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Old 7th December 2012, 10:49 AM   #7991
FdW is offline FdW  Netherlands
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Some research gives the attached graph (for the IRF9610) it shows that Vgs is independent of temperature for about 100mA Id; for the IRFP240 this is at 4.8V at 300mA (about). Somewhere there should be a fit for 30mA (as is needed here).
Or, it should be possible to match a DZtc to the DMOStc and get a temperature neutral system with only a zener and a mosfet No need for the 2 transistors (but keep the resistors and the cap).
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Old 7th December 2012, 10:56 AM   #7992
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Some research gives the attached graph (for the IRF9610) it shows that Vgs is independent of temperature for about 100mA Id; for the IRFP240 this is at 4.8V at 300mA (about). Somewhere there should be a fit for 30mA (as is needed here).
Take a look at the 3LP01M
http://www.onsemi.com/pub_link/Collateral/3LP01M-D.PDF

---Gary
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Old 7th December 2012, 10:58 AM   #7993
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Ok, i try that in the next hour. Was busy with other things.
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Old 7th December 2012, 12:06 PM   #7994
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Almost now if only that was at 2.7V (there the zener is also about Tc neutral).
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Old 7th December 2012, 12:48 PM   #7995
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And I found this http://www.cse.psu.edu/~kyusun/class...S05BiasCkt.pdf it has lots of information about self biasing.
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Old 7th December 2012, 02:25 PM   #7996
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this is a very good one!
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Old 7th December 2012, 02:39 PM   #7997
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Cool
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Old 7th December 2012, 03:00 PM   #7998
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this is a very good one!
It will not work for the purpose, due to the small SOA and 150mW maximum dissipation. At 30mA and Vcc 35V the device to be used must be able to dissipate at the least 20 * 30m = 600mW (and that is not even considering any margin/SOA concerns).
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Old 7th December 2012, 03:22 PM   #7999
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It will not work for the purpose, due to the small SOA and 150mW maximum dissipation. At 30mA and Vcc 35V the device to be used must be able to dissipate at the least 20 * 30m = 600mW (and that is not even considering any margin/SOA concerns).
true! too bad!

I still think that the IRF960 cascoded with the OP-AMP looped should be implemented and tested as it could be a very nice working solution.
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Old 7th December 2012, 03:24 PM   #8000
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Beside just as a thought aside, matchying the CCs for TempCo against the CS input stage might be pain as this might mean that in order to have minimum drift you might want to adjust current flowing through the input stage and thus changing gain and might make it really hard to build one matched 60-61dB matched stereo unit.
At the end it is obviously the most refined solution IMHO but it might be nearly impossible or really impracticle just my two cents.
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