Low voltage mixer biasing using monolithic integrated transformer dc-coupling

Monolithic microstrip transformers are used to perform coupling, biasing, and filtering functions in a CMOS Gilbert cell mixer. The coupling and biasing technique offers important advantages over the traditional stacked transistor biasing arrangement. Foremost among these is a reduction in required operating voltage. Additionally, the new topology allows the designer to easily adjust the bias current present in the Gilbert cell input transistors, while bias currents in other portions of the circuit are unaffected. Mixer linearity can therefore be improved by accurate adjustment of the input MOSFET operating point. The importance of these adjustments with regard to the mixer conversion gain and IIP3 is examined. A transformer was constructed in 0.35 /spl mu/m CMOS and the s-parameters of the transformer were measured over a range of frequencies spanning 800 MHz to 18 GHz. This data was used to design a low-voltage mixer operating at 1.9 GHz.