Searching the "best" CCS - Page 3

dshortt9 · 16th September 2006, 10:11 PM

For something less complex how about using 2 jfets of matched Idss stacked one on top of the other? This would improve the working voltage to twice that of a single jfet and I think would improve the 56.7 db of one jfet to double or 114 db. Or possibly string 3 of them together? I could be daft.

jcx · 16th September 2006, 10:26 PM

cascoding matching fets is wrong (if you run them at Idss):

http://www.diyaudio.com/forums/showt...442#post333442

dshortt9 · 16th September 2006, 11:42 PM

Not what I meant. Imagine stacking 2 CR300 diodes. A good thing or not?

Tim__x · 16th September 2006, 11:44 PM

It would increase output impedance by a factor of two, but that means 56.7dB+6dB=62.7dB, not 56.7dB+56.7dB. Logarithmic addition is linear multiplication.

jcx · 17th September 2006, 01:09 AM

real world tolerance would prevent series ccs crd connection from helping, the crd with infinitesimally (uA) higher limit current would saturate and look like a sorta nonlinear resistor of low value compared to the working crd MegaOhm Z

Tim__x · 17th September 2006, 01:43 AM

Good point. The second JFET (the one with a higher Vto) would be better used as a cascode across the CCS JFET anyway. It would provide a lot more than 6dB improvement in output impedance.

dshortt9 · 17th September 2006, 02:01 AM

Yeah, Thanks. In the future I'll think a bit longer before posting.

Terry Demol · 17th September 2006, 02:02 AM

Quote:

Originally posted by MikeB
I am looking for an "ideal" ccs, i attached a small collection of 9 different ccs circuits, all rated with a psrr. This db value is the voltage ripple visible at the 1k resistor with the power supply contaminated with 1v AC, all ccs giving ~3ma. The LED style is missing because i have no model...

Did i forget other / better circuits ? Right now the cascoded jfet-ccs seems to be the candidate. Number 9 is also very interesting as it is floating style like a single jfet-ccs.
What i want is a ccs with high psrr, but not too complicated.

Mike

Mike

What is it's application? Many different CCS designs with various
strengths and weaknesses.

Application will be best decider of what type to use.

Cheers

Terry

MikeB · 17th September 2006, 10:37 AM

Ah yes, the ccs will be used to feed an input-LTP. In detail, i am trying to build a symasym with optimzed psrr to check the sonic advantages.

Attached is negative psrr of a symasym6, green uses standard 2-bjt-ccs, red cascoded jfet. As you can see, psrr degrades heavily below 100hz, also the normal ccs shows a resonance at 9mhz.
The peak at 13hz down shows that there is an additional problem...

As i want to publish schematic, i should avoid use of exotic parts, they would be substituted anyway.

Mike

kubeek · 17th September 2006, 11:22 AM

MikeB, what simulation software did you use for the initial measurements?

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16th September 2006, 10:11 PM	#21
dshortt9 diyAudio Member Join Date: Apr 2001 Location: Milwaukee, Wisconsin	For something less complex how about using 2 jfets of matched Idss stacked one on top of the other? This would improve the working voltage to twice that of a single jfet and I think would improve the 56.7 db of one jfet to double or 114 db. Or possibly string 3 of them together? I could be daft.

16th September 2006, 10:26 PM	#22
jcx diyAudio Member Join Date: Feb 2003 Location: ..	cascoding matching fets is wrong (if you run them at Idss): http://www.diyaudio.com/forums/showt...442#post333442

16th September 2006, 11:42 PM	#23
dshortt9 diyAudio Member Join Date: Apr 2001 Location: Milwaukee, Wisconsin	Not what I meant. Imagine stacking 2 CR300 diodes. A good thing or not?

16th September 2006, 11:44 PM	#24
Tim__x diyAudio Member Join Date: Jun 2004 Location: Edmonton area, Alberta	It would increase output impedance by a factor of two, but that means 56.7dB+6dB=62.7dB, not 56.7dB+56.7dB. Logarithmic addition is linear multiplication.

17th September 2006, 01:09 AM	#25
jcx diyAudio Member Join Date: Feb 2003 Location: ..	real world tolerance would prevent series ccs crd connection from helping, the crd with infinitesimally (uA) higher limit current would saturate and look like a sorta nonlinear resistor of low value compared to the working crd MegaOhm Z

17th September 2006, 01:43 AM	#26
Tim__x diyAudio Member Join Date: Jun 2004 Location: Edmonton area, Alberta	Good point. The second JFET (the one with a higher Vto) would be better used as a cascode across the CCS JFET anyway. It would provide a lot more than 6dB improvement in output impedance.

17th September 2006, 02:01 AM	#27
dshortt9 diyAudio Member Join Date: Apr 2001 Location: Milwaukee, Wisconsin	Yeah, Thanks. In the future I'll think a bit longer before posting.

17th September 2006, 11:22 AM	#30
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	MikeB, what simulation software did you use for the initial measurements?

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