The model 564 current preamplifier is a versatile gain block for use with photomultipliers, photodiode detectors, and other current source type transducers. It incorporates several features not found on the simpler model 1641. These include switchable ac or dc signal coupling, a detector biasing trimpot, an output buffer amplifier, and an overload indicator. It consists of an input current coverter stage with gain switchable from 104 to 108 V/A followed by an output voltage switchable for 1X or X100 gain. This allows operation either in a high speed (but noisier) mode, or a lower bandwidth mode with near-theoretical noise performance. The output buffer amp stage also makes the model 564 immune to capacitive loading offsets at tht output, resulting in stable operation even when both the input and output have heavy capacitive loads.
The 564 operates with an input virtual ground to mitigate the effects of source and cabling shunt capacitance, and to prevent loading effects on non-linear tranducers such as photodiodes. Like the model 1212, the model 564 is relatively lightly damped, which optimizes it to have the widest bandwidth consistent with lock-in amplifiers, yet not be overpeaked when operated with moderate capacitance tranducers such as small area PIN photodiodes.