Filter before amplification or amplify before filtering

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watertreader

Joined: 02 Dec 2005
Posts: 187
Helped: 6

06 Dec 2007 13:13 filter before amplify

Hi,

Would like to know which one is the common practice? which is better and is there a condition for one to be superior over the other?

I have a PMT(photomultiplier tube) which is currently connected to an amplifier before the amplified signal is then digitised for processing with the computer. I would like to include in a filter to clear up some interfering noise from both the electrical line and ambient source. Where should I place it? what other consideration do we need?

Thanks!

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

06 Dec 2007 16:24 filter or amplify first?

Always amplify BEFORE filtering.

When filtering you attenuate the signal, and when you amplify, you amplify both the signal and the noise.

If you put the filter before the amplifier you first filter your signal attenuating it. This means that the filtered signal level is very close to the noise level. When then amplifying it, you get both signal and noise amplified, thus having almost the same signal to noise ratio (SNR, which will be low) but with amplified levels.

In the other hand if you first amplify you will get both noise and signal amplified, BUT, then you filter the signal, thus eliminating most of the amplified noise, thus getting a much better SNR.

Hope it's clear Smile

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Eugen_E

Joined: 29 Nov 2004
Posts: 358
Helped: 31

06 Dec 2007 17:50 filter before or after lna

It depends on situation.

If the input signal has a limited amplitude range, and the amplifier is linear in that range, you should amplify and then filter the signal, so you don't attenuate the usefull signal and keep a high SNR.

But if the input signal it's very noisy and has random amplitude pertubations superimposed (like the signal from a receiver antenna) it won't have a well defined amplitude range and may overload the amplifier (if it's not linear at high levels). Working in the non linear range will destroy the usefull signal, so in this case it's useful to have a minimal filter at the input to attenuate the perturbations outside the interest bandwidth.

yeechyan

Joined: 14 Jun 2004
Posts: 111
Helped: 1

13 Dec 2007 2:58 filtering or amplifying first

If you are using switching amplifiers, you need filters at the output to filter out the carrier frequency.

laktronics

Joined: 06 Jan 2007
Posts: 945
Helped: 156

13 Dec 2007 15:19 filter effects before amp

Hi,
It is desirable to put the filter before the amplifier for the following reasons:

- Since the level of the input signal is low, the filter components need not handle large signals and therefore need be less power rated.

- The amplifier need not handle noise, which might otherwise have introduced distortions due to clipping of total input (signal + noise), thus improves the usable signal range of the amplifier.

- Assuming amplifier itself is not the source of noise ( like Chopper amplifiers), the SNR of the overall circuit will remain same irrespective of where the filter is introduced.

Regards,
Laktronics.

ehsanelahimirza

Joined: 24 Feb 2006
Posts: 430
Helped: 33

13 Dec 2007 19:15 do i amplify then filter or filter then amplify?

filteration should b first otherwise amplification will amplify the noise level which will be more difficult to tackle as compared to filter before amplification

Old Nick

Joined: 14 Sep 2007
Posts: 445
Helped: 49

13 Dec 2007 20:32 before filter & amplifier

ehsanelahimirza wrote:

filteration should b first otherwise amplification will amplify the noise level which will be more difficult to tackle as compared to filter before amplification

Filters will add noise too though.
In any noise critical system which requires a gain stage, the highest gain part of the sytem should always be placed first (if possible), then subsequent noise additions will be a smaller compared to the signal.

Filtronetics

Joined: 10 Nov 2004
Posts: 34

14 Dec 2007 0:25 filter noise or amplify first

Maybe you can consider using two filters. One before the amplifer as preselect filter which normally has low insertion loss and wide bandwidth. Another one after the amplifier with more selectivity to get the signal you want.

The filter before amplifier needs to be with low insertion loss as the insertion loss will be added to the system noise figure directly.