Re: matching FETs

From Mariano Elizalde
Sent Fri, Sep 13th 1996, 03:26

  Hello;

  The easiest way to match jfets is by selecting those whose Ids (drain to
source curren) is identical (or as close as possible...),  here's how you
can do that:

                ^ +15v
                |
                |
                | Drain
             ___|
            |
  Gate  --->|       jfet under test... 
       |    |
       |    |___   Source 
       |        |
       |        |<---- your meter's + lead (red)
       |        |
       |        /
       |        \ 1K 1%  place you digital volt meter
       |        /         in parallel to this resistor...
       |        \
       |        |<---- you meter's -lead (black)
      ---      ---
        (ground)

   Select the jfets that have a similar voltage reading across the 1K
resistor... 

    A quick breadboarding and three KE4392 jfets revealed.. a) 1.602 volts
b) 2.612 volts and c) 1.876 volts for each jfet... I would use a) and c) as
the closest match...

  note: let the circuit settle a minute or so with each jfet before taking
the final reading... (thermals, you know ..)

   ( I know this post is proper of synt-diy, but I found it here at AH... :]  )


  Hope this helps...
 
  bye...

 someone wrote:
   
>From: Gareth Jones <gareth@swansea.demon.co.uk>
>Date: Thu, 12 Sep 1996 10:26:41 +0100
>Subject: Re: matching FETs
>
>In article <199609120505.WAA19726@taz.hyperreal.com>, analogue-
>owner@hyperreal.com writes
>>Does anyone know how to match FET's for phase shifters specifically the
>>Roland System 100m as it asks for the Fet's to be selected. But doesn't
say for
>>what . I can only assume that they be matched. Is there a circuit I could
build
>>like a jig for matching up Fet's and if they don't match can I alter the
>>resistors>
>
>I would have thought that the selection was just to match the gain. It
>should not be too difficult to breadboard up a little circuit to give
>you the amplification factor of each device. How many do you need? its
>been a while since I've worked with discretes, but trannys can be
>obtained in matched pairs or batches that are pre matched to be within a
>certain (closer) spec.
>It depends on the circuit, but I'd be more inclined not to fiddle with
>the other components (why not put a 3 pin socket in there and just try
>different ones untill it works to your satisfaction?)
>- -- 
>__________________________________________
><Gareth Jones>  Gareth@swansea.demon.co.uk
>                       gl.jones@sihe.ac.uk
>  "Reality sucks - go watch a Star Trek"


   mariano
   
   mariano-elizalde@worldnet.att.net