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Old 24th September 2012, 12:35 AM   #7231
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Quote:
Originally Posted by john curl View Post
... and 'tail poles' to name two, relatively obscure topics. If you want to understand why Bob Cordell and I use resistors in SERIES with our active current sources, think 'tail pole' reduction.
Thanks John

I always thought those were just to reduce voltage or power across the current source.

Thanks
-Antonio
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Old 24th September 2012, 01:02 AM   #7232
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by john curl View Post
To get around the problems, just review (hopefully) 'The Monolithic Op Amp: A Tutorial Study' by Jim Solomon You can get it directly through 'Google'.
Then you will come to understand virtually everything you need to know.
Found it in two versions, As the abstract and as a National reprint. Ha! This is about the first time John has sent us to the books and it did not cost me anything but work. "Tail pole reduction" That is going to need some thinking and reading. The DH-120 used it. My $1 book on FETs helped fill in a few missing pieces.

So much to learn. At least my speaker building has stepped up a couple of notches. I thought my new crossover and switch to Seas tweets in my guest room system had hurt the imaging, then an ad came on where a plane flew in from about 10 feet outside the room. Imaging is still there, but much cleaner and much smoother.
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Old 24th September 2012, 10:10 AM   #7233
DF96 is offline DF96  England
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Is 'tail pole reduction' just a fancy name for reducing the effect of stray capacitance at a high impedance node, such as a CCS, by adding a series resistor? If so, I thought that was common knowledge. Common knowledge can be hidden by giving it a new name, then the gurus can appear superior to ordinary mortals. All very Gnostic!
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Old 24th September 2012, 01:03 PM   #7234
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Of course, I started using the resistor after 1980 in the JC-80 phono stage, what about you?
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Old 24th September 2012, 01:37 PM   #7235
DF96 is offline DF96  England
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I don't do SS these days, but 10-15 years ago when I designed a power amp I remember thinking about stray/parasitic capacitance in the LTP tail CCS. Having estimated its likely effect, I can't remember whether I decided to include a resistor or not. I probably did have a resistor, because I tend to err on the side of caution and a resistor there would seem to do no harm provided it leaves sufficient voltage across the CCS so that device parasitic capacitances don't grow too much.

The same issue can potentially arise whenever a CCS is used so 'tail pole reduction' is an unfortunate name. 'CCS parasitic capacitance isolation' has the merit of being more descriptive and more generic.
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Old 24th September 2012, 04:49 PM   #7236
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Let me explain to everyone WHY I might sometimes use a series resistor and sometimes I might not. I have presumed that Bob Cordell did the same analysis, when he included it in his phono design, in his latest endeavor.
For me, a single differential pair of devices should be fed by a GOOD current source. However, sometimes a PRACTICAL current source might be a simple 2 pin current source made for the purpose and have a rated output capacitance of 2 pf or less. However they can be noisy, so we might make our own current source, instead, with a degenerated low noise jfet, and this effective output capacitance could be MUCH MORE, so what to do. Well, adding a series resistor in series with the output will reduce the effective capacitance, at least at the frequencies where it counts.
Actually, with a limited power supply range, adding this resistor can actually compromise the actual output impedance of the current source by limiting the voltage across the current source jfet, itself. So it is a relatively sophisticated decision what value to use, because a HIGHER value of series R can actually REDUCE the working impedance of the current source, as well as adding a potential noise contribution.
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Old 24th September 2012, 05:59 PM   #7237
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John,

Thanks for the insight.

Also sharing the power dissipation (or voltage) between the CCS and resistor may allow one to choose different components in the CCS.

I wouldn't expect there to be a need for such an isolating resistor with discrete designs of bias currents above a mA (wouldnt it take near a hundred pico-farads to matter).

BTW, personally these are the kinds of exchanges I really like, a few tips for the millions of details.

Is there really a noise penalty?


-Antonio
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Old 24th September 2012, 08:57 PM   #7238
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Let's discuss now who first started using Ohm's law in audio amplifiers.
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Old 24th September 2012, 09:59 PM   #7239
bcarso is offline bcarso  United States
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Quote:
Originally Posted by DF96 View Post
I don't do SS these days, but 10-15 years ago when I designed a power amp I remember thinking about stray/parasitic capacitance in the LTP tail CCS. Having estimated its likely effect, I can't remember whether I decided to include a resistor or not. I probably did have a resistor, because I tend to err on the side of caution and a resistor there would seem to do no harm provided it leaves sufficient voltage across the CCS so that device parasitic capacitances don't grow too much.

The same issue can potentially arise whenever a CCS is used so 'tail pole reduction' is an unfortunate name. 'CCS parasitic capacitance isolation' has the merit of being more descriptive and more generic.
"Tail pole" sounds somehow more potentially salacious This will please some

As far as JFET I sources go, if you can spend enough volts very good results can be gotten up to rather high frequencies with a bootstrapped cascode, which I believe Jung has determined was first shown by Csanky. They operate best when the lower device gm is a good deal more than the reciprocal of the lower source resistor value, as then the upper gate's current mostly flows in the lower source. Watch WJ's website for the details, and possibly some feedback from Csanky himself, who is still with us.
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Old 25th September 2012, 12:29 AM   #7240
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I did not invent the term. You have to read the Solomon tutorial to understand what it is. Of course, I CARE about the small things, perhaps that's why I tend to do a bit better.
Also, a cascode current source is OK too.
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