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Old 21st December 2009, 06:16 AM   #1
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Default Cascode BOZ

I've been trying to get this preamp working and am having problems. The JFets (BF862) keep breaking, best I can tell, when I hook them up in this circuit. I matched all the JFETs before soldering them to the adapter board, and I took precautions with ESD and used a wrist strap. I've never had this problem before so I thought maybe others might see a problem with this circuit.

Click the image to open in full size.

After this last failure I put a 9 volt battery across the drain of the JFETs of one channel and measured ~15mA current draw from every JFET. It should be around 9.0mA, which I also tested independently. In hindsight I wish I had tested them in the adapter board before hooking it up. The other channel is blown I think, and conducts.

Cascode BOZ preamp:
Click the image to open in full size.

BOZ adapter board:
Click the image to open in full size.
The voltage at the gate of the MOSFETs was 15 volts at most when I started this up. So about 12 volts across the JFETs ... not enough to hurt anything.

It's a mystery to me why this has happened twice. And I found no shorts or other problems with the circuit.

I mean I think it's more likely that the JFETs would be blown when I was measuring Idss and checking the bias currents. Any thoughts?
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File Type: jpg adapter_board.jpg (198.0 KB, 653 views)
File Type: png Cascode_BOZ2.png (4.5 KB, 659 views)
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Old 21st December 2009, 07:00 AM   #2
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Offhand, you do have the Source of one JFET attached to
the Gate of another, but most likely that is a schematic
error.
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Old 21st December 2009, 07:26 AM   #3
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Is this a sim? Can you post some voltages at various nodes?
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Old 21st December 2009, 07:59 AM   #4
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could it be a turn-on spike of voltage through the feedback to the gates of the J-Fets? It only has to be above 30 something volts a very short time to damage the J-fets?!
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Old 21st December 2009, 08:05 AM   #5
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I simmed your drawing and used 2SK170 jfets because I don't have spice on yours. Everything checked out OK at first blush.
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Last edited by carpenter; 21st December 2009 at 08:11 AM.
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Old 21st December 2009, 03:28 PM   #6
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I remember reading that Magura encountered a similar problem when he attempted to pair up LU1014D power jfets. They don't share the current evenly, I believe he said. Perhaps low value Drain resistors above the jfets would help? I'll check my sim...


Edit: In the sim I added 10 ohm drain resistors above the jfets with very little change in the performance specs... perhaps it's worth experimenting?
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Last edited by carpenter; 21st December 2009 at 03:37 PM.
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Old 21st December 2009, 03:48 PM   #7
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I simmed your circuit with J310s for kicks and discovered that they were very unhappy in this particular configuration, which of course leads me to wonder if your jfets are encountering a similar fate.
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Old 21st December 2009, 04:39 PM   #8
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Quote:
Originally Posted by Nelson Pass View Post
Offhand, you do have the Source of one JFET attached to
the Gate of another, but most likely that is a schematic
error.
Opps ... forgot about that. Yeah, it was a schematic error ... I fixed that after taking that snapshot of the circuit awhile back.

Quote:
Originally Posted by charpenter
I simmed your circuit with J310s for kicks and discovered that they were very unhappy in this particular configuration, which of course leads me to wonder if your jfets are encountering a similar fate.
Thanks much, for doing all the sims. I actually modeled this circuit in spice but used the LSK170C JFET model. I don't have a spice model for the BF862 either, I haven't found one yet but I may look more for one if they are out there?

I know the BF862 has significantly higher transconductance but I planned on using a bit more feedback to compensate a bit, so the gain is not to high.

I planned on using slightly higher source resistors on the JFETs but forgot. Still, they should only be pulling 9mA, which is close to the 8.6 mA in the sim and should still be biased okay.

Right now they pull 15mA and drop too much voltage across the load resistor of the MOSFET. But the do pull current equally. Which is strange that they would if they were damaged???

I may try this again, maybe with a small drain resistors on each JFET and also slightly larger degeneration resistors. I'll be sure to test the board before hooking it up.

When you say the J310 is unhappy, do you mean it didn't perform well? Didn't bias properly?
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Old 21st December 2009, 05:16 PM   #9
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Quote:
Originally Posted by Johnloudb View Post
....Right now they pull 15mA ...
The Vgs of your JFETs got positive (gates got higher potential than sources).
The possible causes:
- the output coupling cap is leaking;
- you don't have a resistor (100k - 1Meg) from the point where all the gates meet to ground;
- you connected somethnig a wrong way.
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Old 21st December 2009, 05:37 PM   #10
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Quote:
I don't have a spice model for the BF862 either, I haven't found one yet but I may look more for one if they are out there?
Anthing wrong with the manufacturers, NXT, model?

.model BF862 NJF(Beta=47.800E-3 Betatce=-.5 Rd=.8 Rs=7.5000 Lambda=37.300E-3 Vto=-.57093
+ Vtotc=-2.0000E-3 Is=424.60E-12 Isr=2.995p N=1 Nr=2 Xti=3 Alpha=-1.0000E-3
+ Vk=59.97 Cgd=7.4002E-12 M=.6015 Pb=.5 Fc=.5 Cgs=8.2890E-12 Kf=87.5E-18
+ Af=1)

Also you should go for ca 40ohm source-resistors if you want 8mA.

Last edited by revintage; 21st December 2009 at 05:43 PM.
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