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Old 3rd March 2012, 09:49 PM   #4841
Salas is offline Salas  Greece
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Quote:
Originally Posted by pidesd View Post
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.
To avoid current hogging that leads to a catastrophic sequence.

The output impedance will be a function of the error amp's open loop gain and the current follower's transcoductance. R6 gets to be a CCS and the follower's bias gets to be spared higher to trigger more Gfs if we are after decreasing Zo.
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Old 3rd March 2012, 10:56 PM   #4842
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Quote:
To avoid current hogging that leads to a catastrophic sequence.
you mean that we dont want to be in the bottom portion of the graph in the attachment, where linearity is not so good?

Quote:
The output impedance will be a function of the error amp's open loop gain and the current follower's transcoductance. R6 gets to be a CCS and the follower's bias gets to be spared higher to trigger more Gfs if we are after decreasing Zo.
yes i just remembered that a jfet had replaced the resistor in the later versions.how about 2 jfets cascoded to further improve Zo?would you advise against it or should i let myself go?

P.S.: just tell me if i get annoying with my questions and i ll get back to my books
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Old 4th March 2012, 01:00 AM   #4843
Salas is offline Salas  Greece
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We don't want one in a group to draw more current than the others, as it gets hotter it draws even more, vicious circle until it fails and kills some more. So we match them and/or we give them little source resistors like in amps.

To make a cascode is not recommended when you will only have 3.3V and in there you will have to fit one vbe and two Vgs drops. Use a BF245A, its enough low pinch off and low capacitance Jfet good for VHF/UHF amps even.
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Old 4th March 2012, 02:13 AM   #4844
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yes indeed, fitting a cascode within around 2.5V 2sj313 Vgs looks pretty lame. forgot about that .

thanks for showing me bf245a looks better than 2sk170. looks also like it would require very low current to be effective. maybe something like the less available 2sk369 would be a better fit(although capacitance is higher...)?
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Last edited by pidesd; 4th March 2012 at 02:40 AM.
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Old 4th March 2012, 08:31 PM   #4845
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after giving some thought, bf245a might be very nice as current sources in this reg indeed.now i ll try to find a more common shunt mosfet.

thanks a lot salas. now i know better what direction to go to.
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Old 4th March 2012, 08:43 PM   #4846
Salas is offline Salas  Greece
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Whatever sum of parts you choose see they will have adequate phase margin for stability as a circuit. Good luck.
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Old 6th March 2012, 02:54 PM   #4847
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guys the output mosfet is hot when testing without load and colder with load?

is that correct?
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Old 6th March 2012, 02:57 PM   #4848
Salas is offline Salas  Greece
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Correct.
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Old 8th March 2012, 04:17 AM   #4849
pidesd is offline pidesd  Canada
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Whatever sum of parts you choose see they will have adequate phase margin for stability as a circuit. Good luck.
ok. i might just not venture into paralleling shunt devices yet.

what do you think about those?

fqp4n20l
http://media.digikey.com/pdf/Data%20...s/FQP4N20L.pdf
zvn0124a
http://www.diodes.com/datasheets/ZVN0124A.pdf
IRLU024Z
http://www.irf.com/product-info/data...rlr024zpbf.pdf
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Old 8th March 2012, 11:01 AM   #4850
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For 1,2V "disco" (about 150mA):

would it be possible (and benificial) to substitute the v1 "CCS-section" with bc560; - to v1.1 bib with 3 green led? (Then you actually could tweak the bib-board, by mainly substituting the MJE15031 for d45h8 (+ jumper bib-R4, and bib-R6 = 10ohm)
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