Electronics > Beginners
Bandpass Filter Design
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Yansi:
Unless you want a Linkwitz-Riley crossover filter, that just happens to use squared butterworth response.
Rigolon:

--- Quote ---show me the circuit and post your simulation (if it is an LTSpice file)
--- End quote ---
I'm more used to Proteus since I learned with it.
But the image with the circuit and frequency response are attached below.

In the case where fc is not the same for how do I know fc? Let's say I have a LPF with fc1 = 10kHz and another LPF with  fc2 = 50kHz, the final fc it's equal to the smallest one or is there some kind of equation?
Benta:

--- Quote from: Yansi on May 02, 2018, 03:52:41 pm ---Unless you want a Linkwitz-Riley crossover filter, that just happens to use squared butterworth response.

--- End quote ---

Actually, a Linkwitz-Riley can use any squared response. I've built a fourth-order Linkwitz-Riley with Bessel response.
Benta:

--- Quote from: Rigolon on May 02, 2018, 04:26:27 pm ---
--- Quote ---show me the circuit and post your simulation (if it is an LTSpice file)
--- End quote ---

In the case where fc is not the same for how do I know fc? Let's say I have a LPF with fc1 = 10kHz and another LPF with  fc2 = 50kHz, the final fc it's equal to the smallest one or is there some kind of equation?

--- End quote ---

This is where Bode plots and asymptotes come in. Your example would (assuming second order filters) result in a filter with a cutoff at 10 kHz with 40 dB/dec roll off, the roll off increasing to 80 dB/dec at 50 kHz.

I'm suspicious of your Sallen-Key filter, R3 seems way too high. Shouldn't it be 100 ohms?
Rigolon:

--- Quote from: Benta on May 02, 2018, 04:59:39 pm ---his is where Bode plots and asymptotes come in. Your example would (assuming second order filters) result in a filter with a cutoff at 10 kHz with 40 dB/dec roll off, the roll off increasing to 80 dB/dec at 50 kHz.

--- End quote ---
So if I want a better roll-off isn't better to use the same fc? So I get 80 db/dec roll off?
Or are there any advantages or reasons to use different fc?


--- Quote from: Benta on May 02, 2018, 04:59:39 pm ---I'm suspicious of your Sallen-Key filter, R3 seems way too high. Shouldn't it be 100 ohms?

--- End quote ---

I'm using the values that are on the actual circuit, I have an old post that I explain a little how I got to this circuit.
Not sure if the author was knowing what they were doing, because there are a lot of mistakes on the circuit, as resistors and capacitors grounded on both pins.

There are capacitors on the output of each opamp going to ground, which for me is weird since it decreases the slew-rate of the opamp. Not sure if there any motive to use capacitors like that on the output of the opamps.
I will post the whole circuit some time, it's just that I'm taking it by parts to focus and learn in depth about it and electronic in general.  ^-^
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