A HP and a LP filter to make a BandStop filter

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Jiadong Yao

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Hello, everyone.
Today i have analog electronics class. In the class, teacher said that if you want to make an bandstop filter, you have two options. One is combine a lowpass and a highpass filter. The other is 1-BandPass.

The circuit is built with opamp. so it is an active circuit.

My question is, can we use a lowpass filter and a highpass filter to build a bandstop filter???? I tried several times, but i always get bandpass. Bandstop is never reached.
Is my teacher wrong? or do i made some mistake that i did not notice??

Please help.
Thank you!
 

When Low Pass is used in negative feedback path, you get a high pass filter (HPF) same as if you use bandpass in negative feedback , you get bandstop.

There are many types of filters, series and parallel. Passive and active.

Most popular band stop or narrow notch filter is the twin-T which is a parallel LPF//HPF where the frequency that the phase shift cancels at equal amplitude is the notch. This can be passive but higher impedance gain and thus Q desirable using Op Amps.

for series band stop, use band pass in the negative feedback path.
 

well,honestly, I partly understand what you are talking. My question is how to make a bandStop only with a HP and a LP and a buffer. That is the exam question i had last semester.
 

To make a bandstop, the HP and LP filter must be connected in parallel, with a summing amplifier combining both pathes.
 

Jiadong Yao said:
The other is 1-BandPass.
Click to expand...

FvM said:
To make a bandstop, the HP and LP filter must be connected in parallel, with a summing amplifier combining both pathes.
Click to expand...

Along the lines of these ideas...

This bandstop filter is one solution.

With an LC tank, I suppose it would have to be called a 1-bandpass.



The left-hand circuit is a combination of the other two circuits.

The scope traces show the rolloff curves of the three filters.

The bandstop frequency is 1.1 kHz. The bandwidth can be made wider or narrower, by changing the L:C ratio.
 


You are smart guy. But the exam question only gives us RC filters.
So the exact question is to make a bandstop filter with a RC HP filter and a RC LP filter and a buffer.
 

Jiadong Yao said:
You are smart guy. But the exam question only gives us RC filters.
So the exact question is to make a bandstop filter with a RC HP filter and a RC LP filter and a buffer.
Click to expand...

Here is one which is all capacitor-based. It makes use of the principle of phase inversion.

Three capacitors in series provide a phase shift. At a particular frequency they create 180 degree phase shift.

The output is mixed with a portion of the original signal. At the center frequency (the frequency of 180 deg. phase shift), the two signals cancel each other.

(This 180 degree shift is also the operating principle of the phase shift oscillator.)

**broken link removed**

Its bandwidth is large. It does not contain either a high or low pass filter.

Nevertheless it is an RC-based bandstop filter.
 

FvM said:
I told you that the solution is a parallel structure. The details are up to you.
Click to expand...

Sorry, i still didn't get it. I try to write down the system function. But the result is not bandstop.
 

I think most people misunderstand me.

Q: one simple RC HP filter, one simple RC LP filter, one simple buffer. use these three components to build a bandstop filter.
 

one simple RC HP filter, one simple RC LP filter, one simple buffer. use these three components to build a bandstop filter
Click to expand...
The circuit suggested in the previous post does exactly this. Lacking an exact specification I understand "buffer" as an ideal +1 amplifier.

The subcircuits in your diagram are abstracted function blocks rather than "simple RC filters". In so far they are probably confusing the problem (or the problem isn't clearly specified). I understand "simple RC filter" as a real circuit with three terminals (in, out and common ground).

Another option is to add the output of both filters, but an adder isn't exactly a buffer.
 

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