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Old 24th October 2012, 03:17 PM   #1881
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Quote:
Originally Posted by fas42 View Post
For people's information, I just tried the model of the BF862 that's being used in LTspice, and it does add a gentle curve, slope decreasing towards zero gate voltage, to the transconductance characteristic; a simple, minimal spec'd set of parameters for a JFET just traces a straight line, otherwise.

Frank
Maybe a picture would help. I suggest gate shorted to source and sweep the drain voltage plotting Id, I don't see the B parameter doing anything and the curves are still fit as before.
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Old 24th October 2012, 05:22 PM   #1882
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either they set B to zero or to one and then simplified from there.
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Old 24th October 2012, 05:29 PM   #1883
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Quote:
Originally Posted by MiiB View Post
jfets as cascode devices has an impedance advantage over bipolars... so while capacitance is lower with bipolar the impedance is also lower, so the current swing over rail resistors add more distortion due to waring drive current. each type has their over benefits in cascoding
IMO -- unless you are going for the record in low thd, the distortion reduction due to cascoding will be minimal with high gnfb design. Better to spend time on PSR and CMR. Which cascode in a diff amp input will do better for those parameters? Thx-RNM
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Old 24th October 2012, 06:03 PM   #1884
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The "Rush cascode" series differential used in the second stage can help with power supply immunity, and is itself improved with conventional cascoding:

Series differential VAS.jpg

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Chris
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Old 24th October 2012, 11:21 PM   #1885
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Quote:
Originally Posted by scott wurcer View Post
Maybe a picture would help. I suggest gate shorted to source and sweep the drain voltage plotting Id, I don't see the B parameter doing anything and the curves are still fit as before.
Nicely called, Scott, no cigar ...

Original results:
BF862-gm01.jpg
BF862-gm02.jpg

Altered test rig as suggested, and got:
BF862-gm03.jpg

From the LTspice help:

Quote:
The JFET model is derived from the FET model of Shichman and Hodges extended to include Gate junction recombination current and impact
ionization. The DC characteristics are defined by the parameters
VTO and BETA, which determine the variation of drain current with
gate voltage; LAMBDA, which determines the output conductance; and
Is, the saturation current of the two gate junctions. Two ohmic
resistances, Rd and Rs, are included. Charge storage is modeled by
nonlinear depletion layer capacitances for both gate junctions;
which vary as the 1/2 power of junction voltage and are defined by
the parameters Cgs, Cgd, and PB. A fitting parameter B has been
added. See A. E. Parker and D. J. Skellern, An Improved FET Model
for Computer Simulators, IEEE Trans CAD, vol. 9, no. 5, pp. 551
553, May 1990
Frank
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Old 25th October 2012, 12:04 AM   #1886
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Quote:
Originally Posted by fas42 View Post
Nicely called, Scott, no cigar ...

Original results:
Attachment 308141
Attachment 308142

Altered test rig as suggested, and got:
Attachment 308143

From the LTspice help:



Frank
The second graph is not realistic. The sub-saturation curvature transitions abruptly to a linear function. Despite the promising description in Help, it doesn't seem to realize correct behavior.

So it goes.
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Old 25th October 2012, 01:28 AM   #1887
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Quote:
Originally Posted by bcarso View Post
The second graph is not realistic. The sub-saturation curvature transitions abruptly to a linear function. Despite the promising description in Help, it doesn't seem to realize correct behavior.

So it goes.
Exactly ... so, at the moment, "no cigar", not good enough ...

I'll have a bit of a look around, and a play, see what might be possible ...

Frank
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Old 25th October 2012, 03:03 AM   #1888
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what are the actual formulas used in the sim code? whats does the sim software do with the data? If a math major did the code, it might be a lot different than if a device physics major did it. Is curve fitting used?
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Old 25th October 2012, 04:20 AM   #1889
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We may have some good news, people ... the Lambda parameter - channel length modulation - is crucial, and may have been set completely wrong in the published model. Playing with figures pulled out of the air I now get curves that look exactly like they're supposed to for the Ids vs Vds plot, but in the wrong place. So I'll start fiddling with other parameters, and see if I can get it to fully behave ...

Frank
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Old 25th October 2012, 07:25 AM   #1890
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Lambda - even at level 3 model - is often default set to 0.0
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