what decides that you need a low pass filter or a high pass filter

[nalawade]

I have a question which is very serious. When I want to choose a low-pass filter, then what parameters of the signal determine what frequency it should have?

How should I decide the frequency I want ?

 

[crutschow]

It would seem apparent that the frequency response of the filter would be determined by the requirements of the system it is in. It depends upon the signal characteristics and what you want to do with that signal. Without the answer to those questions you can't reasonably choose a filter.

 

[nalawade]

The signal is coming form a resistor. The signal is actually the voltage across the resistor. This resistor is connected between the emitter of IGBT and ground. The IGBT is carrying current. Therefore the current also flows through this resistor.

The frequency of the current flowing through the resistor is 25 kHz.

What I am doing is , the voltage signal across this resistor is given to an non-inverting terminal of an operational amplifier. The gain of the op-amp is set to 100.

Should I use a low pass filter before giving this voltage signal to non-inverting terminal of op amp ?

 

[LvW]

Should I use a low pass filter before giving this voltage signal to non-inverting terminal of op amp ?

How could we know? Why do you consider any filtering? Is the 25kHz signal a wanted signal or not?

 

[crutschow]

...................................
Should I use a low pass filter before giving this voltage signal to non-inverting terminal of op amp ?

Depends upon what signal you want at the op amp output.

 

[BradtheRad]

Should I use a low pass filter before giving this voltage signal to non-inverting terminal of op amp ?

It makes more practical sense, to first remove an unwanted signal, then amplify the wanted part.

Theoretically it is feasible to amplify everything first. However there might be distortions entering into it. If so then you do not need an unwanted signal contributing to distortion.

Or even if the amplified signal is distortion-free, you're liable to have to use higher-rated components when building your filter.

 

[Audioguru]

Why is there this second thread about this comparator used as a very low frequency amplifier??
The circuit has FOUR lowpass filters as I explained in the other thread, and one of the filters is already directly at its input.

It DOES NOT amplify 25kHz, instead it amplifies DC.