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Old 24th November 2009, 10:59 AM   #1
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Default JFET choice

Hello all,

I'm working on a new amplifier, a PP one with jfet frontend and Mosfet output stage.
Nothing new under the sun, a good old topology.
The schematic is attached.
I have simulated it with 2SK170 and 2SJ74 jfets, and it simulates very well.
I have now found a good (cheap) source for 2SK246 and 2sj103 jfets.
Do they perform well? From the datasheet it seems they are even better than the ones I used to simulate the circuit.
Is there anybody here that can send me a spice model for 2SK246/2SJ103 jfets?
I searched in anywhere without success.

Please let me know.
Of course, feel free to comment my schematic.

Ciao,
Giovanni
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Old 24th November 2009, 12:09 PM   #2
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Hi Giovanni,

J-fet models as per your request...I would use 2SK170/2SJ74 cascoded by 2SK246/2SJ103(Erno Borbely style)...and use low Cob transistors instead BD139/140 (like BF472/473 or better)

.model 2SK246 NJF(Beta=1.07m Rs=56.76 Rd=56.76 Betatce=-.5 Lambda=2.8m Vto=-2.638 Vtotc=-2.5m Cgd=10.38p M=.4373 Pb=.3905 Fc=.5 Cgs=6.043p Isr=112.8p Nr=2 Is=11.28p N=1 Xti=3 Alpha=10u Vk=100 Kf=1E-18 Af=1)

.model 2SJ103 PJF(Beta=2.197m Rs=76.76 Rd=76.76 Betatce=-.5 Lambda=735.3u Vto=-2.037 Vtotc=-2.5m Cgd=18.95p M=.5045 Pb=.3905 Fc=.5 Cgs=17.02p Isr=38.48f Nr=2 Is=3.848f N=1 Xti=3 Alpha=10u Vk=100 Kf=1E-18 Af=1)

.model 2SJ74 PJF(Beta=92.12m Rs=7.748 Rd=7.748 Betatce=-.5 Lambda=4.464m Vto=-.5428 Vtotc=-2.5m Cgd=85.67p M=.3246 Pb=.3905 Fc=.5 Cgs=78.27p Isr=129.8p Nr=2 Is=12.98p N=1 Xti=3 Alpha=10u Vk=100 Kf=26.64E-18 Af=1)

.model 2SK170 NJF(Beta=59.86m Rs=4.151 Rd=4.151 Betatce=-.5 Lambda=1.923m Vto=-.5024 Vtotc=-2.5m Cgd=20p M=.3805 Pb=.4746 Fc=.5 Cgs=25.48p Isr=84.77p Nr=2 Is=8.477p N=1 Xti=3 Alpha=10u Vk=100 Kf=111.3E-18 Af=1)
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Old 24th November 2009, 01:29 PM   #3
ra7 is offline ra7  United States
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Regarding the spice models, what parameters must be changed to adjust for different grades of the 2sk170 ie. GR, BL, V? These represent different Idss value, as I understand.
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Old 24th November 2009, 02:43 PM   #4
jony is offline jony  Poland
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Hmm,
Beta=Idss/(Vto^2)
Vto=Vgs(off)
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Old 24th November 2009, 03:09 PM   #5
ra7 is offline ra7  United States
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Thanks Jony!

Just what I needed.
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Old 24th November 2009, 03:17 PM   #6
h_a is offline h_a  Europe
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Quote:
Do they perform well?
Well, they are rather different parts. The K170 is low noise, low pinch-off and high transconductance, the K246 is none of that. Has also its applications, but is usually not used for frontends. If you don't need the gain, why not though.

Have fun, Hannes
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Old 16th February 2010, 10:17 AM   #7
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Hello all,

After a few months of stop, I started again my mumbling on the new PP amplifier; the first schematic evolved into something quite complex.
It became a full differential cascode PP amplifier, which perform amazingly well into the simulator (LTSPICE):
Currently I'm in the process of drawing and, then, realize two PCB in manner to try a stereo version of it.
Before I actually build it, is there anything wrong with it?
I mean, it simulates wonderfully but would it work the same in reality?
I'm afraid this beast should be problematic to stabilize, I think it could oscillate badly.
Any issue? Comments?

Attached the schematic, the LTspice simulation file (029.txt: change its extension to .asc) and a zip file containing the needed TL431 simulation data.

Ciao,
Giovanni
Attached Images
File Type: jpg 029.JPG (102.7 KB, 572 views)
Attached Files
File Type: txt 029.txt (27.8 KB, 54 views)
File Type: zip tl431.zip (2.2 KB, 36 views)
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