A monolithic operational amplifier having input error currents
in the order of 100 pA over a ?55°C to 125°C temperature
range is described. Instead of FETs, the circuit used bipolar
transistors with current gains of 5000 so that offset voltage
and drift are not degraded. A power consumption of 1 mW at
low voltage is also featured.
A number of novel circuits that make use of the low current
characteristics of the amplifier are given. Further, special design
techniques required to take advantage of these low currents
are explored. Component selection and the treatment
of printed circuit boards is also covered.
INTRODUCTION
A year ago, one of the loudest complaints heard about IC op
amps was that their input currents were too high. This is no
longer the case. Today ICs can provide the ultimate in performance
for many applications—even surpassing FET amplifiers.
FET input stages have long been considered the best way to
get low input currents in an op amp. Low-picoamp input currents
can in fact be obtained at room temperature. However,
this current, which is the leakage current of the gate junction,
doubles every 10°C, so performance is severely degraded at
high temperatures. Another disadvantage is that it is difficult
to match FETs closely.1 Unless expensive selection and trimming
techniques are used, typical offset voltages of 50 mV
and drifts of 50 μV/°C must be tolerated.