Analysis of conversion gain in a bipolar Gilbert cell microwave mixer

This work presents a theoretical study of conversion gain in a bipolar Gilbert cell mixer. This double-balanced structure can function at microwave frequencies with the use of HBTs. In order to optimise conversion gain of such mixers, the influence of each transistor non-linearity has been investigated. A simplified model based on transistor equivalent circuit and the conversion matrix method has been used to derive analytical expressions of Gc versus RF input frequency. In case of a down-converter function the main non-linearity which contributes to conversion gain is the transconductance non-linearity and the expressions demonstrate ten typical frequencies introduced by base-emitter and base-collector capacitances