Advantage of a FET-BJT CCS vs BJT only

Bitrex · 9th June 2009, 10:56 PM

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!

gingertube · 10th June 2009, 12:21 AM

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/...a/jung2778.pdf

http://www.audioxpress.com/magsdirx/...a/jung2779.pdf

Cheers,
Ian

Bitrex · 10th June 2009, 03:01 AM

Excellent resources, thank you!

9th June 2009, 10:56 PM	#1
Bitrex diyAudio Member Join Date: Jan 2009	Advantage of a FET-BJT CCS vs BJT only 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!

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10th June 2009, 12:21 AM	#2
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	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/...a/jung2778.pdf http://www.audioxpress.com/magsdirx/...a/jung2779.pdf Cheers, Ian

10th June 2009, 03:01 AM	#3
Bitrex diyAudio Member Join Date: Jan 2009	Excellent resources, thank you!

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