How do you design a low-noise amplifier (LNA)?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

An LNA is the first active stage of a receiver; by the Friis formula its noise figure and gain largely set system NF. Design involves selecting a device with low NF_min at the operating frequency, presenting the optimum source reflection Γ_opt (or close to it), providing an acceptable input return loss, achieving enough gain to suppress later-stage noise, and ensuring unconditional stability and adequate IIP3.^[1]^[2]

How do noise match and gain match differ?

Γ_opt is generally not equal to S₁₁*, so a noise match leaves input mismatch. Constant-noise circles and constant-gain circles on the Smith chart show the trade-off. Inductive source degeneration (a small inductor in the source/emitter) moves the input impedance’s real part toward 50 Ω without adding noise, bringing the gain and noise matches closer; in CMOS, Re{Z_in} ≈ g_mL_s/C_gs = ω_TL_s.^[3]

What topologies are used?

Common-source/emitter with inductive degeneration: narrowband, low NF.
Cascode: improves reverse isolation and stability.
Common-gate: broadband input match, higher NF; noise-canceling variants improve it.
Resistive shunt feedback: broadband, moderate NF.
Balanced LNAs with quadrature hybrids for good match.

^[2]

What about input loss and protection?

Any loss before the LNA adds directly to NF, so use high-Q input matching components, short traces and low-loss pre-filters. ESD protection and limiter diodes add capacitance and loss; budget for them.

Which specifications define an LNA?

NF, gain, input and output return loss, IIP3 and P1dB, reverse isolation, supply current, stability factor, and recovery from overload. Measure NF with the Y-factor method and linearity with two-tone tests.^[4]

Frequently asked questions

What NF can an LNA reach?

Discrete pHEMT LNAs below a few GHz commonly reach well under 1 dB.

Why not maximize LNA gain?

Excess gain hurts downstream linearity; enough gain to suppress later noise is the goal.

Where should the LNA be placed?

As close to the antenna as practical, after any necessary pre-selection filter.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.
“The Design of CMOS Radio-Frequency Integrated Circuits, 2nd ed.”. T. H. Lee, Cambridge University Press. 2004.
“Fundamentals of RF and Microwave Noise Figure Measurements (Application Note 57-1)”. Keysight Technologies. application note.

Categories: Amplifiers

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