I'm looking at a tube differential amplifier schematic, and it has a BJT biased with diodes to form a CCS, and there's a FET between the tube cathodes and the CCS. The FET's gate is connected to the transistor base and diodes, the drain going to the tube and the source connected to the BJT's collector. I guess this is called a cascode current source and I'm wondering what the advantage of putting the FET in there is vs. just connecting the tube directly to the transistor CCS? Also, if anyone can refer me or tell me the basics of how one would design such a current source I would be very appreciative! 
Bitrex,
Some background for your possible interest.
These are 2 of the most commonly quoted articles on CCS circuits. Walt Jung was an engineer at Analog Devices (now retired). These will cover your question.
http://www.audioxpress.com/magsdirx/ax/addenda/media/jung2778.pdf
http://www.audioxpress.com/magsdirx/ax/addenda/media/jung2779.pdf
Cheers,
Ian
Some background for your possible interest.
These are 2 of the most commonly quoted articles on CCS circuits. Walt Jung was an engineer at Analog Devices (now retired). These will cover your question.
http://www.audioxpress.com/magsdirx/ax/addenda/media/jung2778.pdf
http://www.audioxpress.com/magsdirx/ax/addenda/media/jung2779.pdf
Cheers,
Ian
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