  A wide-band current preamplifier based on a composite operational amplifier
is proposed. It has been shown that the bandwidth of the preamplifier can be
significantly increased by enhancing the effective open-loop gain of the
composite preamplifier. The described preamplifier with current gain 10$^7$ V/A
showed the bandwidth of about 100 kHz with 1 nF input shunt capacitance. The
current noise of the amplifier was measured to be about 46 fA/$\sqrt{\rm Hz}$
at 1 kHz, close to the design noise minimum. The voltage noise was found to be
about 2.9 nV/$\sqrt{\rm Hz}$ at 1 kHz, which is in a good agreement with the
value expected for the operational amplifier used in the input stage. By
analysing the total noise produced by the preamplifier we found the optimal
frequency range suitable for the fast lock-in measurements to be from 1 kHz to
2 kHz. To get the same signal-to-noise ratio, the reported preamplifier
requires roughly 10% of the integration time used in measurements made with a
conventional preamplifier.
