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Old 12th September 2012, 07:58 PM   #81
bcarso is offline bcarso  United States
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Yes, most of the time gate stoppers are not necessary but havoc happened to Calvin so in a privat communication i recommended the gate stoppers. I did not get a response so far if that solved the problem. With depletion Mos Fets like DN2540 it is mandatory or they get so hot that they burn.
Yes I would never argue against gate stoppers with DMOS, especially in TO-220 packages! And I recently had a 2N7000 (TO-92) take off even though it had about 400 ohms in one of the leads --- but there was a fair amount of inductance in the gate (this was not a tight layout, I was using it to drive a string of IR emitters) in photovoltaic optoisolators.

If one can get away without them for the lower speed process 51 parts like the 4391, it appears from sim that using them in the cascoded voltage follower leads to a small increase in distortion, so they may be worth losing.

BTW, has anyone used the smaller depletion-mode DMOS part, the DN1509? It looks promising, with a still-high 90V breakdown and lower capacitances.
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Old 12th September 2012, 08:29 PM   #82
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I use other small depletion Mos Fets for cascode CCCS´s. I forgot the part number but up to 50mA they work with high voltage to burn. Usually only one of them has to take the high voltage.
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Old 12th September 2012, 08:40 PM   #83
bcarso is offline bcarso  United States
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I use other small depletion Mos Fets for cascode CCCS´s. I forgot the part number but up to 50mA they work with high voltage to burn. Usually only one of them has to take the high voltage.
Although with tube circuits the low frequency noise of the tube will almost always dominate, as triode plate loads or in other highish-voltage applications, I don't like to rely on the uncertain and unspecified amount of low-frequency noise using the DMOS as the lower device. I will say however that DMOS has gotten so much better in the many years since introduction, that some are quite acceptable, despite the low frequency noise. The noise corner frequency keeps going down --- it would be interesting to graph the typical descent over the years.

At Walt Jung's suggestion I did get some DN2540s in TO-92 and tested them for noise, which was not as bad as I had feared but still high compared to good JFETs. However as a cascode device for low-pinchoff JFETs they will contribute negligible noise, and the high voltage capability, within the limits of device dissipation, is very convenient.
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Old 12th September 2012, 08:47 PM   #84
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Try P channel laterals. I friend of mine made an MC input stage with two in parallel that got down to 0.3nV/qHz. He uses a special way of tackling the 1/F noise though. Willy Sansen says that in his book " Analog Design Essentials ". P - channel lateral Mos Fets are excellent for low noise audio.
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Old 12th September 2012, 09:18 PM   #85
bcarso is offline bcarso  United States
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Try P channel laterals. I friend of mine made an MC input stage with two in parallel that got down to 0.3nV/qHz. He uses a special way of tackling the 1/F noise though. Willy Sansen says that in his book " Analog Design Essentials ". P - channel lateral Mos Fets are excellent for low noise audio.
Interesting. I'd love to learn more, if you can dig up the part numbers.

Wayne of Pass Labs just sent me a couple of the big Interfet parts, which also claim a typical e sub n of 0.3nV/sqrtHz. Someone said that the chip inside of the singles is in fact a dual that they use for another part. That must be one large chip!
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Old 12th September 2012, 09:38 PM   #86
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2SJ162, and it has awesome Gm compared to J-Fet´s like 2SJ74.
This is a secret you should keep !
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Old 12th September 2012, 09:40 PM   #87
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This does not lend to parallel symmetric though, the complimentary 2SK1058 is quite noisy.
What an irony, now we have a P-Channel but not an N-Channel. The world upside down !
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Old 12th September 2012, 09:46 PM   #88
bcarso is offline bcarso  United States
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This does not lend to parallel symmetric though, the complimentary 2SK1058 is quite noisy.
What an irony, now we have a P-Channel but not an N-Channel. The world upside down !
Production will have to be moved to Australia.

(my friends there hate to it being referred to as "down under" )
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Old 12th September 2012, 10:38 PM   #89
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As a child i thought i could drill a hole and come out "on the other side".
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Old 13th September 2012, 04:30 AM   #90
bkdog is offline bkdog  Australia
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Production will have to be moved to Australia.

(my friends there hate to it being referred to as "down under" )
Dunno what you're talking about - we're at the top, see here:

The Upsidedown Map Page - World Maps with South at the Top
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