Noise Match Point
Understanding the Noise Match Point
The noise match point is determined by the internal noise sources of the transistor and their correlation. It cannot be predicted from S-parameters alone; noise parameters must be measured or provided by the manufacturer. LNA input matching network design centers on transforming the 50-ohm source to this optimum impedance.
Noise Parameter Measurement
Four noise parameters are determined by measuring noise figure at multiple source impedance values (typically 8-20 impedance states using a tuner), then fitting to the noise parameter equation.
Noise Figure vs Source Impedance
NF = NF_min + (4 R_n/Z0) x |Gamma_s - Gamma_opt|^2 / ((1-|Gamma_s|^2) x |1+Gamma_opt|^2). NF increases quadratically as the source impedance moves away from Gamma_opt.
NF = NF_min + (4 R_n/Z0) |Gs - G_opt|^2
/ ((1-|Gs|^2)|1+G_opt|^2)
Noise parameters:
NF_min = minimum achievable noise figure
G_opt = optimal source reflection coefficient
R_n = noise resistance (sensitivity)
Frequently Asked Questions
What is the noise match point?
The noise match point (Gamma_opt) is the source impedance that gives minimum noise figure in an amplifier. It is determined by the transistor's internal noise sources and must be measured or provided by the manufacturer. LNA design aims to present this impedance at the input.
How is Gamma_opt different from S11?
Gamma_opt is the source impedance for minimum noise. Conjugate of S11 is the source impedance for maximum gain. They are generally different because noise behavior depends on internal noise source correlation, not signal transfer properties.
What are noise circles?
Noise circles on the Smith Chart show contours of constant noise figure. The center is Gamma_opt (minimum NF). Moving away from center, NF increases. The circles help visualize the trade-off between noise match and gain match in LNA design.