  We implemented a low noise current preamplifier for the readout of resistive
bolometers. We tested the apparatus on thermometer resistances ranging from 10
Ohm to 500 Mohm. The use of current preamplifier overcomes constraints
introduced by the readout time constant due to the thermometer resistance and
the input capacitance. Using cold JFETs, this preamplifier board is shown to
have very low noise: the Johnson noise of the source resistor (1 fA/Hz1/2)
dominated in our noise measurements. We also implemented a lock-in chain using
this preamplifier. Because of fast risetime, compensation of the phase shift
may be unnecessary. If implemented, no tuning is necessary when the sensor
impedance changes. Transients are very short, and thus low-passing or sampling
of the signal is simplified. In case of spurious noise, the modulation
frequency can be chosen in a much wider frequency range, without requiring a
new calibration of the apparatus.
