LNA & RF Amplifier Notes: Noise Figure & Gain

Key points

• ▹Noise figure sets the weakest signal you can use; gain sets how much the following stages matter.
• ▹An early low-noise stage with sufficient gain reduces later stages' noise contribution. Cable, filter and switch losses ahead of it still degrade receiver noise figure.
• ▹More gain is not always better — excessive gain reduces headroom and can compress later stages.
• ▹The input match that minimises noise is often not the match that gives the best gain or return loss.
• ▹Linearity (P1dB, IP3) decides behaviour with strong signals, and is checked separately from noise.

Comparison

What each amplifier parameter actually controls

Noise figure sets the sensitivity floor

An early low-noise amplifier with sufficient gain reduces the noise contribution of later stages. Friis' formula uses linear noise factors and gains: Ftotal = F1 + (F2 - 1) / G1 + (F3 - 1) / (G1 G2) + … . Low first-stage noise factor alone is not enough; cable, filter and switch loss ahead of the LNA also degrades the receiver's noise figure.

Noise figure is frequency-dependent, so compare devices across your actual band rather than at a single spot frequency. A difference of a few tenths of a dB in the first stage can matter more than several dB further down the chain.

Gain, flatness and stability

Gain lifts the desired signal above the noise of subsequent stages, but more gain is not free: it consumes linearity headroom and can push following mixers or ADC drivers into compression. Aim for enough gain to set the noise floor, with margin, rather than the maximum available.

Check gain flatness across the band and confirm the amplifier is unconditionally stable (or that your matching keeps it stable) across the full operating range — instability shows up as spurious oscillation, not just poor specs.

Linearity and dynamic range

In the presence of strong interfering signals, linearity often matters as much as noise figure. The 1 dB compression point (P1dB) and the third-order intercept point (IP3) describe how the amplifier behaves as signals get large. A receiver that must keep working with strong nearby signals needs enough IP3 to avoid intermodulation products landing in-band.

Dynamic range is the span between the noise floor and the onset of compression — improving it means lowering noise figure and raising IP3 together, which is why LNA selection is a balance rather than a single-number contest.

What to specify

Give the supplier your frequency range, target noise figure, required gain and flatness, P1dB/IP3 targets, supply voltage and current, package or connector format, and any need for bypass or gain switching. That is enough to shortlist standard modules without public datasheets.

Frequently asked questions

What noise figure should I aim for?

It depends on band and application. Lower is better for sensitivity, but with diminishing returns and cost. Match the target to your system's required sensitivity and the loss in front of the LNA; a good supplier will recommend a realistic figure for your band.

Can an LNA have too much gain?

Yes. Excess gain reduces linearity headroom and can compress following stages. Specify enough gain to set the noise floor with margin, not the highest available.

What is the difference between an LNA and a power amplifier?

An LNA is optimized for low noise figure at small signal levels at the receiver input; a power amplifier is optimized for output power and efficiency at the transmitter output. They sit at opposite ends of the chain and are specified very differently.

Do I need a bypass or switched-gain option?

Consider it when the receiver sees a wide range of input levels. Bypass or stepped gain protects linearity for strong signals while preserving sensitivity for weak ones.

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