hi Y,
The plot for the two sections of the Sallen should give you a clue where the problem is located.
E

 

hi Y,
The plot for the two sections of the Sallen should give you a clue where the problem is located.
E

Is it because both of the stages are lowpass when it supposes to be highpass on the first stage then lowpass on the second stage?

 

hi,
If you compare the AC response from both stage of your circuit, they are almost identical. 0dB upto approx 1kHz

On the reference plot at 1kHz the response is approx -50dB and at 1Hz it is -170dB.

I appreciate by unloading the 1st stage by disconnecting from the stage will effect the response of the 1st stage, but not to those degrees of attenuation at the lower frequencies,

I would suggest you look at stage 1.
BTW: have you been give the actual written bandpass specifications for the target response
E

 

hi Y,
I created my own from a the web ,made a Auto Gen model [ not very tidy, but it works]
Used AC as 1v in order to the correct plot.
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Tidied up..

I think I'm too curious with Sallen-Key ,so I go to check my MFB first then I found out that my MFB isn't done yet.
The bandwidth suppose to be around 10kHz from 10kHz to 20kHz, but in this case, my bandwidth is so large compare to your simulation. Where did I go wrong?

 

hi,
This is with using a the Old OPA and a High speed OPA,
I have deleted the component values, I cannot give you the answer.

E

 

The old uA776 opamp has a very low slew rate that kills high audio frequencies, especially at high levels.
Its datasheet has no graphs.
Its weak output current cannot drive (at increased levels) the low resistor values shown in the circuits here.

 

hi Y.
Have you prepared a new circuit for posting, based on post #25, so that we can check it for you.?
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Please don't mad at me I know I'm bad at this, but I have already read all that you guys have sent and I appreciate that so much. I still stuck on how do I find the value for each component. Do I need to assume one resistor then use it to calculate further? like I can't even get the same simulation as what you had sent or the reference when I just copied the whole circuit. I would like you guys to see me as a complete newbie, hope you guys understand this. Thank you so much (TwT).

I would like to finish my MFB first then I can go on to the next one, so now I stuck with one question. Is the phase change suppose to be 360 when it is 4th order, not 540 (-180 to 360) like in this picture? If I'm not wrong I remember that my teacher once said that the phase change will increase with the order.

 

hi Y,
As recommended always post your LTS asc file.
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hi Y,
No one is getting mad at you,
But I would ask why have you totally ignored the circuit layout configuration I posted in post #25, which works OK..??

E

 

hi Y,
No one is getting mad at you,
But I would ask why have you totally ignored the circuit layout configuration I posted in post #25, which works OK..??

E

I already build it in my LTspice, I already try to put some value on it, but I don't know how to tune the value ._. to get that beautiful simulation result

 

how do I find the value for each component.

What formula are you using to determine their values?
The values you are using are apparently not correct.

And if you want a bandpass filter the second filter needs to be a low-pass.
Your circuit has two high-pass filters in series.

 

hi,
The order and position of the components in circuit is very important,
Look at this High Pass section, try to calculate the two missing values.

E

 

What formula are you using to determine their values?
The values you are using are apparently not correct.

And if you want a bandpass filter the second filter needs to be a low-pass.
Your circuit has two high-pass filters in series.

Ok I will look it up again
The formular are based on this reference: https://www.iasj.net/iasj/download/ceca8bad4ee05e7c

 

hi,
The order and position of the components in circuit is very important,
Look at this High Pass section, try to calculate the two missing values.

E

Ok I will try to do it again

 

The uA776 opamp is low power, has a poor high frequency slew rate and has a poor high frequency response. The ST Micro datasheet has no graphs with details about it.
Use a newer and much faster opamp for the 15kHz peak then it will produce a proper high frequency rolloff.

Here is your sim but using a good opamp: