I/V stage with a common gate Jfet? - diyAudio
Go Back   Home > Forums > Source & Line > Digital Line Level

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th February 2005, 02:54 PM   #1
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
Join Date: Nov 2002
Location: Grenoble, FR
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
__________________
Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
  Reply With Quote
Old 9th February 2005, 06:51 PM   #2
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.
  Reply With Quote
Old 9th February 2005, 07:05 PM   #3
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
I agree with Charles. One reason why a bjt gives much lower input Z here is it's higher transconductance.

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 9th February 2005, 07:15 PM   #4
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
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
  Reply With Quote
Old 9th February 2005, 07:22 PM   #5
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
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
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 9th February 2005, 07:28 PM   #6
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
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
  Reply With Quote
Old 9th February 2005, 07:32 PM   #7
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
You missed my edit...
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 9th February 2005, 07:33 PM   #8
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
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
  Reply With Quote
Old 9th February 2005, 07:36 PM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
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.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 9th February 2005, 07:37 PM   #10
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Phono stage common ground hum!! ungie Analogue Source 11 3rd January 2008 12:01 AM
Common Emiter output stage. Acoustic Absorb Solid State 16 27th March 2006 11:16 AM
best way to generate Gate-Drain or Gate-Source short in electret capsule JFET? capslock Parts 2 27th September 2002 09:07 AM


New To Site? Need Help?

All times are GMT. The time now is 02:27 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2