In a system where several amplifiers or other functional blocks are cascaded the calculation of noise figure is :

F = F1 +F1/( G1) + F2/(G1*G2) …..

Here the Fi are the Noise figures of the various stages and Gi are the gains ( or losses) of the various stages. F is the cascaded noise figure.This is, of course, a very basic textbook result but instructive nonetheless. Please visit our website for more technical and other information.