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Zen -> Cen -> Sen, evolution of a minimalistic IV Converter
Zen -> Cen -> Sen, evolution of a minimalistic IV Converter
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Old 18th May 2018, 05:07 PM   #2101
EUVL is offline EUVL  Europe
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Nice piece of work. Happy for you.

What is U2 at the back of the PCB ?


Patrick
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Old 18th May 2018, 05:13 PM   #2102
Alex_twn is offline Alex_twn  Taiwan
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All the greeting & thanks goes to you !
This is an opamp, actually the servo which I am taking about. TL074, standard stuff.
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Old 9th July 2018, 11:31 AM   #2103
torb is offline torb  Finland
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Default Input impedance of cascoded bf862

Hi!

I am thinking about building a Sen I/V for my buffalo 3 dac using bf862 cascoded with j111 as shown previously in this thread (since I have these transistors already.)

In deciding how many jfet-pairs to use, I have been trying to understand how/if the cascode affects the input impedance of the I/V. Is the impedance of one "pair" the sum of the inverse transconductances of the upper bf862 and j111?

Best regards,
Tor-björn

Last edited by torb; 9th July 2018 at 11:33 AM.
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Old 17th July 2018, 01:02 PM   #2104
torb is offline torb  Finland
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Quote:
Originally Posted by torb View Post
In deciding how many jfet-pairs to use, I have been trying to understand how/if the cascode affects the input impedance of the I/V. Is the impedance of one "pair" the sum of the inverse transconductances of the upper bf862 and j111?
So after thinking about it I guess this was a quite stupid question.

I'm planning to build a sen as in the attached schematic, but am uncertain about where to use gate stoppers. The "upper" bf862:s will get some, but should there bee stoppers on the lower ones/on the cascodes as well? My google skills fail me, and I have no proper education in electronics (learning by magic smoke as I go along...)
If there is no advice regarding this, I'll just include pads for resistors and jumper them so that I can insert resistors if needed.

Best regards,
Tor-björn
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Old 17th July 2018, 06:32 PM   #2105
EUVL is offline EUVL  Europe
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I always use gate stoppers.


Patrick
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Old 18th July 2018, 12:49 PM   #2106
torb is offline torb  Finland
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Quote:
Originally Posted by EUVL View Post
I always use gate stoppers.


Patrick
Ok, thanks! I take that to mean that I should put some on the j111's gates as well.
Will continue matching jfets and make a layout, this will take some time but will report when things progress

Best regards,
Tor-björn
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Old 18th July 2018, 02:57 PM   #2107
jborden is offline jborden  United States
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Zen -&gt; Cen -&gt; Sen, evolution of a minimalistic IV Converter
You realize that you've got to match these ... its going to be a *lot* of work ...
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Old 18th July 2018, 03:04 PM   #2108
EUVL is offline EUVL  Europe
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> its going to be a *lot* of work

Why ?
He does not to match a reel (3000).
We match regularly 200 in one go.


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Old 18th July 2018, 03:23 PM   #2109
torb is offline torb  Finland
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Quote:
Originally Posted by jborden View Post
You realize that you've got to match these ... its going to be a *lot* of work ...


The ones i've got are actually quite close, the Idss of the j111 are mostly +/- 0,5 mA.

It will take a while but feel nice when done.
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Old 18th July 2018, 04:43 PM   #2110
jborden is offline jborden  United States
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Zen -&gt; Cen -&gt; Sen, evolution of a minimalistic IV Converter
Quote:
Originally Posted by EUVL View Post
> its going to be a *lot* of work

Why ?
He does not to match a reel (3000).
We match regularly 200 in one go.


Patrick


Oh! You must have an efficient system, what workflow technique do you use? I find putting those little bugs into the template etc. to be very tedious -- and need to match 8 -- but perhaps there is a better way?


I've also found through dumb trial and error that sometimes you don't need gate stoppers and other times you do, can depend reel-reel on the factory the BF862s come from, so use stoppers always.

Last edited by jborden; 18th July 2018 at 04:48 PM.
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