Transistors as low-noise amplifiers

The noise properties of a transistor considered as a circuit element (physical origins of noise are not discussed) are outlined, and an equivalent circuit containing two correlated noise generators is described. A method of measuring the magnitudes of the two generators and their correlation coefficient is given, and the results of measurements of these quantities in the audio-frequency band for a germanium junction transistor over a range of collector operating conditions are reported. Further measurements on batches of junction transistors of different types at fixed operating conditions are reported, showing the variation of noise characteristics among individuals of the same type and between different types. A discussion of the application of such measurements to the design of low-noise transistor amplifiers follows.