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Old 8th July 2012, 04:06 PM   #211
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Since folks are curious how to modify the amp camp amp for depl SemiSouthes, I will attach an example (basic idea) where it belongs.
Clearly, the difference is with the front end circuit.
You will need 2SJ74 V, BL.
You will need an additional neg volt supply, probably up to -10V,
and it will be used to feed the neg volt reference, where the lower end of P1 will be connected.
So, I have marked a few test points to give the ballpark values.
For Q2 SemiSouth depl transistor connection, pick the solution
that works for you (discussed in this thread).
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File Type: jpg SS+input Jfet.JPG (30.0 KB, 369 views)
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Old 8th July 2012, 06:08 PM   #212
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I will try original circuit, then add FE to see what it may offer in improvement. Not getting much done, as i have matching left that is consuming my time. Doe sthe BJT addition offer improvement because of stable nature of Vbe drop?
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Old 10th July 2012, 01:56 AM   #213
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Don't know if I have posted this before, but here is a trace of the R085 at Vds of 25V, no Rs. Very pretty, I think and based on this raw data, -5v seems to be the sweet spot. I will have to test with Rs and also at higher currents.
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Old 10th July 2012, 02:52 AM   #214
LBHajdu is offline LBHajdu  United States
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I have not simulated this yet so I donít know if it will work as drawn. I know this is a depletion Jfet thread but it may be more advantageous to do the Camp Amp like I have drawn it here.
  1. The SJEP120R100A only costs $28 each vs. $41.08 for the SJDP120R085 at newark.com
  2. The top MOSFET can also be swapped out without any changes for the JFET making this an all JFET amp.
  3. There is no need to use P-type Jfets which tend to be harder to find. Moser sells the 2SK170 but not the 2SJ74
  4. You can replace the two LEDs with an ultra-low noise shunt like ADR530 if preferred (Just watch the current limit).
  • I am not sure if the charging of the cap will cause turn on thump?
  • The LEDís should be selected to get a drop of about -3.0~-3.5 volts.
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Old 10th July 2012, 11:55 AM   #215
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buzzforb, your graph was at 25C? The threshold will shift down with temp going up. It would be nice to know the threshold at 75C, or something like typical operating temp.
Also, Id up to 2A with Rs bias would be nice. Thx
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Old 10th July 2012, 12:26 PM   #216
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Ill put one in the oven and then test. Will be interesting when the kids ask what i am cooking for dinner That graph shows response up to almost 3A. Rs testing will be tougher with current setup. May have to do the old fashion way. I tested this way for two resons, it was easy, and i plan to try without Rs for the fun of it. Got boards almost redrawn and should be able to move through the different iterations. Might be a week or two, as i haev some other matching to finish this week.....hopefullly!
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Old 10th July 2012, 04:40 PM   #217
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Buzz, your DR085 data looks very suspicious. Gm at ~1A is only (0.3A / 0,1V =) 3S, and drops with current very quickly. And this is @25V Vds? Seems very low.
I am judging only by the d/s fig 4.
Are you sure that 25V stays stable throughout the measurement process? Needs monitoring with an additional voltmeter between D and S.
Are you sure that Vgs steps are coming stable in value (and no excessive gate leakage and high gate drive output resistance to affect this)? Needs monitoring
with an additional voltmeter between G and S.
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Old 10th July 2012, 04:47 PM   #218
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I was interested in the temp. relative numbers. But, now that you mention it, how did you get these data points?
Actually, I expect something on the order of -4Vgs for Ids of 2A at temp??? 2 ohm Rs w/maybe 15Vds - 25Vds for a self bias test...
Thx
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Old 10th July 2012, 05:01 PM   #219
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Quote:
Originally Posted by StevenOH View Post
Buzz, your DR085 data looks very suspicious. Gm at ~1A is only (0.3A / 0,1V =) 3S, and drops with current very quickly. And this is @25V Vds? Seems very low.
I am judging only by the d/s fig 4.
Are you sure that 25V stays stable throughout the measurement process? Needs monitoring with an additional voltmeter between D and S.
Are you sure that Vgs steps are coming stable in value (and no excessive gate leakage and high gate drive output resistance to affect this)? Needs monitoring
with an additional voltmeter between G and S.
Could be possible, as this is particluar measurement is pushing the curve tracer beyond its preset current limits. Trying to cheat the system. I will test again using standard settings and see what happens. The Yfs did seem low to me. Should have set off alarms.
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Old 10th July 2012, 05:02 PM   #220
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BTW buzzforb, FETs should do their best with no Rs(feedback). Ideally, I think, they should produce only the first few even order hamonics and nothing else.
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