Electronics > Beginners
Bandpass Filter Design
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Rigolon:
Thx Wimberleytech and Benta, it's this kind stuff that I want to learn.  ;D

Another question, maybe this one is obvious:
- When analyzing a circuit with several stages of filtering, can I look at each stage individually or should I just look at the Input on the first stage and the output from the last?
Benta:
It's much easier to extract the poles if you look at each first/second order stage separately.
Wimberleytech:

--- Quote from: Rigolon on May 02, 2018, 11:27:40 am ---Thx Wimberleytech and Benta, it's this kind stuff that I want to learn.  ;D

Another question, maybe this one is obvious:
- When analyzing a circuit with several stages of filtering, can I look at each stage individually or should I just look at the Input on the first stage and the output from the last?

--- End quote ---

You can analyze each stage by itself and take the product of the impulse response of all stages to get the final transfer function IF each stage is isolated by a unity-gain buffer with infinite input impedance and zero output impedance.  Does not have to be really infinite and really zero, but high compared to the other impedances in the circuit.  When active biquad filters are cascaded, you have this situation because the output of each stage has essentially zero output impedance.
Rigolon:
I see, If the stages are not isolated what I will have to do to get the TF?

About that modified Sallen-key, I used the values that are on the circuit where I saw this kind of filter and found the poles approximately  s' = - 840 and s"=-120,000. Which gives the cutoff frequencies of approximately fc' = 133.56Hz and fc" = 18,965Hz. But simulating it I got the -3dB at 12.7kHz, at 18.9kHz it's -4.3dB. It's this plausible?

One more (maybe obvious) question:
- When cascading filters I have to use the same corner frequency for each?
Wimberleytech:

--- Quote from: Rigolon on May 02, 2018, 02:49:24 pm ---I see, If the stages are not isolated what I will have to do to get the TF?

--- End quote ---
Yes, you will have to analyze the whole circuit

--- Quote ---
About that modified Sallen-key, I used the values that are on the circuit where I saw this kind of filter and found the poles approximately  s' = - 840 and s"=-120,000. Which gives the cutoff frequencies of approximately fc' = 133.56Hz and fc" = 18,965Hz. But simulating it I got the -3dB at 12.7kHz, at 18.9kHz it's -4.3dB. It's this plausible?

--- End quote ---
show me the circuit and post your simulation (if it is an LTSpice file)

--- Quote ---
One more (maybe obvious) question:
- When cascading filters I have to use the same corner frequency for each?

--- End quote ---
No, see my sketch
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