# Why should LNA also care about IIP3?

1. ## Why should LNA also care about IIP3?

Usually LNA works in linear region, but some text books and documents say that sometimes there can be "blocker" signals which may force the LNA into non-linear region.

My question is what's source of the blocker signals? In my view, when your mobile phone is very near to the base station, the received signal may have very large power that will make your LNA non-linear. Or when there's a large crowd of people, the PAs of other people's phones can make your LNA non-linear? Are these true? Also, some documents say that harmonic outputs from transmitters who don't even work in the same band with your phone can also be that blocker. Is it because even if these harmonics are well-rejected by filters in the transmitting system, its power may still be larger than the desired received signal because the received signal is usually very small?

Any way, I just want to know why we should care about IIP3(or non-linearity) for LNA.

Also, should we always put a filter before the 1st stage LNA in receiver? If true, why should we do that?

Thanks!

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2. ## Re: Why should LNA also care about IIP3?

Assume three mobile phones, A, B and C.
Their Distances from base station are different.
Distance of A is small.
Distance of B and C is large.

Consider tx power control from base station.
TX power for A is small.
On the other hand, TX power for B and C is large.
So A undergoes large two interferences.

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3. ## Re: Why should LNA also care about IIP3?

Originally Posted by pancho_hideboo
Assume three mobile phones, A, B and C.
Their Distances from base station are different.
Distance of A is small.
Distance of B and C is large.

Consider tx power control from base station.
TX power for A is small.
On the other hand, TX power for B and C is large.
So A undergoes large two interferences.
Thanks!

Do you mean that the base station is transmitting signals for A, B and C simultaneously? So all these three signals will arrive at A, which means signals for B and C may be blockers for A?

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4. ## Re: Why should LNA also care about IIP3?

Originally Posted by sqx
Do You you mean that the base station is transmitting signals for A, B and C simultaneously?
Yes........

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