Low voltage modified Gilbert cell mixer: An addendum

This brief note is an addendum to an earlier post on a low voltage modified Gilbert cell mixer. If the power supplies ( VCC ) do not vary too much ( typically 5%), then the low voltage mixer can be used as is. However, if the power supply variation is greater ( say 3.0 to 6.0V ), then the earlier design's current dissipation will be uncontrollable. In such a case it is best to revert to the standard Gilbert cell mixer. The saving grace is, that if the design is being done in a Bi-CMOS process, then instead of using a bipolar as the tail current in the mixer, a MOSFET can be used. Note that in this case the drain to source voltage of the MOSFET can be very low ( VDS=0.1V or lower) depending on size. In this case the power supply voltage will have a minimal effect on the performance.