An experimental study of 1/f noise in transistors

Mathematical models of thermal and shot noise characteristics of transistors are adequate for the determination of the optimum operating point and source impedance for the mid-frequency region. However, in the case of 1/f noise, which occurs at low frequencies and has a spectral distribution proportional to 1/fⁿ, models presently available do not describe the phenomenon sufficiently for practical design purposes. This paper describes an experimental approach to the determination of optimum operating point and source impedance for transistor circuits in applications where l/f noise predominates. Results of a number of tests which measure the noise spectrum as a function of operating point and source impedance are described. Results of such tests are presented as parametric plots of noise factor versus frequency, and conclusions are drawn as to the choice of parameters for optimum noise performance at low frequencies. A description of the special measurement techniques is included. A procedure is developed for rapid determination of the entire noise spectrum of a transistor for practical design purposes.