Analysis of a composite amplifier technique for optimum noise performance

A technique for combining two voltage-operational amplifiers to create a single composite-voltage operational amplifier (VOA) is analysed. The composite amplifier can be treated essentially as a single VOA which gives an optimum combination of the DC and AC performances of the two constituent amplifiers. Specific practical examples of the technique have been previously reported, but in this paper we present an analysis and establish the optimum combination conditions. To support the analysis a further example of the composite technique is presented in which a high-performance chopper-stabilised VOA (MAX 430) is coupled with a low-noise bipolar VOA (OP 27). Measurements have shown that the composite device exhibits premium qualities from both OA, with low noise and very low input-offset voltage drift