Noise Figure Circles

Noise Figure is defined by

Definition of Noise Figure

In terms of Circuit Parameters, Noise Figure can be written as

Formula for Noise Figure

Here

F_min = Minimum Noise Figure,
G_s = Input source reflection coefficient,
G_o = Input source reflection coefficient when F=F_min,
R_N is an equivalent noise resistance, sometimes given as normalized r_n=R_N/Z_o.

These parameters are either measured or supplied by the manufacturers.

Since the noise figure equation given above is a function of the sourse reflection coefficent G_s, we can then raise the questions:
For what values of G_s, we can obtain the same Noise Figure?
Manipulation of the equation for constant F gives a locus of points for G_s on the Smith Chart which lie on a circle.

If we define a parameter

gp definition

The center and the radius of the circles for constant F are given by

Center

Here F(dB) has to be converted into a number

The following example gives the constant available gain circles for NE76000 transistor at 10 GHz.
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