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.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
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.