Using the inductive degeneration method gives a noise figure of Fmin, but how can I calculate the reduced gain of the amplifier once inductive degeneration has been performed?
There is no simple S-parameter method for calculating gain after inductive degeneration has been used. We can recalculate S-parameters with the inductors added by using network parameter conversions, but we didn’t cover that in this course. A simpler but less accurate method would be to calculate the small signal voltage gain from a small signal model (like we used it the notes). If we do that we find that the voltage gain is reduced due to a portion of the input voltage appearing across the source degeneration inductor L_s. Hence we trade off lower noise figure for lower gain, just as we do when we use noise and gain circles.
