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Old 7th November 2011, 09:59 PM   #1341
Salas is offline Salas  Greece
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Nice Andrej, but K170 & J74 tend to positive TC in my experience when selecting them for IDSS. Also high Ciss. I have in hand a few BF245C and BF245B. Tested them, 10mA ones they go lower current about 0.3 mA relatively fast when left on 9V battery in G&S shorted IDSS mode. The delicate part is to find some already near target, because the source resistor (or trimmer) slows down the -TC time as I have seen with 100R. A 10mA one goes 7.7mA and moves negatively slower for just 0.1mA. Nice is that the C are almost 15mA all of them and the B are 7mA and 10mA groups. So a small trimmer can just hold the 10mA ones to trim around 8mA centre. We don't need PJFET, we can orient drain of NJFET towards the input transistor and keep them same die characteristics. Now if the steady current will be the main stabilizing force and not the supplementary TC tendency, maybe we can use many different JFET types and trimmers. Remains to be seen in practice. Soon.
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Old 7th November 2011, 10:24 PM   #1342
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Yes, I agree, it can be N channel j-fet only but it was so tempting to put complementary ones in the schematic hehe. Small Rgs resistance will do the job, so BF245B will be the right one to suit the need.
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Old 7th November 2011, 10:42 PM   #1343
Sheldon is offline Sheldon  United States
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DN2540?
http://www.supertex.com/pdf/datashee...upertex.com%22
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Old 7th November 2011, 10:50 PM   #1344
Salas is offline Salas  Greece
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Thought of them, I have 5 matched pairs and about 20 random pieces, but see the Ciss. 300pF VS 4pF of the BF245. And this is an amp we like to keep fast. They also need gate stoppers that will build on time constant. Nicely -TC non the less.
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Old 7th November 2011, 11:06 PM   #1345
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Quote:
Originally Posted by Lazy Cat View Post
Yes, I agree, it can be N channel j-fet only but it was so tempting to put complementary ones in the schematic
Sometimes, aesthetic of the schematic can provide benefits to the sound. I understand-you well. Harmony is the target.
I mean, every *love* or pleasure you bring in a work will be given back a way or an other. Reason why no computer can design a "musical" power amp, neither a good text translation ;-)
More than that, under some reptilians reflexes we can have, we can discover sometimes some hidden intuitive and not analyzed knowledge witch will go to the surface later.
That makes the differences between our works, can bring to new ideas...
Well, i use to make symmetrical passive filters for my speakers. It first look stupid as it cost more components. it makes a difference, i tried to explain and now, i understand why and stay on it.
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Last edited by Esperado; 7th November 2011 at 11:15 PM.
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Old 7th November 2011, 11:24 PM   #1346
Bigun is offline Bigun  Canada
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I had to discard the use of vertical MOSFETS and BJTs, because of their high temperature drift. I can assure you that I've spent hundreds of hours (I'm not exaggerating) and tried many output configurations. None of them had stable quiescent current and no modification or change in the schematic values gave me reliable results.
Ruwe, Apart from the unruly idle current how bad was the dc offset stability ? - did you physically tape together the two input devices ?

Thanks.
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Old 7th November 2011, 11:55 PM   #1347
Salas is offline Salas  Greece
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Quote:
Originally Posted by Lazy Cat View Post
... If this solution would prove to be adequate, than you can simply omit j-fet from your VAS stage.
It will mean asymmetric cascode load resistors (need be trimmers). It will shoot fast the output Iq one way or opposite polarity if steady same resistors. Much as Ruwe had. Latfets have no symmetric Vgs and as the thing changes tempco current difference in the bridge branches it takes 4 hands to trim fast. In practice the drivers common bias CCS kicks at about 150mV minus that homogenizes in the Latfes Vgs bracket enough.
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Old 8th November 2011, 12:35 AM   #1348
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Salas View Post
Thought of them, I have 5 matched pairs and about 20 random pieces, but see the Ciss. 300pF VS 4pF of the BF245. And this is an amp we like to keep fast. They also need gate stoppers that will build on time constant. Nicely -TC non the less.
Does the speed really matter for a current source here?

Sheldon
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Old 8th November 2011, 12:49 AM   #1349
Salas is offline Salas  Greece
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Not sure. Intuition. At least not as good as a 4pF one for CCS PSRR. Choosing to test with the BF because I have both. Against DN price, rarity in Europe, size, big resistor to get down the Idss, probably noisier as Mosfet VS Jfet... But for basic current stability test it will do it too. As LM334 will and other we may have.
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Old 8th November 2011, 01:28 AM   #1350
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Salas View Post
Not sure. Intuition. At least not as good as a 4pF one for CCS PSRR. Choosing to test with the BF because I have both. Against DN price, rarity in Europe, size, big resistor to get down the Idss, probably noisier as Mosfet VS Jfet... But for basic current stability test it will do it too. As LM334 will and other we may have.
My question of speed was based on weak intuition. I guess that gate to source capacitance would affect high frequency PSRR. At lower frequencies PSRR would be just a function of transconductance? I asked about the DN part, as I have a bunch here (TO92 versions, so size not an issue). Would we bond these thermally to the input devices?

Sheldon
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