Rn circles for series-feedback amplifiers

Constant equivalent noise resistance R_n circles of a series-feedback amplifier are presented. They visualize on the series-feedback element plane, which is the region where a series feedback can be selected in order to decrease the value of R_n. The R_n circles demonstrate that lossy series-feedback impedances can lower the equivalent noise resistance. This is an important point because it demonstrates that the feedback element can make the noise figure less sensitive to changes in the input mismatch; and careful selection of the feedback element may reduce the noise figure of the device, despite the loss in the feedback branch. The use of R_n circles along with circles for a constant magnitude of scattering parameter S_ij on the feedback plane is explained