wrote:

I'm wanting to use a FET as a current sink for a differential longtail.
This will be a balanced input front end using a 6922 with about a -60VDC on
the tail.
All the FETS I have around here seem to have the reverse protection diode
built in.
Can anyone recommend a good part number for this? I've been experimenting
with
the MJE350 bipolar part, but I want to try the FET and see if it will be
more stable
with temp and better PSRR. Any ideas appreciated.

You don't need a PNP fet.

NPN fet is what you want, but they are fragile and expensive and no
more effective than am MJE340, an npn bjt, as seen in the
schematic at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

The bjt CCS is seen in the middle of the schematic about 3/4 the way down the
image.

There would be zero benefit in using any other device because the
stability of the current flow value etc is all very immune to temp changes
since the current is set by the emitter resistor negative current feedback
developed by the emitter resistor.

And the simple MJE340 CCS is a better CCS than a pentode,
unless the cathode resistor is indeed a large value, since the actual
typical value of the current source resistance is above 20megohms which is
enough for the source to be called a constant current source since the current
changes such a tiny and totally insignificant amount.

Patrick Turner.



Thanks!
RonL