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)

 

Hello,

When you look in post #2 I posted some time a go, you will see a link to a lot of filter schematics and designs.
Here is the link again : http://www.educypedia.be/electronics/analogfil.htm
And a link for filter calculators : http://www.educypedia.be/electronics/electroniccalculatorsfilter.htm

Greetings,
Bertus

 

We ask you to always start a new thread with a new topic.

The thread this was posted in - http://forum.allaboutcircuits.com/showthread.php?t=15866

TI has good application notes on active filters.

 

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?

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?

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)

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.

 

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?

Quite true if the filters are connected in series.

 

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.

 

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??

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.

 

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.

Yes. Good point...

 

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)

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

 

Hey Whats d application for which u are designing d dual band pass filter..??

 

Hey Whats d application for which u are designing d dual band pass filter..??

 

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

Hi Eric,

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

 

Hi Eric,

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

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.

 

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

 

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

No argument there. I didn't understand what you meant by

You can use two series circuits in parallel, or two parallel circuits in series.