Noise of field-effect transistors at very high frequencies

Abstract

The minimum noise factor of a field-effect transistor has been computed at high frequencies on the basis of the thermal noise of the real parts of the equivalent circuit. A treatment of the intrinsic FET is followed by a consideration of the influence of feedback, parasitic output impedance and parasitic impedance in series with the source on the noise factor. Moreover, the difference between common-gate and common-source configuration has been considered. For frequencies smaller than the gain-bandwidth product f gb the factor F_{\min} - 1 varies linearly with the frequency, whereas at higher frequencies this factor varies with f2. The computed results are compared with measurements on both JFETs and MOSFETs in the frequency range 100-1500 MHz at different conditions of operation. The agreement is rather good. For the JFET the value of F_{\min}(f_{gb}) \approx 2.5 ; for the MOSFET somewhat higher values are found due to the presence of substrate depletion effects.