Skip to content

LNA β€” Low Noise Amplifier Design

Last reviewed: 2026-09-17

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.

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 Sβ‚‚β‚‚ 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 β€” Lowest noise, highest cost (satellite, aerospace)
  • SiGe HBT β€” Good noise + integration (cellular, radar)
  • CMOS β€” Lower performance, lowest cost, integrates with digital
  • GaN β€” High power handling, higher noise (base stations)

Impedance Matching

  • LNA input must be matched to the source impedance (usually 50Ξ©)
  • Noise matching β‰  power matching β€” 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, Ξ” < 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 + (Fβ‚‚ - 1)/G₁ + (F₃ - 1)/(G₁·Gβ‚‚) + ...

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

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

Resources

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