filter followed by amplifier vs amplifier followed by filter

[aggpankaj2]

Hi,

Can somebody tell me is there is any difference in output when a LPF follows and amplifier stage vs amplifier follows the LPF stage? If no, then where to use which stage?

 

[LvW]

Hi,

Can somebody tell me is there is any difference in output when a LPF follows and amplifier stage vs amplifier follows the LPF stage? If no, then where to use which stage?

If both units are ideal as far as the input/output impedances are concerned (no interaction due to loading effects) it does not matter which unit comes first.
From this it follows that your question cannot be answered without information about input/output impedances. More than that, even the impedance of the signal source as well as the input impedance of the load connected to the last stage (whatever it is) matters.
Example: If you speak about a passive filter and an opamp with a rather high input impedance, the best solution most probably will be: filter first, then amplifier. As mentioned:...most probably - but it depends also on source and load properties.

 

[biff44]

yes, it really depends on what you are trying to do.

If you have a wide bandwidth to an input signal, but only want to use a small portion of it, lowpass filtering before an op amp can keep the higher frequency components from mixing back into the lower frequencies through compression/3rd order effects.

If you are inputing a signal to an analog to digital converter, you want the lowpass after the op amp so that ANY 2nd harmonics formed (either in the original signal or in the op amp itself) are eliminated, and do not cause aliasing.

 

[aggpankaj2]

thanks everybody! i got the idea...

 

[Phytech]

The noise figures are different!

 

[aggpankaj2]

how, could you kindly explain this???

The noise figures are different!

 

[Phytech]

how, could you kindly explain this???

When the filter is placed before the amplifier, the overall NF is the NF of amplifier plus the insertion loss of the filter. If the filter is placed after the amplifier, the overall NF is the NF of the amplifier only provided the amplifier has sufficient gain.

However, it doesn't mean it is always good to put the filter after the amplifier, if you have interference signals which will also be amplified and cause intermodulation, you want to filter them out first before you amplify them.