Bandpass filter why we are cascading HPF+LPF why not LPF +HPF,both are LTI systems

[hioyo]

Dear Team,

For the bandpass filter why we are cascading HPF and LPF why not LPF and HPF.

Below is the circuit of a BPF.

My question is for an LTI system if we put LPF 1st and HPF 2nd of HPF 1st and LPF 2nd, we need to get the same response.

But in all places I searched I found 1st stage is HPF and 2nd stage is an LPF.

May I know why it is like that?

Regards

HARI

 

[Audioguru]

The second very simple passive filter affects the first one.

 

[hioyo]

The second very simple passive filter affects the first on

You mean HPF+LPF combination will have better frequency response.Please correct me if I am wrong.
May I know about the noise performance of the combinations HPF+LPF and LPF+HPF

 

[FvM]

You mean HPF+LPF combination will have better frequency response

Both have different frequency response. What is better depends on your specification.
To reduce the load effect, you can simply increase the impedance of the second RC circuit by a factor, the difference between both frequency responses almost disappears.

 

[Audioguru]

The passive filters have very gradual cutoffs at each side of the peak frequency. If that is not good enough then you should use active filters with an opamp like this:

 

[d123]

Hi,

My interpretation of bandpass as high pass followed by low pass:

High pass = NOT under x (lower) frequency
+
Low pass = NOT over y (higher) frequency

Bandpass = only pass signals that are NOT under x lower frequency & NOT over y higher frequency

I guess if they were configured the other way round (LPF + HPF), the LPF would also cut off the frequency range of the signal of interest.

 

[Audioguru]

Active bandpass filter details:

 

[FvM]

I guess if they were configured the other way round (LPF + HPF), the LPF would also cut off the frequency range of the signal of interest.

No. As stated correctly in post #1, ideal LPF and HPF can be cascaded in any order without changing the total frequency response. Problem is that the ideal frequency response is only achieved with zero source and infinite load impedance of each first order filter. Inserting a buffer between both filters fixes the problem, also scaling the impedance of the second filter as shown in post #4.

 

[BradtheRad]

My simulation reveals little or no difference in response between the two configurations.

However the CRRC arrangement is easier to customize.
Example, greater voltage and Amperes are conveyed by using larger input C and lesser output C.

 

[danadakk]

Regards, Dana.