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Old 12th December 2013, 12:44 PM   #31
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i'll have to look at the test data but the NE5534 wasn't in the same class with the AD825 or AD797. I would suggest that you try the OPA604 which will do +/-24V, has low noise and a reasonably high slew rate. As we know from experience, the AD797 tests wonderfully, (most of the time), but is prone to instability.
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Old 12th December 2013, 06:58 PM   #32
UMarcus is offline UMarcus  Germany
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Quote:
Originally Posted by jan.didden View Post
Not a stupid question, rather to the point.
I would say about 1 A. If you really want to go to such high currents, you probably want to select a high-Hfe pass transistor.
Also keep an eye on the dissipation - with 5V across the series transistor at 1A that's 5W. Check the degrees/watt figure of the heasink you use to avoid too much temp rise.
For instance, with a heatsink with 5 degree/W that 5 watt causes a 25 degrees temp rise. That's probably as high as you want to go, so a taller (lower degrees/W) heatsink would be better.

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Old 12th December 2013, 07:54 PM   #33
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> Can you expand on that?

The pass device is a BJT, which means the base voltage is higher than the emitter (= output) voltage by about 0.65V (Vbe). Else no current will pass.

The opamp is powered by the output. So even if the opamp is Rail to Rail, it cannot possibly supply the required voltage to the base of the pass device directly. So you need a start-up Zener to make up some voltage. In Jan's schematics, that zener is 6.8V.

If the output is higher than 44V, you would either have to use a HV opamp, or you need to reduce that output voltage to something the opamp can take, say 30V. So let's say output is +50V, and we supply the opamp only with 30V. That supply can be made up by a TL431 configured to 30V, plus a CRD (say 5~10mA).

Now things get interesting. You can hang that 30V opamp supply and hence the opamp itself at the "top", i.e. at the output, or you can put it at Gnd level. I personally do not like hanging the opamp to a "floating" voltage, even though it should be regulated. So I want my opamp to see 0V / +30V for a positive regulator. Now even if the opamp is R2R output, its output can only be 30V max. But the pass device base needs to be at +50.65V. So you would need a ZD of say 24V, ideally ~30V, before the regulator will work at all.

Not for beginners.....

Clear enough ?


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Old 13th December 2013, 10:53 AM   #34
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Default 400V 50mA J-D Pos Super Reg

To take it to the extreme, and for the sake of stimulating discussions,
this is a concept (i.e. not-proven, build-at-own-risk) schematics for a 400V 50mA Jung Didden Positive Super Regulator.

At least in Spice simulations, performance is excellent.


Patrick

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File Type: pdf 131213 400V Jung Didden Pos Concept Schematics Public.pdf (106.3 KB, 227 views)
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Old 13th December 2013, 12:06 PM   #35
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Nice Patrick, that should work well!

Two things to watch out for: start up may give issues with the opamp locking up at zero output, and on the other hand the supply to the opamp may have switch on transients above the save value.
These things are not always modeled well in Spice.

The lock up may be fixed with a large resistor across the pass device, while the opamp can be protected with a zener across it's supply pins.

Since the MOSFET pass device will have lower transconductance than a BJT performance may be slightly less, but will be of no consequence.

If you use depletion mode MOSFETs like the 2540, be aware that it has a peak Id limit not much higher than its avarage Id limit. That means that if you have large capacitors at the output, the switch-on (high current loading of the output cap) may destroy the pass device.
That's the reason that in my HV supply I went away from a depletion mode device; it did increase the circuit complexity somewhat but made it much more robust.


Jan
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Old 13th December 2013, 12:07 PM   #36
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Like the TL071, the OPA140 uses JFETs on its inputs. (Pchannel for the TL071, Nchannel for the OPA140).

Like the TL071, the OPA140 can tolerate a very large delta-V across its inputs; there are no emitter-base junctions to worry about, and no VEBO breakdown voltage to fear.

This is why the TL071 is the unanimous choice for generating triangle waves in PWM switchers: the (integrator + SchmittTrigger) triangle circuit applies more than 2/3rds of the supply across the opamp's input pins. That would be disastrous for a BJT input opamp, but no problem at all for a JFET input device like the TL071. Or the OPA140.

So why include diodes D6 and D7 in the schematic of post #34? They are across the input pins of a JFET opamp.
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Old 13th December 2013, 12:19 PM   #37
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Not wanting to talk for Patrick, but I find this good practise. It keeps the opamp from driven hard into saturation even if for protection purposes this would not be required.
And, very important, don't forget many people will try this with their fav opamp, FET or BJT input.

Jan
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Old 13th December 2013, 12:26 PM   #38
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Quote:
Originally Posted by Mark Johnson View Post
So why include diodes D6 and D7 in the schematic of post #34? They are across the input pins of a JFET opamp.
Because people "roll" opamps.
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Old 13th December 2013, 12:50 PM   #39
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For my own use, I normally use a red LED there.
BUT for the public, better be safe than to be accused of negligence.

Who dares to build ?



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Old 13th December 2013, 12:58 PM   #40
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I may try it with a 6V6 preamp based on how the smaller version does with the Riaa stage I will be trying it with.
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