How do I select the right LNA for a receiver front end based on noise figure, gain, and IP3?
LNA Selection Criteria
The LNA is the first active element after the antenna (or antenna filter) and has the greatest impact on receiver noise figure and dynamic range. By the Friis equation, the system noise figure is dominated by the first stage: NF_system ≈ NF_LNA + (NF_2-1)/G_LNA, where NF_2 is the noise figure of everything after the LNA and G_LNA is the LNA power gain. A high-gain, low-NF LNA minimizes the contribution of subsequent stages.
However, high LNA gain also amplifies strong interferers, driving the mixer and ADC toward their compression points. The spurious-free dynamic range (SFDR) of the receiver depends on both the noise floor (set by NF) and the linearity ceiling (set by IIP3). Increasing LNA gain improves NF but degrades the upper end of dynamic range by amplifying interferers.
Technology options: GaAs pHEMT provides the lowest noise (0.3-1 dB NF at 2 GHz) with moderate linearity. SiGe BiCMOS provides low noise (0.5-1.5 dB) with good linearity (IIP3 to +10 dBm). GaN HEMT provides the highest linearity (IIP3 to +30 dBm) but higher noise figure (1-3 dB). InP HEMT achieves the lowest noise of all (0.1-0.5 dB NF at cryogenic temperatures) for radio astronomy and deep space communication.
Spurious-free dynamic range:
SFDR = ⅔(IIP3 - NF - 10log(BW) + 174) dB
Example: NF=1.5 dB, IIP3=-5 dBm, BW=10 MHz:
SFDR = ⅔(-5 - 1.5 - 70 + 174) = 65 dB
Frequently Asked Questions
What noise figure do I need?
For cellular base stations: 1-2 dB NF is standard. For satellite earth stations: 0.5-1 dB. For radio astronomy: 0.1-0.5 dB (cryogenic). For general purpose receivers: 2-4 dB is adequate. The required NF depends on the noise floor target and the losses before the LNA (cable, filter).
How much gain is optimal?
Enough to suppress the noise contribution of the following stage by 10+ dB: G > 10·log(NF2/NF1). Typically 15-20 dB. Too much gain (>25 dB) risks saturating the mixer or ADC with strong signals. Variable-gain LNAs (with a bypass mode) provide flexibility for varying signal environments.
What about P1dB?
The 1-dB compression point (P1dB) limits the maximum signal the LNA can handle before gain drops. P1dB should be 10+ dB above the maximum expected signal level at the LNA input. For IIP3, the rule of thumb is P1dB_in ≈ IIP3 - 10 dB.