you read it, or if your class covered this material. But you at least should have the textbook.

My textbook for this type of class was: Electric Circuits, 7th Edition by James W. Nilsson and Susan A. Riedel. Chapter 15 has the stuff.

#### #12

Joined Nov 30, 2010
18,190
As the task is described(and to be a nitpick/stickler ). Any filter regardless order, with the correct spec. Will be a correct answer
So many ways to do this! Chebychev, Butterworth, a couple of others, any rate of slope...Your instructor has turned you lose in a world of options. Pick one and make the best of it. HOW you do it is what will make your grade.

#### t06afre

Joined May 11, 2009
5,934
i thank all for try'n to help

the problem is my teacher don't want to help me with anything
and to be honest i don't know anything in this subject .
every time i go ask him he said No i can't help it worth 20% of the overall mark you should do it by yourself .
i really don't know how explain the question better so i will attach what he asked us to do .
all i need is a hand draw of what the circuit will look look and i will try to draw in in crocodile Tech
thank you very much all
In this task your teacher want you work as you would in any work situation Your filter can be made by combining the correct low-pass and high-pass filter. If you use opamps you can connect the output of one "stage" to the input of the other stage. Look at the link I gave you. That can be a big help in your task. And also remember Google is your friend https://www.google.com/search?q=band+pass+filter
Also I hope you understand that we will help with giving a nudge in the correct direction. But we will not do all the work for you. Being an engineer is much the same as being an explorer. Go exploreing do not just sit on your thumbs because your books do not give you a ready made solution