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Old 29th October 2014, 04:02 PM   #1
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Default Messing about with JFET input JLH

I have been messing with the idea of building a JLH with a Jfet input. This is what I came up with. I am only planning on a low powered version so +/-12V at 1A bias is OK. Seems to measure OK.
If I build it I will be using some 2SD718's rather than the 2N3055, which have twice the gain and a much higher Ft at 15mhz and hence the base stoppers to attempt to suppress instability. Couldn't find a suitable spice model so ran with the 2N3055's.

Seems the response is very much SE rather than PP with the bottom output transistor doing about half as much work again as the top one, and the current delivery far more typical of SE than PP.

Anyone care to critique.

Shoog
Attached Images
File Type: png Jfet_JLH.png (70.9 KB, 241 views)
File Type: png Plot1.png (28.6 KB, 236 views)
File Type: png FFT.png (26.7 KB, 221 views)
File Type: png Plot2.png (25.9 KB, 215 views)
File Type: png FFT2.png (28.5 KB, 208 views)
Attached Files
File Type: asc JLH_v5.asc (4.0 KB, 12 views)

Last edited by Shoog; 29th October 2014 at 04:15 PM.
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Old 29th October 2014, 05:45 PM   #2
knutn is offline knutn  Norway
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There is a thread with all-FET versions:
All FET JLH Class A
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Old 29th October 2014, 10:07 PM   #3
Bigun is offline Bigun  Canada
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Old 29th October 2014, 11:35 PM   #4
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JLH bottom has to work harder because the base current gets dumped into a rail rather than the load.
Nothing you can do about it but use complimentary devices or insulated gates, I suggest neither.
Simply with higher gain output devices, the work imbalance is a lesser fraction.

I prefer the sum of output currents be measured by the sum of voltage drops across sense resistors,
one to either side of the output rail. Set the bias current purposely a little too high. When the voltage
drops (in the sensing totem) sum to 0.7V, a shunt transistor (across the bias) bleeds off the excess.
The output class is forced to behave by strict Class A rule of linear emitter current vs linear collector
current, even though drive current requirements for each transistor's position in the circuit will differ.
Big advantage over the standard biasing scheme: its automatic, and it can't thermally run away...

Its good that yours has less than unity bootstrap. Not by itself in any real danger of becoming positive
feedback, but bootstrap too close to unity here can make the upper half less stable for something else
to tip it into oscillation. I used to do that part wrong and cause myself all sort of troubles.

Whats the purpose of r8?

Last edited by kenpeter; 30th October 2014 at 12:05 AM.
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Old 29th October 2014, 11:41 PM   #5
Nisbeth is offline Nisbeth  Denmark
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Old 30th October 2014, 10:01 AM   #6
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R8 is simply a carry over from the original schematic - used to sense bias current. I can probably drop it. R10 will be variable and will be the bias adjustment.
Will think about other suggestions. Kenpeter maybe you could sketch what you mean by a bias transistor as I am having a little trouble conceptualizing it.

Took me a bit to work out what each component does, clever little circuit the original JLH.

Shoog

Last edited by Shoog; 30th October 2014 at 10:03 AM.
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Old 14th December 2014, 11:18 PM   #7
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So I haven't forgotten this project, but I have been messing about with making it a PNP version because i have more 2SJ170 and should get better matches than for the K74's.
When reading the schematic notice that it is effectively upside down in terms of the voltage sources.
The overall distortion is lower than the original, but the profile is not quite so benign.

Hopefully will start building this in the next few weeks.

Shoog
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File Type: png JLH_PNP_FINAL.png (104.8 KB, 42 views)
File Type: png Screenshot from 2014-12-14 23:16:00.png (26.4 KB, 40 views)
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