Optimum Noise Performance of Transistor Input Circuits / Transistor AC and DC Amplifiers with High Input Impedance

Abstract

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.

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