The noise figure of junction transistors

An expression for the noise figure N of a junction transistor is derived from a noise equivalent circuit based on the theory of shot noise in p-n junctions, and on a transistor small-signal equivalent circuit. The thermal or Johnson noise generated in the a.c. extrinsic base resistance, rbb¿, is included, while `excess¿ or 1/f noise sources are neglected. N is found to be essentially identical for the three basic transistor configurations: common base, common emitter and common collector; it is strongly dependent on the generator or source admittance, Ys, on the injection level and on the intrinsic and extrinsic elements of the transistor equivalent circuit. However, at any operating point and frequency, N can be minimized with respect to Ys and can be written in the separable form N = Nmin + Rn/gs|Ys ¿ Y0|2 In this expression Ys and gs characterize the source, and Nmin, Rn and Y0 characterize the transistor noise. The dependences of Nmim Rn and Y0 on injection level, rbb¿, Ic0, ¿0 and frequency are discussed in detail for typical transistors, as are the roles of ¿ cut-off frequency and transition capacitance. Measurements are described which indicate good agreement with predictions up to frequencies beyond the transistor a cut-off frequency. A method of minimizing Nmin with respect to injection level is given. Finally the conflict between the generator admittances required to realize optimum power match and noise match is discussed, together with the problems associated with the design of low-noise transistor circuits as far as choice of operating point and transistors is concerned.