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Old 27th February 2012, 11:54 AM   #4831
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What would be part voltage-reference-part, with the lowest voltagedrop available wahtsoever?
(probably a silicium diode - but which silicium diode then?)
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Old 2nd March 2012, 11:42 PM   #4832
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hi everyone,

is there a reason why a power jfet could not be used in the 1.1SSLV as a shunt device so to have a low voltage output, instead of a BJT?

i just began understanding how the circuit(and electronics in general) work and would like some more insight.

also what charateristics should a good shunt device have? what i understand (if i m not mistaking) is that the higher the transconductance of the shunt device, the higher the gain and the lower the output impedance. i guess it s all a matter of stability,bandwidth,etc. but for instance what does the irfp9240 do well?

not sure that has been discussed before.

Last edited by pidesd; 2nd March 2012 at 11:47 PM.
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Old 3rd March 2012, 12:18 AM   #4833
Salas is offline Salas  Greece
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It does well decent gfs in decent Crss. A SIT could be tried out, there are considerations of its gfs, Crss, and impact to stability to get to terms as with any other chosen shunt device. Which one you got in mind?
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Old 3rd March 2012, 12:21 AM   #4834
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Quote:
Originally Posted by pedefede View Post
What would be part voltage-reference-part, with the lowest voltagedrop available wahtsoever?
(probably a silicium diode - but which silicium diode then?)
You probably mean a silicon diode? We have used 1N4XXX series with success in some low Vo builds.
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Old 3rd March 2012, 12:46 AM   #4835
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well i was looking at the sje120r100 datasheet and thought it may give lower noise, higher Yfs, lower Vgs than a mosfet and i saw it behaved like a mosfet. i think there is a PNP counterpart but i m not sure. i also have a few lu1014d which are less pricey and less available but behave differently.

i m not sure how reverse capacitance can affect anything, i ll have to study what is is first...
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Old 3rd March 2012, 12:55 AM   #4836
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In follower mode the reverse capacitance fully counts. You may apply them parts and test, certainly not out of place. But the SITs are pricey as you said to be relegated in such a hum drum duty I guess. MOSFETs are noisier but there is no voltage gain in that position. Mainly the Vref noise dominates.
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Old 3rd March 2012, 04:15 AM   #4837
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what about paralleling 2sk2013 or 2sj313 for 3.3v output?does this make sense?
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Old 3rd March 2012, 04:50 AM   #4838
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also,

what do you mean by "decent Gfs in decent Crss"? do you mean Gfs vs Crss?

Last edited by pidesd; 3rd March 2012 at 04:54 AM.
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Old 3rd March 2012, 04:54 PM   #4839
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About paralleling rather low transcoductance devices don't know, capacitance and parasitic pin inductance accumulating, cost, Rs for current balance upping impedance, doesn't sound like more than a practical way for using up some stock, than specifying a single proper part.

I meant something like a ''figure of merit'' for the job. The possibly higher transconductance along the possibly lowest capacitance penalty.
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Old 3rd March 2012, 09:28 PM   #4840
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ok got it for high Gfs with low Crss.

about paralleling, i thought it might keep Vgs lower at a given total current and maybe alleviate the need for heatsinking ...but i ll take advices of more experienced and knowing person than me .

i may have an idea on how capacitance and inductances may be harmful(resonance?) in this situation, but i m not sure why we would like the bias to be the same in each shunt device. i also remember having read multiple times that it is desirable to match parallel output devices in the output stage of amps but i m not sure why...could you (or someone else) care to explain? i understand that heat load will be shared even but that is it...

also, i understand that having the lowest possible bias through the bjt (see pic) gives lower output impedance, so i m wondering what obstacles do you encounter when lowering to much the bias. is there a sweet spot? in the pic it is around 4mA depending on the mosfet.
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Last edited by pidesd; 3rd March 2012 at 09:41 PM.
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