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Old 21st December 2009, 06:32 PM   #11
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There's a mistake somewhere, and the JFETs are croaking
either from overheating or more likely high voltage.

I bet oscillation, if you don't have a scope.

You don't have a resistor in series with the output, and the
value of R1 might be too low.


Last edited by Nelson Pass; 21st December 2009 at 06:34 PM.
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Old 21st December 2009, 09:37 PM   #12
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R1... Yes, we generally use 100 to 220 ohms. I miss that one altogether.
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Old 22nd December 2009, 12:16 AM   #13
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Thanks for input for all the input guys!

Quote:
Originally Posted by juma
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.
Thanks, I've been investigating the circuit more carefully, and it does appear the JFETs are toast. Just to double check I put a 9v battery across them again with input grounded and they are each pulling 18.0mA through drain and 12uA through the gate. Should be 9.1mA. And, it's highly unlikely do to electrostatic damage given that I haven't killed one yet on my test board (below).

Click the image to open in full size.

I neglected to put any load at the output when I fired it up. That may have affected stability, or maybe a transient voltage at turn on damaged the JFETs. Don't know.

Quote:
Originally Posted by revintage
Anthing wrong with the manufacturers, NXT, model?
I didn't look too hard I guess. Thanks for the model.

Quote:
Originally Posted by Nelson Pass
There's a mistake somewhere, and the JFETs are croaking
either from overheating or more likely high voltage.

I bet oscillation, if you don't have a scope.

You don't have a resistor in series with the output, and the
value of R1 might be too low.
I'll start over, do some spice sims and hook up the circuit without feedback first and see if how that goes. And, I'll be sure to put, say, 20k load resistance at the output.

I used a 49.9 ohm resistance for R1 because in another cascode circuit using the IRF510, putting a larger resistance there caused a large peak at 10Mhz, I think. But, I'll do some spice sims and see what happens.

I'll put a 220 ohm resistance at the output too.

Thank You.
Attached Images
File Type: jpg JFET_testboard.jpg (173.5 KB, 344 views)
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Last edited by Johnloudb; 22nd December 2009 at 12:33 AM.
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Old 22nd December 2009, 07:26 AM   #14
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Quote:
I neglected to put any load at the output when I fired it up.
But I suppose you had the 10k from output to ground connected. Otherwise this might be the cause as it is also the ground-reference for the jfet gates.
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Old 22nd December 2009, 07:41 AM   #15
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Quote:
Originally Posted by revintage View Post
But I suppose you had the 10k from output to ground connected. Otherwise this might be the cause as it is also the ground-reference for the jfet gates.
Which begs the question: if you remove the feedback loop, will you provide another ground reference for the jfet gates?
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Old 22nd December 2009, 08:03 AM   #16
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A 470k from the JFETs gate to ground could do no harm.
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Old 22nd December 2009, 12:40 PM   #17
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Quote:
Originally Posted by revintage View Post
But I suppose you had the 10k from output to ground connected. Otherwise this might be the cause as it is also the ground-reference for the jfet gates.
I had the 10k at the input grounded, but I didn't have the 10k ohm load at the output. So, I agree that this is likely the cause of the problem.

I see what you mean about that being the ground reference for the JFETs.

I'll add that 470k resistor while I'm at it.
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Old 22nd December 2009, 09:14 PM   #18
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The circuit should not require an output load for stability.

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Old 22nd December 2009, 09:51 PM   #19
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IŽd say the output resistor wether it is 10k or some other value is absolutely necessary, but not for stability. This to still have a ground-reference for the JFET gates in case you remove the source. But as I suggested before you could also add a resistor directly from the JFETs gate to ground to get this.
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Old 22nd December 2009, 11:28 PM   #20
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Quote:
Originally Posted by Nelson Pass
The circuit should not require an output load for stability
.

Okay, good to know!!! I didn't think so at first either (hence no resistor), but not sure and I'm kind of stumped here.

Here's the circuit schematic again but simmed with the bf862 JFET model. I added a small cap to get rid of a response peak at 1MHz. I added the 220 gate stopper to the MOSFET, which had no affect on the response in this circuit.

Click the image to open in full size.

Here's the simmed high frequency open loop response and shows almost 20 degrees of phase margin. That's not really very good though is it?

Click the image to open in full size.

If this all looks okay, I'll go order some more JFETs.
Attached Images
File Type: png Cascode_BOZ_bf862.png (4.9 KB, 275 views)
File Type: png C_BOZ_openloop.png (3.4 KB, 277 views)
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