RF Amplifier (LNA / PA)
Amplifier
Two very different jobs in one name: the LNA (low-noise amplifier) lifts faint received signals without adding hiss, while the PA (power amplifier) muscles the transmit signal up to real wattage. Every radio has both.
What it looks like
RF Amplifier (LNA / PA)
A typical amplifier — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Frequency range
MHz / GHzThe band over which gain and matching are guaranteed.
Gain
dBSignal multiplication: 10–25 dB per stage typical. Chained stages add dB.
Noise figure
dBThe LNA's defining spec — how much hiss it adds (0.5–1.5 dB is excellent). The first stage's NF dominates the whole receiver.
Output power (P1dB)
dBm / WThe PA's defining spec — power at 1 dB compression. Beyond it, distortion and splatter.
Supply & efficiency
V / %PAs draw serious current; class of operation (A/AB/C/E) trades linearity against efficiency.
Markings
MMIC blocks are tiny SOT-89/SOT-363 parts with cryptic codes (e.g. BGA420 marks 'AN'). Module boards print band and gain, e.g. 'LNA 50–4000 MHz 21 dB'. Datasheet lookup is the rule.
Standard values
Hobby staples: LNA4ALL/SPF5189Z LNA boards (50 MHz–4 GHz), 2.4 GHz 1–2 W WiFi boosters, GSM/LoRa PA modules, 88–108 MHz FM PA kits, Mini-Circuits gain blocks (ERA/MAR series).
How to choose
1) Receiving weak signals (SDR, GPS, satellite): pick lowest noise figure and mount the LNA at the antenna, before cable loss. 2) Transmitting: pick P1dB above your target power with headroom, add a harmonic low-pass after it, heatsink it, and mind licence limits. 3) Wideband MMIC blocks are the easy path — 50 Ω in/out, one supply resistor. 4) Never point a PA into a mismatched/absent antenna.
Pinout & package
RF in, RF out, DC supply, ground. Modules: SMA in/out with a DC jack or bias-tee feed (power sent up the coax — how masthead amps and active GPS antennas are fed). Bare MMICs: input, output/bias, ground pins.
Example circuits
- RTL-SDR + LNA at the antenna pulling in weather satellite images
- Active GPS antenna: patch + SAW + LNA powered via 3.3 V bias tee on the coax
- LoRa node with PA/LNA front-end module (e.g. SX1262 + PA) for extra kilometres
- TV masthead amplifier overcoming a long coax run's loss
Common failures
PA burnout from transmitting into a bad VSWR or missing antenna (instant), LNA front-end blown by ESD/static from the antenna or by a nearby transmitter, oscillation from poor layout (amp becomes a transmitter), and thermal death of undercooled PAs.
How to test
Gain check: known signal in, compare RSSI/power out vs bypassed (should rise by the rated dB). An LNA that adds noise but no signal lift is damaged. PA: measure output into a dummy load with a power meter — never test on-air. Current draw far from datasheet typical signals trouble.
Substitutes
Any same-band amp with equal/better NF (receive) or P1dB (transmit) and 50 Ω interfaces. Wideband MMIC blocks substitute across a huge range at moderate performance. For marginal links, better antennas or lower-loss coax often beat adding an amplifier.