4.2 Front End Receiver Noise FIgures
Figure 1. Block diagram of receiver front-end
Front End Receiver
Consider the typical front-end of a receiver shown in Figure 1, where the output impedance of the signal source is denoted as Zss. The low-noise amplifier (LNA) represents the device that is modeled with noise parameters. Zin denotes LNA input impedance. A passive matching network is in general needed between the signal source and the LNA. This network has a different purpose in different systems. However, it is most often used to convert Zss to Zs such as Zs is complex conjugate of Zin.
In such a case, all output power of the signal source is delivered to the LNA. In other words, no power is reflected back to the signal source, and the reflection coefficient of the Matching Network and LNA is near zero. If Zopt of the LNA is known, we can determine whether the LNA noise figure (NF) comes close to NFmin. If it happens that Zs = Zopt, then NF does equal to NFmin and the LNA is both power matched and noise matched!
If, however, this matching network creates Zs≠Zopt, then the LNA noise figure will be larger than NFmin and the LNA is power matched but not noise matched. Alternatively, the matching network could noise match the LNA but cause a power mismatch, which results in some input power reflected back into the signal source and a reduction in power gain. When this occurs, the power gain is referred in datasheets as the “associated gain.”