RF amplifiers: why an LNA and a PA solve different problems

September 29, 2026

Try the idea

Each stage has a different job

AntennaLNAMixerDemodulator

What does the LNA do?

It amplifies a weak received signal while limiting additional noise. It also amplifies existing noise and can overload on strong signals.

Where is the PA?

The power amplifier is in the transmit path, which is not shown in this simplified receive chain. It needs suitable loading, power and filtering.

Teaching model only. No radio transmission, measured coverage or delivery guarantee.

“Add an amplifier” sounds like a simple way to improve a radio link. In practice, you must first identify which end of the link is limited and by what. A receiver struggling with noise, a receiver overloaded by a strong nearby transmitter and a transmitter with a poor antenna need different remedies. More gain is not a universal solution.

LNA: protect the weak signal early

A low-noise amplifier, or LNA, raises a weak received signal while adding as little noise as practical. It also amplifies noise already present at its input; it cannot recover information that has already been lost. Its position near the beginning of the receive chain matters because early losses and noise affect everything that follows.

Noise figure describes degradation of signal-to-noise ratio through a device under specified conditions. Gain describes amplification. They are different specifications. An LNA with impressive gain but poor noise figure is not automatically a good choice. Analog Devices’ selection note also stresses compression, distortion and frequency range.

A simple gain budget

Suppose an illustrative signal reaches an amplifier input at −100 dBm and the amplifier provides 15 dB gain in its linear region. Its signal output is then approximately −85 dBm. A following 3 dB cable loss reduces that to −88 dBm. This arithmetic says nothing by itself about whether the decoder can recover the message: noise, interference, bandwidth and modulation still matter.

An amplifier before a lossy receive cable may help offset the influence of later stages, but it cannot undo loss before its own input. Nor is an external LNA always beneficial: an integrated radio may already have an appropriate front end, and adding gain may overload it. Check maximum input level and the complete signal chain before connecting equipment.

Illustration: RF amplifier and oscilloscope signals. Gain, noise and output power are different quantities.

Too much signal is also a problem

Real amplifiers stop behaving linearly at sufficiently high levels. Compression means that increasing the input no longer produces the expected proportional output increase. Multiple strong signals can also generate intermodulation products that fall into the frequency band you want to receive.

This explains the apparently strange case where adding an amplifier makes reception worse. A strong nearby signal may consume the amplifier’s usable range even if it is not your desired transmission. Filtering, less gain, a better antenna location or a more linear front end may be the remedy. A filter before the LNA helps reject unwanted signals but adds loss; receiver design balances those effects.

PA: deliver transmit power

A power amplifier, or PA, provides the RF output power on the transmit side. It must operate with a suitable load, supply, matching network and thermal conditions. Efficiency determines how much supplied energy becomes useful RF output rather than heat. Linearity and filtering influence unwanted emissions.

More transmitter power helps only one part of a two-way conversation. If the distant device cannot send back successfully, your stronger outgoing signal has not completed the link. Antenna placement and receiver performance remain important. External amplifiers may also change radiated power and compliance; they are not an automatic upgrade for a finished LoRa board.

Low noise is not the same as low power

“Low-noise amplifier” describes a receiving performance goal. “Low-power amplifier” may refer to low DC consumption or modest RF output, depending on context. Always ask which quantity is meant. For battery operation, compare supply voltage, idle and active current, enable/sleep behaviour and how long each state lasts.

As an illustrative power calculation, a circuit drawing 50 mA from 3.3 V consumes 165 mW of electrical power. That is not its RF output power. Keep DC input, RF output and gain in separate columns when comparing specifications.

A useful selection checklist

Identify transmit or receive, operating band and expected signal levels.
Check gain, noise figure where relevant, compression/linearity and maximum input/output ratings.
Verify impedance, connector, supply voltage, current and control requirements.
Determine filtering, shielding and transmit/receive switching needs.
Test the existing antenna and feed line before adding another active stage.

Follow the manufacturer’s reference design rather than placing an arbitrary amplifier in the antenna cable of a transceiver. A receive-only LNA may not tolerate the radio’s transmit power. Read antenna mismatch and PA protection and mixers and frequency conversion to complete the picture. Contact eBits with the actual radio and signal-chain diagram when discussing a project.

Reviewed 28 September 2026. Calculations are teaching examples, not specifications or modification instructions for a stocked board. Further reading: receiver front-end trade-offs.

The radio workshop

Understand the radio. Build the network.

LoRa, Meshtastic and Reticulum: learn what each layer does, choose suitable parts and test your own links. Start with a learning path, then explore the electronics underneath.