I/V stage with a common gate Jfet?

Bricolo · 9th February 2005, 01:54 PM

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

Charles Hansen · 9th February 2005, 05:51 PM

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.

jan.didden · 9th February 2005, 06:05 PM

I agree with Charles. One reason why a bjt gives much lower input Z here is it's higher transconductance.

Jan Didden

Bricolo · 9th February 2005, 06:15 PM

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?

jan.didden · 9th February 2005, 06:22 PM

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

Bricolo · 9th February 2005, 06:28 PM

I just wanted to try with jfets

And no, feedback isn't what I want

jan.didden · 9th February 2005, 06:32 PM

You missed my edit...

Bricolo · 9th February 2005, 06:33 PM

you like me, really?

I was more into discrete, but I'll think about your idea, thanks!

SY · 9th February 2005, 06:36 PM

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.

Bricolo · 9th February 2005, 06:37 PM

are you talking about this?

AD844 as a common base stage in the I/V converter

9th February 2005, 01:54 PM	#1
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	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 __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

9th February 2005, 06:05 PM	#3
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	I agree with Charles. One reason why a bjt gives much lower input Z here is it's higher transconductance. Jan Didden __________________ /Yes! Its out: Linear Audio Vol 5! I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

9th February 2005, 06:15 PM	#4
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	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? __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

9th February 2005, 06:22 PM	#5
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	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 __________________ /Yes! Its out: Linear Audio Vol 5! I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

9th February 2005, 06:28 PM	#6
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	I just wanted to try with jfets And no, feedback isn't what I want __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease


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9th February 2005, 05:51 PM	#2
Charles Hansen diyAudio Member Join Date: May 2003 Location: Colorado	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.

9th February 2005, 06:32 PM	#7
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	You missed my edit... __________________ /Yes! Its out: Linear Audio Vol 5! I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

9th February 2005, 06:33 PM	#8
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	you like me, really? I was more into discrete, but I'll think about your idea, thanks! __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

9th February 2005, 06:36 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

9th February 2005, 06:37 PM	#10
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	are you talking about this? AD844 as a common base stage in the I/V converter __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

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