RF LNA
An RF LNA amplifies an antenna or filter output by 15–25 dB while presenting a noise figure (NF) of 0.5–2 dB across its specified band. The Friis cascade equation makes its NF the dominant contributor to total receiver NF, so the LNA's quality directly sets receiver sensitivity. Examples: Mini-Circuits PMA3-83LN+, Qorvo QPB9335, Skyworks SKY67159-396LF.
An RF LNA is built around a low-noise transistor (CMOS, GaAs pHEMT, or SiGe HBT) biased at its minimum-noise current, with input matching that simultaneously approaches the conjugate match for power and the optimum-NF source impedance Γ_opt. Inductive source degeneration on FET LNAs aligns these two impedances at one frequency, producing both low NF and good return loss. Output matching transforms the transistor's output back to 50 Ω; ESD-tolerant input pads protect the gate.
In plain terms
A whisper amplifier in a quiet library: it makes the soft voice loud enough to hear without adding any rustling paper of its own — because once noise is added in the first stage, no later stage can remove it.
Why designers use it
- Receive faint GPS satellite signals (typically −127 dBm) with sufficient SNR to demodulate.
- Boost cellular RX signals from a weak in-building cell before they hit a lossy filter chain.
- Recover Wi-Fi packets at the cell edge by lowering total RX NF below the modem's required SNR.
- Pre-amplify radio-astronomy or radar return signals just microseconds before they're swamped by switching noise downstream.
Best for
- GPS/GNSS
- Cellular RX
- Radio astronomy
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When the selected amplifier's input, output, stability or thermal limits cannot accommodate the signal environment.
- When preceding losses already compromise the required system noise performance; evaluate the complete chain.
Common mistakes
- Ignoring compression and intermodulation when strong unwanted signals reach the amplifier.
- Routing bias and RF without following the selected device's reference network and layout.
- Treating one amplifier's biasing arrangement as universal.
Where you will find it
- A u-blox NEO-M9N GNSS module places an LNA right after the patch antenna: the chip's PMA3-style LNA contributes ~0.7 dB NF to the front end, recovering the −127 dBm satellite signal to a level where the digital correlator can lock onto it; without the LNA, the SAW filter and trace loss would push system NF above 6 dB and the receiver could not lock indoors.
- A Starlink user terminal's phased-array antenna uses dozens of GaAs pHEMT LNAs (Qorvo TGA2624) — one per element: each LNA conditions the 12 GHz Ku-band element signal with NF < 1.5 dB before downconversion, allowing the array to detect the satellite's −95 dBm spot beam through atmospheric attenuation.
- A Hubble Space Telescope ground station at Goldstone uses cryogenically cooled HEMT LNAs at 14 K: chilling the FET reduces channel thermal noise so the system NF drops to ~0.3 dB, recovering the spacecraft's −165 dBm telemetry from 540 km altitude with bandwidth left over for the gigabit science data stream.
A short history
A low-noise amplifier is an RF circuit placed close to an antenna to boost weak received signals while adding as little extra noise as possible. Because it sits early in a receiver chain, its noise and gain performance strongly influence the overall signal-to-noise ratio, so designers aim for low noise, stable gain, and proper impedance matching across the intended operating band.