A micropower precision single-supply instrumentation amplifier

Conventional 2- and 3-opamp instrumentation amplifier (IA) structures are not able to function with close to the supply rail input common-mode voltage even using rail-to-rail opamps, due to the saturation of the intermediate opamps. To avoid this problem, the emitter followers that level shift the input voltage are used in a 2-opamp structure. Such an IA is operable with common-mode input voltage V/sub cm/ close to the negative rail if V/sub ref/<(V/sub sh/+V/sub cm/)R2/R1, where V/sub ref/is the reference voltage, V/sub sh/is the input level-shift voltage, assuming opamps with rail-to-rail output. Matching of the input voltage shifters and stability of the shift voltage versus input voltage are the key elements of the overall IA accuracy. Emitter followers with feedback loops for the stabilization of collector-emitter voltage are used as level shifters.