  An ultrastable low-noise current amplifier (ULCA) is presented. The ULCA is a
non-cryogenic instrument based on specially designed operational amplifiers and
resistor networks. It involves two stages, the first providing a 1000-fold
current gain and the second performing a current-to-voltage conversion via an
internal 1 MOhm reference resistor or, optionally, an external standard
resistor. The ULCA's transfer coefficient is extremely stable versus time,
temperature and current amplitude within the full dynamic range of +/-5 nA. A
low noise level of 2.4 fA/sqrt(Hz) helps to keep averaging times low at small
input currents. A cryogenic current comparator is used to calibrate both input
current gain and output transresistance, providing traceability to the quantum
Hall effect. Typically, within one day after calibration, the uncertainty
contribution from short-term fluctuations of the transresistance is below one
part in 10^7. The long-term stability is expected to be better than one part in
10^5 over a year. A high-precision variant is available that shows a
substantially improved stability at the expense of a higher noise level of 7.5
fA/sqrt(Hz). The ULCA also allows the traceable generation of small electrical
currents or the calibration of high-ohmic resistors.
