Active Devices › LNA Design
LNA Noise Match Designer
Design LNA input matching networks for minimum noise figure. Enter transistor noise parameters (Fmin, Rn, Γopt) and the tool computes NF at any source impedance, available gain, noise-gain trade-off table and L-network matching component values.
Transistor Noise Parameters
From datasheet at operating frequency and bias point:
MHz
dB
Ω
°
(0=50Ω)
°
Ω
dB
Noise Figure Theory
Noise figure at any ΓS:
F = Fmin + (4Rn/Z0) × |ΓS − Γopt|² / ((1−|ΓS|²)|1+Γopt|²)
Rn sensitivity:
High Rn → NF rises steeply away from Γopt
Low Rn → NF stays near Fmin even with ΓS far from Γopt
Optimal source impedance:
ZS,opt = Z0 × (1+Γopt)/(1−Γopt)
Noise match vs power match:
Γopt ≠ ΓMS in general
Match to Γopt for best NF — sacrifice some gain
Compromise: simultaneous noise and gain match (SANG)
F = Fmin + (4Rn/Z0) × |ΓS − Γopt|² / ((1−|ΓS|²)|1+Γopt|²)
Rn sensitivity:
High Rn → NF rises steeply away from Γopt
Low Rn → NF stays near Fmin even with ΓS far from Γopt
Optimal source impedance:
ZS,opt = Z0 × (1+Γopt)/(1−Γopt)
Noise match vs power match:
Γopt ≠ ΓMS in general
Match to Γopt for best NF — sacrifice some gain
Compromise: simultaneous noise and gain match (SANG)
Design rule: After choosing your NF budget, find the gain circle that intersects the noise circle for that NF — the intersection gives the source impedance that simultaneously satisfies both constraints. Tools like Keysight ADS show these on a Smith chart; this calculator gives you the key numbers to start from.
Noise Match Results
Noise Figure at Selected Source
Fmin (best possible NF)—
NF at current ΓS—
NF penalty vs Fmin—
|ΓS − Γopt|—
Optimal Source Impedance
ZS,opt (real part)—
ZS,opt (imaginary part)—
Actual ZS (current match)—
L-Network to Γopt
Series element—
Shunt element—
Q factor of match—
Match bandwidth (est.)—
Gain
GA at Γopt (noise match)—
Estimated GA at ΓS—
Noise-Gain Trade-off Table
Moving ΓS from Γopt toward ΓMS sacrifices NF but gains GA:
| NF (dBc above Fmin) | NF total | GA (est.) | Match type |
|---|
Noise figure circles: constant NF circles on Smith chart centred at Γopt. Radius rN = √(N(N+1−|Γopt|²)) / (1+N) where N = (F−Fmin)·|1+Γopt|² / (4Rn/Z0)
Available gain circles: centred at specific ΓS on Smith chart, shrink as gain increases toward MAG.
Available gain circles: centred at specific ΓS on Smith chart, shrink as gain increases toward MAG.