I am new here and it is my first thread, so... first of all: Hello everybody!!!

I have some questions (doubts) and I would like a second opinion. Thanks in advance!

1. Active Band Pass Filter
I would like to build an Active Band Pass Filter with f1 = 20Hz and f2 = 500Hz. I have selected Multi-Feedback topology instead of Voltage-Controlled Voltage Source due to the advantanges that we get in Multi-Feedback. Here is the schematic:

Figure 1. Active band Pass Filter (one stage)


I have been simulating the circuit in PSPICE and this is the frequency response curve:


Figure 2. Frequency response curve.


As we can see, f1 and f2 have dropped 3 dB of the mid-band gain. So the simulation and element values are right.

The questions are:

• Which Op-Amp would you use? I have selected the IDEAL because I really don't know which amplifier is better for this project.

• I would like to know if it is possible to have a gain equal to 1 (0 dB) with flat frequency response in the 20Hz - 500Hz range (as shown below) with just ONE STAGE:

Figure 3. Desired frequency response.

I have used "FilterPro" from TI to know the stages needed and I have obteined 5 stages. But I would like to discuss about this. I believe that 5 stages is excessive...

 

• I would like to know if it is possible to have a gain equal to 1 (0 dB) with flat frequency response in the 20Hz - 500Hz range (as shown below) with just ONE STAGE:

Not really.
Typically you can do no more than 3-poles in a single stage.

Why did you select a design which such a low passband gain?

Below is the LTspice simulation of an example 3-pole LP Sallen-Key type filter followed by a 3-pole HP filer, which gives a response similar to what you want.
It gives an 18dB/octave (60dB/decade) rolloff above and below the -3dB corners.
It is not necessarily optimized for your requirements.
Here and this are calculators for the 3-pole circuit values.

For those frequencies, just about any general purpose op amp will suffice (avoid the old 741 as it's rather noisy), but a low-noise one, such as the NE5532 would likely be good if you are concerned about noise.

 

Yes - I fully agree with Crutschows recommendation. For a bandpass with such a small Q.value (Q=100/(500-20)= 0.21) it makes no sense to use the one-stage-bandpass MFB topology. The best solution is a highpass-lowpass combination.

 

This discusses OpAmp selection -

http://www.ti.com/lit/an/sbfa001c/sbfa001c.pdf


Regards, Dana.

 

The multiple-feedback-bandpass-filter produces plenty of output at 10kHz even when its -3dB cutoff is 500Hz because it is much too simple with very gradual slopes.

 

Deleted a double-post.

 

The multiple-feedback-bandpass-filter produces plenty of output at 10kHz even when its -3dB cutoff is 500Hz because it is much too simple with very gradual slopes.

A second-order bandpass provides - far above the corner frequencies - a first order slope (20dB/dec).
The same can be achieved with a lowpass-highpass series combination - each of first order.
Using low- and highpass stages each of second order, gives a rising resp. falling slope of 40dB/dec.

 

Not really.
Typically you can do no more than 3-poles in a single stage.

Why did you select a design which such a low passband gain?

Below is the LTspice simulation of an example 3-pole LP Sallen-Key type filter followed by a 3-pole HP filer, which gives a response similar to what you want.
It gives an 18dB/octave (60dB/decade) rolloff above and below the -3dB corners.
It is not necessarily optimized for your requirements.
Here and this are calculators for the 3-pole circuit values.

For those frequencies, just about any general purpose op amp will suffice (avoid the old 741 as it's rather noisy), but a low-noise one, such as the NE5532 would likely be good if you are concerned about noise.

View attachment 169077

Hello, do you have the procedure to obtain the resistance and capacitance values for that particular frequency range (20-500Hz)?

 

Yes
http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm