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Old 12th November 2012, 05:50 PM   #11
Salas is online now Salas  Greece
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Yes there is R6 also.
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Old 12th November 2012, 05:53 PM   #12
roger57 is offline roger57  Canada
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Quote:
Originally Posted by jcx View Post
N type (only type available for MOSFET) depletion mode devcies conduct Idss with Vgs=0, are progressively cutoff by Vgate lower than Vs

the Vgate "comes from" the current flowing in the R in series with the source - allows selecting an operating current that is < Idss

as pointed out this is the same as N jfet ccs, CRD

Thanks for both replies. I'll read those Siliconix notes so I have a better understanding of the mechanisms involved. Some folks use just one Dn2540, but the article points out better performance using the cascoded arrangement, so that's what I will try.

In any case, after I last posted, I closed LTSpice and restarted; it now seems to be OK!

Just a couple of further questions.
1 - What should the gate stoppers optimally be for a CCS of about 10mA?

2 - If either of you has modeled this, why does the current through the devices look so funky?

I would use a fixed resistor plus a pot for current trimming.

Thanks again,
Gary
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Old 12th November 2012, 06:06 PM   #13
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The gate stoppers won't play any role in current setting, the source leg resistor does that. You put enough value for gate stoppers until the thing is stable for oscillations in a given app and layout but you don't overdo it because too much makes it slower losing higher band PSRR.
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Old 12th November 2012, 06:30 PM   #14
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cascoding with the exact same device may not give very good results - you really want ~2x Vp across the D-S for the "lower" device to be a decent current regulator
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Old 12th November 2012, 09:42 PM   #15
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Quote:
Originally Posted by jcx View Post
cascoding with the exact same device may not give very good results - you really want ~2x Vp across the D-S for the "lower" device to be a decent current regulator
Thanks again for the replies. I appreciate the sound advice in this area to get me going!
When you say "exact same device", do you mean the same family, or the same lot?

For example, if I were to use a DN2540 in the top of the pole, 10M45S in the bottom, would that be OK since the 10M45S is a CCS device by design?

My cathode AC voltage is 2.65Vp (sitting on 92VDC). So I should seek a device that is linear Idss at that Vds?

Sorry about the questions...learning is asking and the sooner I get the bigger picture, I'll read more and pursue the details to get a better understanding.

Gary

Last edited by roger57; 12th November 2012 at 09:43 PM. Reason: forgot details! oops!!
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Old 13th November 2012, 02:20 PM   #16
roger57 is offline roger57  Canada
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Quote:
Originally Posted by roger57 View Post
Thanks again for the replies. I appreciate the sound advice in this area to get me going!
When you say "exact same device", do you mean the same family, or the same lot?

For example, if I were to use a DN2540 in the top of the pole, 10M45S in the bottom, would that be OK since the 10M45S is a CCS device by design?

My cathode AC voltage is 2.65Vp (sitting on 92VDC). So I should seek a device that is linear Idss at that Vds?

Sorry about the questions...learning is asking and the sooner I get the bigger picture, I'll read more and pursue the details to get a better understanding.

Gary
too many questions I guess? sorry to bother, and thanks for the help so far.
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Old 15th November 2012, 09:29 AM   #17
jcx is offline jcx  United States
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your circuit, diff pair tail ccs doesn't require a self biasing 2 terminal circuit

so unless the "eye candy" factor trumps price/performance, or you simply can't stand still for a "wasted" few 100 uA to bias Vref or regulation circuits you should use one of the many "3-terminal" ccs circuits

likewise BJT have good performance in this app - if you aren't imposing artificial conditions like the silly "only fets sound good for audio"
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Old 15th November 2012, 03:03 PM   #18
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Quote:
Originally Posted by jcx View Post
your circuit, diff pair tail ccs doesn't require a self biasing 2 terminal circuit

so unless the "eye candy" factor trumps price/performance, or you simply can't stand still for a "wasted" few 100 uA to bias Vref or regulation circuits you should use one of the many "3-terminal" ccs circuits

likewise BJT have good performance in this app - if you aren't imposing artificial conditions like the silly "only fets sound good for audio"

I took the time to read the AN103 Siliconix article on CCS devices, and now have a much better understanding.

I have installed right now a power supply fed bipolar CCS, using an MJE340, BC550, and 2 LED reference. It sounds...OK, not remarkable. More detail, but a little stiffer (maybe harsher) but I didn't have a pot in the Rset leg and I really needed to tweak that to see the effect on distortion. I still want to try:
1) the 10M45, and
2) The cascode pair. I've already made the boards in my shop so they're basically read to test. As per your suggestion, I plan to test a different IXYS depletion mode in the lower half of the CCS, but easy enough to use the 2 DN2540's as well to see if I can measure and/or hear a difference.

Before I complete my amplifier re-work, I will also look at a CCS for the input driver stage in the plate circuit. I understand there could be a significant different there as well.

thank you for taking the time to answer,
Gary
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Old 15th November 2012, 06:19 PM   #19
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the dn2540 can cascode the bjt in the LED Vref+BJT ccs - bjt usually work quire well with even 1 V C-E
then you an use lower V, lower power small signal bjt

may still need gate stopper R - have to look with 100+ MHz scope - but often oscillation changes current draw, make it depend on even hand waving parasitic C

I am highly dubious of anyone's ability to "hear" decent ccs, not oscillating - once the ccs performance gets good enough the sensitivity of circuit properties to further ccs "improvement" are small
I would much more believe fft harmonic, IMD distortion plots, frequency sweeps, Audio DiffMaker comparisons Audio DiffMaker

Last edited by jcx; 15th November 2012 at 06:23 PM.
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Old 15th November 2012, 08:06 PM   #20
roger57 is offline roger57  Canada
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Quote:
Originally Posted by jcx View Post
the dn2540 can cascode the bjt in the LED Vref+BJT ccs - bjt usually work quire well with even 1 V C-E
then you an use lower V, lower power small signal bjt

may still need gate stopper R - have to look with 100+ MHz scope - but often oscillation changes current draw, make it depend on even hand waving parasitic C

I am highly dubious of anyone's ability to "hear" decent ccs, not oscillating - once the ccs performance gets good enough the sensitivity of circuit properties to further ccs "improvement" are small
I would much more believe fft harmonic, IMD distortion plots, frequency sweeps, Audio DiffMaker comparisons Audio DiffMaker
Not sure what you mean by "decent" CCS. I guess that means it could be bipolar or FET, both set up correctly.

Lots of individuals touting FET as a better CCS; many of them indicating sound quality. Just maybe they haven't hear a bipolar CCS set up according to what you are saying. I agree that there should not be a difference, but why are there different topologies (not just with CCS, but in general) with one being readily discernible to some and not others?

Because audio is so subjective to the listener, I never dismiss what others say they hear, even if an analysis proves causal or not.

Thanks
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