The ultrastable low-noise current amplifier (ULCA) was developed as an advanced instrument for the improved measurement of currents generated by single-electron transport (SET) devices. It was optimized for direct currents in the picoampere range, and achieves an uncertainty of <;0.1 μA/A for a current of 100 pA typically generated by SET pumps. This paper summarizes our efforts in extending the ULCA's current range and minimizing the measurement uncertainty over a wide range. Two ULCA variants with improved shortterm uncertainty of about 0.02 μA/A or improved noise level of 1.4 fA/√Hz are presented. Combining these devices allows the construction of a novel standard for direct currents below 50 μA, which has superior performance to previous methods for small-current generation or measurement. ULCAs involving √ thick-film resistors achieve noise levels down to 0.43 fA/√Hz. A low-bias variant features an effective input bias current within ±100 aA and a temperature coefficient within ±10 aA/K, which enables uncertainties below 10 aA in direct current measurements without reversing or switching ON/OFF the signal current.