Typically, FET input operational amplifiers have desireable characteristics
such as good frequency response, low input bias current, and low input-bias-current
noise compared to bipolar input amplifiers.
On the other hand, bipolar operational amplifiers offer such noteworthy characteristics
as low input offset voltage, low input-offset-voltage drift, and low voltage
noise.
This composite amplifier circuit (see
the figure) offers the best of both worlds. It achieves low current
and voltage noise, combined with 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, A11 is a high-speed FET input op amp
with a closed-loop gain of 100 (the source impedance was arbitrarily chosen
to be 100 kΩ). A2 is a Super β eta bipolar input
op amp. It has good dc characteristics, biFET-level input bias current, and
low noise. A2 monitors the voltage at the input of A1
and injects current to A1'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.
