Some results are presented for optimum noise performance of transistor input stages when fed from resistive or reactive sources. Standard theory has shown that a common-emitter transistor fed from a resistive source presents a minimum noise figure F_m when the source resistance has a certain value R_gm in the order of lkΩ. In this
paper, expressions are developed for minimum noise figure and optimum
source resistance in the presence of base bias resistors, emitter degeneration
resistance, and various kinds of feedback. Results are in
terms of F_m and R_gm only, and do not contain other functions of the
transistor internal noise sources. It is shown that the minimum noise
figure is never less than F_m, but the optimum source resistance can be
either greater or less than R_gm.
In the case of reactive sources, noise figure is meaningless and
the quantity of interest is signal-to-noise ratio over the passband.
It is shown that for an inductive source, such as a magnetic tape head,
there is a maximum signal-to-noise ratio obtainable with an optimum
source inductance, and that a Figure of Merit can be assigned to the
source which is independent of its inductance.
Experimental results presented for both resistive and inductive
sources show good agreement with the theoretical predictions
