Based on the En-In noise model of cascade circuits

not only the limitation of Friis theorem is analysed

but also the En-ln noise mathematical model for the cascade cireuits is developed. This model with simple formulas and easy parameter’s extraction will facilitate the implementation of the model into the simulation programs for the circuit noise. The analsis also focuses on the minimum noise conditions for the cascade precucuits. Because the correlativity between the En and In is considered

the pnyision of noise calculation for the cascade circuits can be guaranteed effectively.