# Bandpass filters

Discussion in 'General Electronics Chat' started by kivir, Dec 17, 2008.

1. ### kivir Thread Starter New Member

Dec 17, 2008
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Hello,I want to design a dual band filter.Do you think that using 2 band pass filter and connect them in parallel is a good idea to achive this goal?

Another question about bandpass filters: Can I obtain a band pass filter without using an opamp and inductor, and how?(I think I should use transistors in my project)

Apr 5, 2008
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Last edited: Dec 17, 2008

Apr 20, 2004
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4. ### PRS Well-Known Member

Aug 24, 2008
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What is a dual band filter? How can you possibly wipe out one band and retain it's passband after wiping it out with the last filter? What are you trying to do?

Yes, you can use transistors to make active filters. One way is to use a FET or a BJT to drive a tank -- that is, an inductor in parallel with a capacitor. This is a winding on a toroid with a capacitor in parallel with the winding. Then, put another winding on the same toroid and you have the secondary of a transformer. The secondary drives the next stage.

The beauty of this is that the input resistance of the next stage is reflected through the transformer windings by the square of the turns ratio. If you use 2:1 step down, then the first stage sees 4 times the input resistance of the second stage. If you wind 3:1 then the resistance seen by the first stage is 9 times greater than the resistance looking into the second stage.

This may be a bit confusing, I know. But more if you want it.

Last edited: Dec 17, 2008
5. ### eblc1388 AAC Fanatic!

Nov 28, 2008
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Quite true if the filters are connected in series.

6. ### hobbyist AAC Fanatic!

Aug 10, 2008
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I'm just kicking this around, but what if you had say 4 different bandpass filters connected in parrallel, and the output of each filter would be connected to a seperate load, the signal coming in would be applied to each filter simultaneouisly, so each filter does not interact with each other OR does one filter hog most of the signal if it is lesser impedance to ground through it's load??

But if you had 4 signals of differnet frequencies each filter would pick up it's band of frequencies much like a radio reciever as you tune it.
Instead of having one filter the tank circuit ,you would have 4 filters.
Like having 4 radios tuned to 4 seperate stations.

So yeh. you could have filters in parrallel to pick up different frequencies in each of it's own bandwidth.

Last edited: Dec 18, 2008
7. ### leftyretro Active Member

Nov 25, 2008
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Yes the output of each filter would drive it's load from signals within it's bandpass. Keep in mind that paralleling the inputs changes the total input impedance of the combined filter and that would require rematching to the source driving the combined filter.

8. ### hobbyist AAC Fanatic!

Aug 10, 2008
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Yes. Good point...

9. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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But of course! You can use two series circuits in parallel, or two parallel circuits in series. Or combinations of both.

Little historical note.....this was common practice in many early A.M./F.M. radios where they used the same I.F. strip for both 10.7 MHZ and 455KHz.

Now you know!

Eric

10. ### aloksurve New Member

Dec 18, 2008
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Hey Whats d application for which u are designing d dual band pass filter..??

11. ### aloksurve New Member

Dec 18, 2008
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Hey Whats d application for which u are designing d dual band pass filter..??

12. ### Ron H AAC Fanatic!

Apr 14, 2005
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Hi Eric,

Not sure what you mean. Two bandpass filters in series is a "nopass".

13. ### eblc1388 AAC Fanatic!

Nov 28, 2008
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It is common practice in AM/FM radio that the I.F. transistor collector load consists of two I.F. transformers in series, one resonant on 455KHz and one on 10.7MHz.

But they are not band pass filter in this context. They resonance at the IF frequency and present a load for the transistor while the other one is just a near short circuit.

14. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Hi, Mr. Chung.

This depends on the topology of the filters. Indeed, two cascaded bandpass filters, where the bandpasses don't overlap, would be a "no-pass" filter. But how about those two bandpass filters in PARALLEL? We now have two independent paths for the signals.

Eric

15. ### Ron H AAC Fanatic!

Apr 14, 2005
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No argument there. I didn't understand what you meant by