The noise performance of microwave transistors

Expressions for the noise parameters of microwave transistors are derived. The theory is based on a small-signal common-emitter equivalent circuit which includes a new basic noise equivalent circuit and the dominanting header parasitics. The theory is verified experimentally in the L-band (1 to 2 Gc/s) frequency range using Ge and Si microwave transistors. It is found that the header parasitics have little influence on the minimum noise figure, but do have large effects on the equivalent noise resistance and the optimum source admittance in the frequency region above about one-half of the series-resonant frequency resulting from the parasitics in conjunction with wafer parameters. For a quick evaluation of the noise performance, new approximate expressions are also given for the noise figure and for the optimum current which produces the lowest value.