LNA — Low Noise Amplifier Design

Last reviewed: 2026-05-29

A Low Noise Amplifier (LNA) is an electronic amplifier designed to amplify very weak signals (typically from an antenna) while adding as little noise as possible. LNAs are critical components in radio frequency (RF) and wireless communication systems: Wi-Fi, cellular, GPS, satellite, and software-defined radio (SDR).

Overview

The LNA is typically the first active stage in a receiver chain. Its noise figure (NF) dominates the overall receiver sensitivity, making LNA design a key skill in RF engineering. Key parameters include gain, noise figure, linearity (IP3), impedance matching, and power consumption.

Training Content

Key LNA Parameters

Parameter	Symbol	Description	Typical Value
Noise Figure	NF	Degradation of signal-to-noise ratio	0.3–3 dB
Gain	S₂₁	Forward power gain	10–25 dB
Input Return Loss	S₁₁	Impedance match at input	< -10 dB
Output Return Loss	S₂₂	Impedance match at output	< -10 dB
Reverse Isolation	S₁₂	Isolation from output to input	< -20 dB
IIP3	IP3	Third-order intercept point (linearity)	0–20 dBm
P1dB	P1dB	1 dB compression point	-20 to +10 dBm

Design Considerations

Transistor Selection

GaAs pHEMT — Lowest noise, highest cost (satellite, aerospace)
SiGe HBT — Good noise + integration (cellular, radar)
CMOS — Lower performance, lowest cost, integrates with digital
GaN — High power handling, higher noise (base stations)

Impedance Matching

LNA input must be matched to the source impedance (usually 50Ω)
Noise matching ≠ power matching — optimal noise match often differs from conjugate match
Matching network typically uses microstrip/stripline transmission lines or lumped elements (inductors, capacitors)

Stability

LNAs must be unconditionally stable (K-factor > 1, Δ < 1)
Instability can cause oscillations, destroying performance
Stabilization: resistive loading, feedback networks, ferrite beads

LNA in a Receiver Chain

Antenna
   ↓
Bandpass Filter (image rejection)
   ↓
LNA (lowest noise contribution)
   ↓
Mixer (downconversion to IF)
   ↓
IF Amplifier + Filter
   ↓
Demodulator / ADC

The LNA's noise figure directly adds to every subsequent stage's effective noise (Friis formula):

F_total = F₁ + (F₂ - 1)/G₁ + (F₃ - 1)/(G₁·G₂) + ...

Where stage 1 is the LNA. This is why the LNA must have BOTH low noise and moderate gain.

Practical Applications

Application	Frequency	Typical NF	Typical Gain
GPS L1	1.575 GHz	0.8 dB	20 dB
Wi-Fi 2.4 GHz	2.4–2.5 GHz	1.5 dB	15 dB
Cellular LTE	700–2600 MHz	0.5–1.0 dB	18–25 dB
SDR (HF/VHF/UHF)	0–6 GHz	0.5–3 dB	15–30 dB
Satellite (X-band)	7–12 GHz	0.3–0.6 dB	25–30 dB

Design Flow

Define system requirements (frequency, NF, gain, P1dB)
Select transistor technology
Bias circuit design (DC operating point)
Input noise match (source pull simulation)
Output conjugate match (load pull simulation)
Stability analysis across all frequencies
Layout (PCB microstrip, via stitching, shielding)
EM simulation + prototyping
Measurement (VNA, spectrum analyzer, noise figure meter)

Resources

Pozar — Microwave Engineering (standard textbook)
Gonzalez — Microwave Transistor Amplifiers
Keysight ADS / NI AWR for simulation
Mini-Circuits, Qorvo, Skyworks — LNA product app notes