Minimum noise-setting of transistors

It is shown that the noise factor (F) in a transistor, by setting the emitter current at its optimum value, is given by the ratio of source resistance (R
s
) and base spread resistance (r
b
) and a factor that depends on the current-gain (β). This results generally in a lower noise factor (F) than that found by differentiation of F with respect to the source resistance.