Q: MAX2090 is specified to have a 4dB noise figure at maximum gain. How does this change across the gain range?
A: The internal amplifier has a fixed 26.1dB gain, and the control voltage adjusts the chip's gain by controlling an attenuator block in front of the amplifier. The 4dB noise figure is for the internal amplifier only, and is measured with the control voltage at its max (no attenuation). If you begin to attenuate the signal, the noise figure of the chip will rise accordingly, 1dB for every extra dB of attenuation. Because of this, the part wasn't intended for use as an LNA, and is best suited to having some gain stages before it in the signal chain to offset the noise impact of the attenuation.