Noise factor contours for field-effect transistors at moderately high frequencies

This analysis of noise behavior is based on an equivalent circuit for the junction field-effect transistor (FET) that was previously published [10]. Since all noise sources in this equivalent circuit are uncorrelated and all significant parasitic elements are already considered, one may apply an easy and direct calculation for the noise factor, which is carried out for the common source, common gate, and common drain configurations. As a result, the constant noise factor contours are represented as circles in the plane of the complex source admittance Y8. This plot of circles is valid above 1/f noise up to moderately high frequencies, and is arrived at by a frequency-dependent normalization of the axis. It is shown that the plot is fully characterized by two frequency-dependent values: the optimum source admittance and the minimum noise factor Fmin, both being derived from the small-signal equivalent circuit of the FET and the bias condition. The results are in agreement with the commonly held opinion that the noise factor does not differ very much for the three basic configurations [9]. Finally, the theoretical results are verified by measurements at 30 and 60 MHz.