Noise in transistor mixers

Starting from the physical sources of noise in junction transistors, an equivalent noise circuit for a high frequency mixer circuit is presented. By means of formulas which have been derived for normal amplifiers and mixers the noise current components in the collector circuit are computed. These noise current components at the intermediate frequency are due to diverse amplifying and mixing processes. The dependence of the derived noise factor upon working point, generator output resistance, frequency and oscillator voltage is verified. The noise figure exhibits a minimum not only as a function of the generator output resistance but also as a function of the collector direct current. The noise figure may be optimized by choosing appropriate values for the circuit components and the operating point. Finally, the application of the noise formula to a dc-stabilized mixer stage is presented.