Don's favorite source follower

[Don Tillman]

   Date: Wed, 28 Jun 2000 08:29:41 -0700 (PDT)
   From: Don Tillman <don at till.com>

      From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
      Date: Wed, 28 Jun 2000 16:16:15 +0200

      So ... the circuit is very "musical", and the only remaining
      drawback would be the not-so-low output impedance.

   Is the output impedance really a problem? 

[Replying to myself...]

Maybe the output impedance isn't all that high.  For the applications
I'm working on right now I've been using low Idss FETs because I don't
need to drive significant loads.  

I've been using the 2N5457-2N5462 series (low, medium and high Idss in
N-channel and P-channel versions) because they seem like a nice set of
devices with low noise, they're readily available, and because I have
a lot of them in my parts collection.  It wouldn't surprise me if
there were better devices available.

The 2N5457/2N5460 devices should give you an output impedance around
250 ohms in a complementary source follower circuit without the source
resistors.  The 2N5459/2N5462 devices should give you around 140 ohms.
By selecting them for transconductance you could get down to 85 ohms. 

The 2SK170/J74 that Erno B uses seems to be very high transconductance
FETs.  That should give you about an 18 ohm output impedance.  He
claims an output impedance of 2.3 ohms (!!!), maybe he's hand-
selecting higher transconductance devices.

I don't know anything about Erno's FETs -- the 2SK170, 2SK246, and the
2SJ74.  Are they readily available?

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com