  The capacitance-charging method is a well-established and handy technique for
the generation of dc current in the 100 pA range or lower. The method involves
a capacitance standard and a sampling voltmeter, highly stable devices easy to
calibrate, and it is robust and insensitive to the voltage burden of the
instrument being calibrated. We propose here a range extender amplifier, which
can be employed as a plug-in component in existing calibration setups, and
allows the generation of currents in the 1 nA range. The extender has been
employed in the INRIM setup and validated with two comparisons at 100 pA and 1
nA current level. The calibration accuracy achieved on a top-class instrument
is 10 ppm at 1 nA.
