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Old 14th February 2012, 01:54 AM   #11
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I think one other additional key to the design is to bias the fet fairly hot for high transconductance. This, coupled with the current source loading, gives one a lot of gain to feed back. Using the IRF610 with a good heat sink, one could really go for the gusto and use 50-100ma of bias current for even more transconductance. With a little higher gain, you'd get that headphone amp that Mr. Newton was talking about...
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Old 14th February 2012, 02:47 AM   #12
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No Schaded single curveset accurately mimics a triode.
The way real triode curves "lean to the right" is probably
aggregate of many parallel triode paths of differing cutoff.
I think was Smoking who proposed this?

Anyways, I tested that theory by throwing many different
Schaded curvesets into a blender. I found that it seems to
require bare minimum of four curvesets to make a triode
curveset that leans in believable fashion.

Faking a SIT might be one curve only, I don't know...
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Old 14th February 2012, 03:09 AM   #13
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I've always said "triode-ish", unlike the unmodified device, which is "pentode-ish".
Anyway, the composite device stands on its own. I'll find out how good it actually is when I hook it up to my distortion analyzer. It could be 100X worse than the simulation in real life and still be pretty damn good, considering the parts count.
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Old 14th February 2012, 03:09 AM   #14
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That why it's called fake.

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Old 14th February 2012, 03:11 AM   #15
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Anode follower?
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Old 14th February 2012, 03:11 AM   #16
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Yupper (for the "fake")....

Edit - Call it Rupert if you get your jollies that way. Build it if you like it, ignore it if you don't....

Last edited by wrenchone; 14th February 2012 at 03:16 AM.
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Old 14th February 2012, 03:32 AM   #17
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Current works as well as voltage.

What I mean is: I didn't need a base
resistor to convert voltage to current.
The divider is also the base resistor.

It doesn't matter the voltage is wrong
due to the base current, only matters
the base current is right. And it is...

High impedance gate unnecessary.
The leakage current causes no error.
Attached Images
File Type: gif SillySIT.gif (34.1 KB, 990 views)

Last edited by kenpeter; 14th February 2012 at 03:40 AM.
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Old 14th February 2012, 03:38 AM   #18
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That's nice - why don't you build something like that with large-ish bipolars and start a thread with the results? Bipolars get a bum rap these days. A simple Class-A amp with bipolars would be interesting.
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Old 18th February 2012, 02:54 AM   #19
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I've done some messing about in simulation with the bipolar version shown a couple of posts ago with results so far that are unimpressive compared to the original Jfet/mosfet circuit posed at the beginning of this thread. That one appears to satisfy all my requirements for a relatively low gain line amp except for being inverting. I may have time to make some distortion measurements on my breadboard this weekend, if I can find the BNC cables I just bought a week and a half ago...
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Old 19th February 2012, 04:41 AM   #20
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Nice work, thanks for posting.

I'd like to see what happens if you went for less gain, say 2-3? I assume you do that by increasing degeneration/the bottom resistor on the Mosfet. Also, any idea what output impedance is as shown?

What about the BS170? They are cheap but should handle 20ma, 830mW total. I have a bunch here for something else.
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