Low Noise And Drift Composite Amp Circuit

This circuit offers the best of both worlds. It can be combined with a low input offset voltage and drift without degrading the overall system`s dynamic performance. Compared to a standalone FET input operational amplifier, the composite amplifier circuit exhibits a 20-fold improvement in voltage offset and drift. In this circuit arrangement, A1 is a highspeed FET input op amp with a closed-loop gain of 100 (the source impedance was arbitrarily chosen to be 100 kfl). A2 is a SuperBeta bipolar input op amp. It has good dc characteristics, biFET-level input bias current, and low noise.
Low Noise And Drift Composite Amp Circuit - schematic

A2 monitors the voltage at the input of A1 and injects current to Al`s null pins. This forces A1 to have the input properties of a bipolar amplifier while maintaining its bandwidth and low-input-bias-current noise.