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Old 25th October 2012, 10:15 AM   #1891
fas42 is online now fas42  Australia
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Just to show how close one can go to matching the JFET curves, below is a grab from the BF862 datasheet, and LTspice's version thereof with modified parameters. There is still a problem unresolved, because the constant Vgs curves shown have been fudged, LTspice keeps insisting on equal spacing of curves between equal stepped values -- those shown represent other than Vgs of -0.1 through to -0.5. Which means that the curve of drain current as a function of gate source voltage doesn't match the datasheet.

This can be circumvented by adding a behavioural voltage at the gate, but maybe there's some fine tuning elsewhere that can be done ...

Frank
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Old 25th October 2012, 01:52 PM   #1892
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This is looking better, but there also still seems to be a kink at the transition. The problem for me was simulating the null of seconds for a simple phase splitter. It always walks down to the kink, where a real device has a sweet spot with a very deep null.

The level 3 model has an ALPHASP parameter that fits the entire curve smoothly with a tanh function which has a continuous derivative.

What did you end up using for lambda and B, and did B start doing something?

You might ask why something as fundamental as lambda is so far off in the extracted model.
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Last edited by scott wurcer; 25th October 2012 at 01:57 PM.
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Old 25th October 2012, 02:07 PM   #1893
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IMO curve fitting as used here is often a sub for an incomplete model. but often necessary and useful.
what model is being used and is it complete?

Last edited by RNMarsh; 25th October 2012 at 02:10 PM.
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Old 25th October 2012, 05:48 PM   #1894
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I would like to see the schematic of a simple circuit to match jfets BF862 at the voltage they are gona work most linearly in the circuit. I have one for BJTs. Thanks, Ray
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Old 25th October 2012, 05:50 PM   #1895
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Both p channel and n channel? Thanks, Ray
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Old 25th October 2012, 05:53 PM   #1896
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they dont come in a Pchannel
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Old 25th October 2012, 06:17 PM   #1897
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But obviously if I'm gona built a circuit to match jfets then it would be nice to match p channel as well. All I'll probably have to do is reverse voltage polarity??? Thanks, Ray
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Old 25th October 2012, 06:39 PM   #1898
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It would be nice to measure pinch off voltage on a p channel jfet. I've just never done it. Thanks, Ray
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Old 25th October 2012, 06:49 PM   #1899
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I did a google search on "Testing jfets", lots of information!!
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Old 25th October 2012, 07:06 PM   #1900
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Been there! Done that! I'm interested in people who have DONE IT and interested in avoiding the pittfalls and interested in the circuit parameters of the SWOPA that I should be looking for; what optimum voltage, what optimum Idss of the BF862. They are probably all over the map out of a sample of 100. Thanks, Ray
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