A rail-to-rail amplifier input stage with +-0.35%g/sub m/ fluctuation

A new scheme for achieving rail-to-rail input to an amplifier is introduced. Constant g/sub m/ is obtained by using tunable level shifters and a single differential pair. Feedback circuitry controls the level shifters in a manner that fixes the common-mode input of the differential pair, resulting in consistent and stable operation for rail-to-rail inputs. As the new technique avoids using complimentary input differential pairs, this method overcomes problems such as commonmode rejection ratio and gain-bandwidth product degradation that exist in many other designs. The circuit was fabricated in 0.5-(mu)m process. The resulting differential pair had a constant transconductance that varied by only +-0.35% for rail-to-rail input common-mode levels. The input common-mode range extended well past the supply levels of +-1.5V, resulting in only +-1% fluctuation in g/sub m/ for input common modes from -2 to 2 V.