Active Band Pass Filter (20Hz - 500Hz)

Thread Starter

TelecoESP

Joined Jan 29, 2019
1
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:
bandpass_filter.JPG
Figure 1. Active band Pass Filter (one stage)


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

frequency_response_curve.JPG
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:
requested_freq_curve.JPG
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...
 

crutschow

Joined Mar 14, 2008
23,735
  • 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.

upload_2019-1-30_19-39-30.png
 
Last edited:

LvW

Joined Jun 13, 2013
887
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.
 

Audioguru

Joined Dec 20, 2007
11,251
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.
 

LvW

Joined Jun 13, 2013
887
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.
 
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