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 Z_ss. 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 Z_ss to Z_s such as Z_s is complex conjugate of Z_in.

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 Z_opt of the LNA is known, we can determine whether the LNA noise figure (NF) comes close to NF_min. If it happens that Z_s = Z_opt, then NF does equal to NF_min and the LNA is both power matched and noise matched!

Matching Challenges

If, however, this matching network creates Z_s≠Z_opt, then the LNA noise figure will be larger than NF_min 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.”