LNA operating in its roll off region, filtering action of a strong interferer

Question

I have an LNA and a strong interferer hitting it. Knowing the in band input IP3 and input P1 dB of the LNA, that strong interferer would saturate the amplifier if it were in band.

However the interferer happens to be in the roll off region of the LNA, so the LNA gain is reduced in that region. If I consider the roll off to be acting like a filter and if I assume that in the roll off region the input IP3 and input P1 dB remain unchanged, then the interferer would not saturate the LNA.

Now can I assume that the roll off of the LNA gain will really act like a filter? and is my assumption that the input IP3 and input P1 dB remain the same in the roll off region (like in band) a reasonable assumption?

Answer 1

You make a mistake: the assumption you need is not that the input IP3 and input P1dB remain unchanged. That would not help you at all: since the input signal remains the same it would saturate the LNA just as much as before.

What you need is that the output IP3 and output P1dB remain unchanged, because that means that with lower gain the input IP3 and input P1dB actually increase, and therefore the LNA can handle larger input signals.

Whether this assumption (or any assumption) is reasonable depends of course on the specifics of the LNA, which you did not show us. But we can look at two reasonable possibilities:

1. The gain roll-off happens at the input, for instance due to input capacitance, and the gain compression happens at the output (e.g. by exceeding the possible voltage swing). In that case you are fine, with higher f you will have less output signal, so less problems with distortion. The input IP3 increases. This might happen naturally at high frequencies where the roll-off is caused by approaching the fT of the device.
2. The gain roll-off happens at the output, for instance by a tuned tank circuit. In that case it is like post-filtering, the LNA linearity will not improve, the input IP3 will remain the same. The output IP3 will become lower.

Of course you can have a situation in between those two cases.