Source Impedance: Z_s is the impedance seen by the input terminals of the transistor looking toward the source (toward the Input Matching Network). The Input Matching Network transforms the Generator Impedance to an equivalent Source Impedance. This impedance is usually a complex quantity. In the actual design of the amplifier, it plays a major role in determining the overall gain as well as the noise figure of the amplifier. It is interchangable with Source Reflection Coeffcient G_s