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Attached is a small file describing a hybrid charge sensitve amplifier I built this week using a discrete source follower front end and an AD841JN opamp. The AD841 is currently available from All Electronics, and is a downright steal at the price. Other N-channel FETs could be used instead of the PN4393 shown on the schematic. The FET used must have an IDSS of at least 3ma to be used with the circuit as shown. R4 can be bumped up in value to accomodate a FET with a lower IDSS. The circuit fits on a 2" X 2" printed circuit board with room to spare. An alternate construction method would be to use plated through pad-per-hole prototyping board, with copper tape for power and ground connections. The results are not as pretty as a real PCB, but are functional and durable. A practical expedient not shown in the schematic would be to use protection diodes from the input to each DC rail. These deteriorate the response of the amplifier somewhat, but will save you from many a blown FET gate... Use a fast signal diode such as the 1N914 or 1N4148, or preferrably its higher current cousin the 1N4448.
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