Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 32

The formal definition of noise figure for an amplifier defines the noise figure only for the impedance or reflection coefficient of current source termination, but this noise figure is not the 50‐Ω noise figure. Rather, it is the noise figure of the amplifier for the impedance of the source. In general, one cannot compute the 50 Ω noise figure from this value without additional information about the amplifier. If one considers the amplifier in Figure 1.5, with internal noise sources, the effect of the noise sources is to produce noise power waves that may be treated similarly to normalized power waves, a and b.

Figure 1.5 An amplifier with internal noise sources.

The source termination produces an incident noise wave a_NS and adds to the internal noise created in the amplifier, which can be represented as an input noise source a_Namp. There are scattered noise waves represented by the noise emitted from the input of the amplifier, b_N1, and the noise incident on the load is b_N2. From this figure, one can make a direct comparison to the S‐parameters and see that reflected noise power might add or subtract to the incident noise power and affect the total noise power. However, at the input of the amplifier, the noise generated inside the amplifier is in general not correlated with the noise coming from the source termination so that they don't add together in a simple way. Because of this, the noise power at the output of the amplifier, and therefore the noise figure, depends upon the source impedance in a complex way. This complex interaction is defined by two real valued parameters and one complex parameter, known collectively as the noise parameters. The noise figure at any source reflection coefficient may be computed as

(1.70)

where N_Fmin is the minimum noise figure; Γ_Opt, called gamma‐opt, is the reflection coefficient (magnitude and phase) that gives the minimum noise figure; and R_N, sometimes called the noise resistance, describes how the noise figure increases as the source impedances varies from the gamma‐opt. The characterization required to determine these values is quite complex and is covered in Chapter 6.