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Bandpass active filter

Ioanna Deli
Prodigy 20 points

Hello,

I am trying to design a bandpass active filter with the following specifications:

Pass-band BW-3dB =  200Hz

Center frequency = 3.5KHz

Gain =1

It seems to me that using the multiple feedback topology is a good design.

I  used the FilterPro program and this gave me  a lot of help.

But I cannot understand why it uses 10n capacitors....

Thanks you in advanced,

Ioanna

  • 0
    TI__Guru* 93105 points

    Hi Ioanna,

    FilterPro selects reasonable, standard value capacitor values whenever possible and 10 nF is a practical value for the frequency range that you are specifying. Note that you can type over one of the 10 nF capacitor values shown on the FilterPro filter schematic, enter a new capacitor value, press ENTER and FilterPro will recalculate all of the filter component values based on that new capacitor value.

    I did notice that you are attempting to create a narrow Butterworth band-pass filter using FilterPro. The circuit "Q" is 17.5 which is really quite high for the simple MFB and Sallen-Key topologies. Most accurate filter results will result with a "Q" of 10, or less. FilterPro places an asterisk by a "Q" value above 10 indicating that could be a limitation on performance. The resulting "Q" of 17.5 filter is quite high and the resulting response may not be precise. Often, for higher "Q" filters a two or three op-amp per stage topology is required for most accurate response. A bi-quad filter topology is an example of a topology that supports a high "Q" filter. The bi-quad is not supported by FilterPro. If your application isn't critical you may be able to get by with the simpler MFB or Sallen-Key pressed into the high "Q" role.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Thank you Thomas.

    I 'll try to design the bi-quad filter topology...

    Also, I would like to ask some help about the setup.

    I 'll put a signal generator in the Vin and an analyzer or oscilloscope in the output. Am I right??

    The generators' output depends on the op-amp of the filter??? In which values (generator's Vout, frequency), I have to set my generator.

    Sorry if my questions are very silly. This is my first design.

    Ioanna

  • 0 in reply to Ioanna Deli
    TI__Guru* 93105 points

    Hi Ioanna,

    You would apply the audio generator output to the filter input. Make sure the generator is terminated at the filter input with the correct termination impedance; usually 50 Ohms. Otherwise, the amplitude at the filter input will be different than what you have set the generator output level to. Connect the analyzer or O-scope at the output of the filter. The operational amplifier won't likely drive a 50 Ohm input impedance so make sure the equipment is set to the high input impedance setting. O-scopes usually have a 50 Ohm, or 1 Meg-ohm setting; use the 1 Meg ohm setting. Other analyzers may only have a 50 Ohm input impedance. You an place a resistor (~1 k to 5k) between the operational amplifier output and analyzer input to increase the load resistance the amplifier sees. That series resistance, in conjunction with the 50 Ohm input impedance forms a voltage divider. That gain reduction should be taken into account when determining the actual amplitude at a particular frequency.

    I have provided an example of a Fliege, second-order band-pass filter having a 3.5 kHz center frequency, 200 Hz bandwidth (-3 dB), and a a gain of 1 V/V. This filter topology requires two operational amplifiers, but doing assures an accurate filter response for this particular application. A dual operational amplifier would be suitable for this application. The operational amplifiers should have a gain-bandwidth of at least 5 MHz to assure good filter performance. You didn't mention your supply voltage and whether you would be using a single, or dual polarity supply. That will affect the operational amplifier you select and how the filter is practically implemented.

    Regards, Thomas

    PA - Linear Applications Engineering