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I'm looking at the RF3858 datasheet and on page 2 it shows the gain of the integrated LNA. There are two modes. The high-gain mode has typically 21dB gain. But the low-gain mode has typically -6dB gain.

Now what's up with the low-gain? Does that mean it's actually making the signal weaker? If that's true, what is the purpose?

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    \$\begingroup\$ Link to datasheet, please - in the question and not in the comments. \$\endgroup\$
    – Transistor
    Aug 9 '16 at 17:35
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In low gain, the low noise amplifier can deal with much bigger signals so, this allows the designer to deal with a wide dynamic range of potential signal powers coming from the antenna. For instance the input IP3 in low gain mode is typically 17 dBm whereas in high gain mode it's about -1 dBm. This means, that for a big signal (greater than about 0 dBm the low gain mode will progressively become the better option in avoiding non-linearities (harmonics and intermodulation).

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Yes, negative dB gain means attenuation.

The purpose is to make the signal weaker. Imagine if the transmitter is sitting right beside the receiver. You don't want to overload the front end circuitry in that situation.

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