Choose an RF low noise amplifier by working backward from your receiver’s noise, signal-level and interference budgets. The useful shortlist contains devices that cover the required band, supply enough gain for the following stages and remain sufficiently linear in the presence of the strongest expected signals.
Start with the receiver requirements
Write down these requirements before comparing catalog rows:
- Lowest and highest operating frequency, occupied signal bandwidth and any tuning range.
- Receiver noise-figure target, minimum wanted signal and required output signal-to-noise ratio.
- Loss ahead of the LNA, including cables, switches, protection devices and filters.
- Required gain and allowable gain variation, plus the input range of the next stage.
- Strongest wanted signal and blockers, including their frequencies, levels and modulation.
- Available supply, current budget, operating temperature, connector interface and mounting space.
Use the RFComp low noise amplifier catalog to find candidates, then check each original datasheet against the same requirements.
Check performance across the actual operating band
A frequency-range label is a starting point. Review noise figure, gain, input/output match and linearity at the band edges as well as the center. A specification at one frequency does not establish the same result across a broadband LNA’s entire range. Record the supply, temperature and whether each value is typical, minimum or maximum. Analog Devices’ LNA selection note explains these interdependent selection parameters.
For a receiver that tunes widely, repeat the budget at its most demanding frequencies. For a narrowband receiver, decide which nearby blockers the amplifier will see after any preselection filter. Avoid choosing extra bandwidth without checking the resulting interference exposure.
Balance noise figure and gain in the complete receiver
Noise figure describes degradation of signal-to-noise ratio under defined reference conditions. Lower LNA noise figure can help receiver sensitivity, but the receiver also depends on bandwidth and the signal-to-noise ratio needed for detection. Use the receiver requirement rather than an arbitrary “good NF” threshold. See Analog Devices’ receiver sensitivity discussion.
Gain reduces the contribution of noise from later stages. For two matched stages, the cascade relationship is Ftotal = F1 + (F2 − 1) / G1, where noise factors F and power gain G are linear ratios, not dB values. Convert the result back to dB. Mini-Circuits’ noise-parameter application note provides the cascade relationship.
Hypothetical calculation: assume matched stages in their linear operating range. An LNA with 2 dB NF and 20 dB gain followed by a stage with 8 dB NF gives approximately 2.14 dB total NF. Adding a matched 1 dB passive loss at the standard 290 K reference temperature before that chain raises the result to approximately 3.14 dB. These are calculated example values, not RFComp product measurements. The same cascade equation accounts for the passive loss.
Include every input-side loss in the budget. If a filter is needed ahead of the LNA to reduce blockers, evaluate its sensitivity cost alongside its protection against overload. Also check how the chosen gain changes signal levels at the mixer, converter or other following stage.
Use P1dB and IP3 for different linearity checks
P1dB is the point where amplifier gain has fallen 1 dB below its small-signal value. Confirm whether a quoted number is input- or output-referred. Use a signal-level budget and an application-specific margin below compression; P1dB is not a damage rating. A receiver carrying a modulated signal also needs a check of its peaks and distortion requirements. Analog Devices’ compression discussion distinguishes compression, saturation and input survivability.
Third-order intercept, IP3, indicates intermodulation behavior. Two tones can produce unwanted responses at 2f1 − f2 and 2f2 − f1, potentially inside the wanted band. IP3 is an extrapolated intercept, not an allowable operating power. Compare input-referred IIP3 with IIP3 or output-referred OIP3 with OIP3, using compatible gain and test conditions. Check tone spacing and per-tone power before treating two quoted values as equivalent. Analog Devices’ IP3 guide explains the distinction.
Read the limits and installation details
Typical curves describe expected behavior under their stated conditions. They do not establish guaranteed worst-case performance. An operating-temperature entry in an absolute-maximum section also does not establish that every electrical specification is guaranteed throughout that range.
Check supply tolerance, RF input limits, polarity, thermal mounting, connector details and the mechanical drawing before ordering. Keep absolute-maximum stress limits separate from the usable linear signal range. Confirm the specified installation conditions and any required derating in the original model document; request clarification where a limit or test condition is missing.
Send a model specific inquiry
Before requesting a quote, keep this short datasheet checklist with your shortlist:
- Correct model number, current datasheet and drawing.
- Required band, NF, gain and linearity, with minimum/maximum limits separated from typical values.
- Input/output reference, test frequency, supply and temperature for each deciding specification.
- Blocker levels, input survivability, connector configuration and thermal installation.
- Quantity, required delivery date and any evidence needed for design approval.
Send your LNA requirements to RFComp. Put the candidate model number in the inquiry message and attach your requirement list where supported. Ask RFComp to confirm the applicable specification conditions, current commercial status and quotation details.
