How do I choose between GaAs, SiGe, and CMOS LNAs?

GaAs pHEMT offers the lowest noise figure (0.3 to 0.5 dB at 2 GHz) and highest linearity (OIP3 above 35 dBm), making it the standard for base station and military receivers. SiGe BiCMOS provides moderate noise (0.8 to 1.2 dB) with excellent integration (LNA plus mixer on one die), making it cost-effective for consumer devices. CMOS is cheapest and integrates entire transceivers, but NF is 1.5 to 3 dB and linearity is limited, suitable for WLAN, Bluetooth, and IoT where sensitivity requirements are relaxed. At mmWave (28 to 39 GHz), SiGe and InP are preferred because GaAs pHEMT gain drops off.

What gain should an LNA have?

Enough to suppress the noise of subsequent stages but not so much that strong signals drive the LNA or mixer into compression. For a cellular base station with a 0.5 dB NF LNA followed by an 8 dB NF mixer: 15 dB of LNA gain makes the mixer's contribution less than 0.1 dB to system NF. Adding more gain beyond this point does not improve noise figure but does reduce the system's ability to handle strong interferers. Typical LNA gain is 15 to 25 dB for base stations, 10 to 15 dB for handsets (where strong signals from nearby towers are common), and 30 to 40 dB for satellite earth stations where signal levels are very weak.

Active Components

Low-Noise Amplifier

Q: Why is noise matching different from power matching?

The source impedance that minimizes the transistor's noise figure (Gamma_opt) is generally different from the source impedance that maximizes power transfer (complex conjugate of S11). Presenting Gamma_opt to the transistor achieves minimum noise figure (Fmin) but may result in a significant input mismatch (high S11, poor return loss). The designer must choose a compromise: accept 0.2 to 0.5 dB worse noise figure to achieve 10 to 15 dB return loss, or use a source degeneration inductor (common in CMOS LNAs) that rotates Gamma_opt closer to the center of the Smith Chart, allowing simultaneous noise and impedance matching.

LNA

The signal arriving at a cellular base station antenna from a handset 2 km away is approximately −90 dBm: one picowatt. The thermal noise floor in a 10 MHz bandwidth is −104 dBm. The LNA must amplify this signal by 20 dB without adding more than 0.5 dB of noise, or the SNR will be too low for 64QAM demodulation. Simultaneously, a nearby handset 10 meters away is blasting −10 dBm into the same antenna. The LNA must not compress or generate intermodulation products from this signal while faithfully amplifying the −90 dBm signal. This dual demand, near-zero noise and high linearity, is why LNA design is one of the most exacting disciplines in RF engineering.

Category: Active Components

NF Range: 0.3 to 3 dB

Key Trade-off: Noise vs. Linearity

Choosing the Right Technology for the Job

Technology	NF (2 GHz)	OIP3	Gain	Cost	Best For
GaAs pHEMT	0.3 to 0.5 dB	+35 to +45 dBm	15 to 25 dB	$2 to $8	Base station, military, radar
GaN HEMT	0.5 to 1.0 dB	+42 to +50 dBm	12 to 20 dB	$5 to $15	High-dynamic-range, EW
SiGe BiCMOS	0.8 to 1.5 dB	+15 to +25 dBm	10 to 20 dB	$0.50 to $2	Handset, consumer, integrated RX
CMOS (65 nm)	1.5 to 3.0 dB	+5 to +15 dBm	10 to 15 dB	$0.10 to $0.50	WLAN, BT, IoT, fully integrated
InP HEMT (cryogenic)	0.04 to 0.1 dB	+10 to +15 dBm	25 to 35 dB	$500+	Radio astronomy, deep space

Noise figure from noise parameters:
F = F_min + (4R_n/Z₀) × |Γ_S − Γ_opt|² / ((1 − |Γ_S|²) × |1 + Γ_opt|²)

F is minimized when Γ_S = Γ_opt. The noise resistance R_n determines how quickly NF degrades as Γ_S deviates from Γ_opt. Low R_n devices are more forgiving of imperfect noise matching.

Common Questions

Frequently Asked Questions

Why is noise matching different from power matching?

Γ_opt (minimum NF) differs from S₁₁* (max power transfer). Compromise: accept 0.2 to 0.5 dB worse NF for 10 to 15 dB return loss, or use source degeneration inductance (CMOS) to rotate Γ_opt toward 50 Ω for simultaneous noise and impedance match.

GaAs vs. SiGe vs. CMOS?

GaAs: lowest NF (0.3 dB), highest IP3, for base stations. SiGe: moderate NF (1 dB), integrates LNA + mixer, for consumer. CMOS: cheapest, 1.5 to 3 dB NF, fully integrated, for WLAN/IoT. At mmWave, SiGe and InP dominate.

How much gain does an LNA need?

15 to 25 dB for base stations (suppresses mixer NF to <0.1 dB contribution). 10 to 15 dB for handsets (strong nearby signals). 30 to 40 dB for satellite earth stations (very weak signals). More gain improves NF but reduces strong-signal handling.

Related Terms

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Front-End Design

LNA Selection Guide

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