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Old 10th September 2013, 06:27 AM   #2971
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Quote:
Originally Posted by EUVL View Post
>
As I posted earlier, our team built a 10x10 prototype of the Kaneda circuit.
But it is too small to solder for most, and too tidious to adjust to make it a public project.

While I am certain the latter designs are equally nice in performance, they become too complex and hence large to build in 10x10 footprint.
This is not quite 10x10 - it's about 12.7mm by 15 mm, but with no additional vias and almost entirely on a single side (the rail-to-rail bypass on the solder side is optional, in which case this is a pure single-sided layout). It could be crunched down a bit more by using SOT323, 363 and dual JFETs, but as it stands now it represents a reasonable tradeoff between ease of assembly and size. The dual BJTs shown are Rohm duals that are readily available.

The schematic is the Kaneda variant shown in the previous post.
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Old 11th September 2013, 10:34 PM   #2972
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Default High Voltage Depletion FETs

Guru Wurcer, I believe one of your original plans for SWOPA was to use the input stage for a 100W 8R amplifier.

This would need High Voltage Depletion FETs for the input cascodes.

I think some suitable devices including some depletion MOSFETs were unearthed.

Would you or anyone else remember what these were ... especially if they have good SPICE models?
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Old 12th September 2013, 05:07 AM   #2973
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Quote:
Originally Posted by kgrlee View Post
I think some suitable devices including some depletion MOSFETs were unearthed.

Would you or anyone else remember what these were ... especially if they have good SPICE models?
Supertex makes (or used to make) a line of depletion MOSFETs like the DN2540, etc. There were probably a few in TO92 packages. Other vendors were Ixys and Clare, but not sure of current production status.

Edit: Apparently still in production at Ixys (which owns Clare), in very nice SMT packages:

http://www.ixysic.com/Products/FET.htm

Infineon also has some BSS family parts like the BSS126, 129, 149, 159, 169...

All of these are n-channel only - I don't think there has ever been a depletion-mode p-MOSFET fabricated.

There are also a few Japanese n-channel depletion JFETS with (relatively) high Vds(max) ratings - Toshiba 2sk373 at 100V, and a Sanyo part that does 80V, among others. Both are nearly obtainium now.

Last edited by linuxguru; 12th September 2013 at 05:23 AM. Reason: addendum
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Old 12th September 2013, 06:32 AM   #2974
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ALD has a helpful paper on depletion mode FETs. I tried to upload it but its just too big for the filesize limit. Google will find it (Advanced Linear Devices).
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Old 3rd October 2013, 12:48 PM   #2975
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Hi all,

A potentially "quick" question: Any of these designs have GBW of >= 100 MHz and low noise data?

Thanks for looking and maybe replying ;-)

Jesper
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Old 3rd October 2013, 01:09 PM   #2976
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Quote:
Originally Posted by gentlevoice View Post
A potentially "quick" question: Any of these designs have GBW of >= 100 MHz and low noise data?
SWOPA and kgrlee's FET990 variant could get to 50 MHz GBW, maybe more - but 100 MHz is probably outside their envelope. All the others are slower, AFAIK.

Low noise is not a problem - any of the JFET-input designs can use low-noise devices and surpass almost any monolithic.
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Old 3rd October 2013, 08:07 PM   #2977
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Quote:
Originally Posted by linuxguru View Post
SWOPA and kgrlee's FET990 variant could get to 50 MHz GBW, maybe more - but 100 MHz is probably outside their envelope . . . .
I recall playing with simulations that got GBW up to the 50 MHz - 60 MHz range by using fast transistors and adjusting passive component values. I could tell it was starting to get sensitive to active device parameters, so might require some pre-selection or matching of active devices to reach that performance level on a physical assembly.

100 MHz might be possible if you're willing to give up unity gain stability. The ability to modify the compensation scheme to suit an application is one of the attractions of a discrete design like this. That's why I brought out the main compensation node to an external connection when I did my PWB layouts. (See Posts #2909 and #2656.)

Dale
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Old 4th October 2013, 04:38 AM   #2978
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Quote:
Originally Posted by gentlevoice View Post
Any of these designs have GBW of >= 100 MHz and low noise data?
What application needs this?
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Old 4th October 2013, 08:58 AM   #2979
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An oscillator of course! Why, you can do that with just one Vfet (not sure about the low noise part).

But, it would be rather interesting to try to design an audio circuit with that goal.
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Old 4th October 2013, 07:05 PM   #2980
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Hi all,

& thanks for replying. An oscillator would be interesting although I don't really know how to design one with low THD which would be my aim here. In this case I'm mostly curious - maybe you have a part of your mind that often "searches" for new & better solutions - this is what happened here as I've found this thread's designs interesting though haven't had the time to read all of it.

Greetings,

Jesper
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