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Old 21st May 2004, 10:32 PM   #31
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Lightbulb What I got........ beside tired after all this

Actually a level shifting circuit like the CCS jfet and resistor are only necessary when driving the base of the pass transistor to voltages near are above the rail voltage for the op amp. With a 12 Volt raw supply feeding the op amp it can swing to about 10 volts, in the process changing from sinking current to sourcing it. The base of the transistor is sitting at about 5.6 Volts with the current source supplying base current. The op amp will swing to about 10 Volts (the op amp supply minus a couple of volts). so we have 10 volts minus 5.6 volts divided the 2.4 K ohm resistor or 1.8 mA. This added to the current source current which is all going to the base. The collector current is Hfe times the sum of these two current. Pulling a number out of the air for the Idss of the CCS fet of 3.2 ma for quick math and assuming hfe of a 100 for more lazy math, you have (1.8mA plus 3.2 ma) times 100 or 500 mA. If one is really going to take advantage of this level shifting on the output like "the regulator who dare not speak its name"; one should run the op amp off regulated 5 volts rails for better PS rejection. If you are going to power the op amp from the raw 12 volt rails you might as well leave out the level shifting circuit and put a 10 ohm resistor between the op amp output and pass transistor base for both simplicity and lower output impedance.

Perhaps some reverse engineering with more reversals than engineering resulted in what we see on the schematic. I would go for the zener- jfet level shifter and power the op amp from the regulated output voltages like the "Engineer whose name we dare not speak" did on the super regulator. Just a little humor WJ, please don't be offended.
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Old 22nd May 2004, 12:27 AM   #32
mfr is offline mfr  Canada
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Some more measurments for sake of clarity:

Load is the working circuit, probably 50-100mA for the 5v regulators, a bit more on the 15v regulator (it gets a little hot)

1st 5v regulator

Output : +5.2 and -5.2 v
Opamp Out: -3.75 and +3.82
FET (after Zener): +5.8 and -5.82

2nd 5v regulator

Output : +5.18 and -5.25 v
Opamp Out: -4.10 and +3.70
FET (after Zener): +5.79 and -5.85

3rd regulator 15v

Output : +14.26 and -14.49v
Opamp Out: +4.91 and -5.34
FET (after Zener): +5.8 and -5.82

----

I'm uzing the zeners, as you suggested. Are you also suggesting to power the Opamps from the regulated side of the rails? Whould that still be ok in terms of output current spare?
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Old 22nd May 2004, 07:57 PM   #33
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Default Re: What I got........ beside tired after all this

Quote:
Originally posted by Fred Dieckmann
Actually a level shifting circuit like the CCS jfet and resistor are only necessary when driving the base of the pass transistor to voltages near are above the rail voltage for the op amp. With a 12 Volt raw supply feeding the op amp it can swing to about 10 volts, in the process changing from sinking current to sourcing it. The base of the transistor is sitting at about 5.6 Volts with the current source supplying base current. The op amp will swing to about 10 Volts (the op amp supply minus a couple of volts). so we have 10 volts minus 5.6 volts divided the 2.4 K ohm resistor or 1.8 mA. This added to the current source current which is all going to the base. The collector current is Hfe times the sum of these two current. Pulling a number out of the air for the Idss of the CCS fet of 3.2 ma for quick math and assuming hfe of a 100 for more lazy math, you have (1.8mA plus 3.2 ma) times 100 or 500 mA. If one is really going to take advantage of this level shifting on the output like "the regulator who dare not speak its name"; one should run the op amp off regulated 5 volts rails for better PS rejection. If you are going to power the op amp from the raw 12 volt rails you might as well leave out the level shifting circuit and put a 10 ohm resistor between the op amp output and pass transistor base for both simplicity and lower output impedance.

Perhaps some reverse engineering with more reversals than engineering resulted in what we see on the schematic. I would go for the zener- jfet level shifter and power the op amp from the regulated output voltages like the "Engineer whose name we dare not speak" did on the super regulator. Just a little humor WJ, please don't be offended.
Nice analysis Fred! Still, I feel the 2.4k is too large. Maybe replace with a smaller R and a zener of say 5V in series. Waddayathink? (And please, try to answer in less than 500 words, if at all possible...)

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Old 22nd May 2004, 08:52 PM   #34
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Default Surely your joking Mr. Didden.......

"And please, try to answer in less than 500 words, if at all possible...'"

Are you kidding? I am just getting warmed up at 500 (Grey hits his pace at about 1000, from what I can tell). Besides,I heard that writers get paid by the word.
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Old 22nd May 2004, 09:43 PM   #35
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Default Re: Surely your joking Mr. Didden.......

Quote:
Originally posted by Fred Dieckmann
[B[snip]Besides,I heard that writers get paid by the word. [/B]
Oh no, Fred got word that writes get paid...

Jan Didden
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Old 24th May 2004, 03:17 PM   #36
mfr is offline mfr  Canada
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Got some info on the 2sk246 FET:

The Idss of the

2SK246Y: 1.2~3.0 mA,
2SK246GR: 2.6~6.5 mA
2sk246BL: 6~14 mA


I'm using the BL version but I also have some Y's.

Which one is more desirable in this circuit?
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