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I/V stage with a common gate Jfet?
I/V stage with a common gate Jfet?
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Old 9th February 2005, 01:54 PM   #1
Bricolo is offline Bricolo  France
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Default I/V stage with a common gate Jfet?

Hi,


I'm working on a Jfet based I/V converter. Open loop, common base, maybe folded cascode if I can get it work.

But based on my simulations, the input impedance is high. A few dozen ohms, compared to the few ohms of a bjt or mos based common gate/base stage, this seems high.

Have I done something wrong, or is this normal with jfets? (making them unappropriate for common gate )

Thanks
Alex
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Old 9th February 2005, 05:51 PM   #2
Charles Hansen is offline Charles Hansen
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Presumably you mean common gate and not common base. The input impedance in this case will be the reciprocal of the transconductance. For a 2SK170 running at 5 mA or so, this will typically give 33 ohms input impedance.
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Old 9th February 2005, 06:05 PM   #3
jan.didden is offline jan.didden  Europe
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I agree with Charles. One reason why a bjt gives much lower input Z here is it's higher transconductance.

Jan Didden
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Old 9th February 2005, 06:15 PM   #4
Bricolo is offline Bricolo  France
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ok, so I can forget the Jfets for my IV

you both agree that the input impedance is dependant of the gm
but Mosfets have a lower gm than BJTs, and my sims still show a low input impedance (a few ohms)

no way to have a Jfet IV with a low enough impedance?
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Old 9th February 2005, 06:22 PM   #5
jan.didden is offline jan.didden  Europe
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Bricolo,

Well, why do you insist on a jfet??

it can be done: wrap some feedback around the jfet stage. But I guess that's not exactly what you are waiting for...

You want a real novel solution, something others haven't thought about, open loop, excellent quality, simple etc, in other words, worlds best i/v?

Just for you, because I like you:

Get an AD844. Connect the + input to ground. Connect the DAC current output directly to the -input. Put 1k from pin 5 to ground. At pin 6 you take off the signal at 1V/mA. One chip, 1 resistor.

Jan Didden
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Old 9th February 2005, 06:28 PM   #6
Bricolo is offline Bricolo  France
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I just wanted to try with jfets


And no, feedback isn't what I want
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Old 9th February 2005, 06:32 PM   #7
jan.didden is offline jan.didden  Europe
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You missed my edit...
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Old 9th February 2005, 06:33 PM   #8
Bricolo is offline Bricolo  France
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you like me, really?

I was more into discrete, but I'll think about your idea, thanks!
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Old 9th February 2005, 06:36 PM   #9
SY is offline SY  United States
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I/V stage with a common gate Jfet?
Is 33 ohms really too high? I remember the old 1541As being happy with anything under 100 ohms or so. In any case, you could always go parallel on the jFETs.
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Old 9th February 2005, 06:37 PM   #10
Bricolo is offline Bricolo  France
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are you talking about this?
http://www.diyaudio.com/forums/showt...206&highlight=
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