Don's source follower

[Don Tillman]

   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.

Yes, it's good to see that that Erno Borbely, who has a great
reputation in audio circles, likes it and has performed more
sophisticated measurements than I'm set up to do.  

I guess I'm just surprised I haven't seen the circuit around more.

Is the output impedance really a problem?  Well, how would we use this
thing?  These are the uses I have in mind:

1. Very high impedance buffer amp for my Wurlitzer piano pickup.  10
Mohm input impedance and low enough output impedance to drive a
passive resistor mixing network.  So output impedance can be over 1
kohm without a problem.  I just built this and it sounds pretty nice,
but a Wurlitzer piano isn't an especially discriminating situation.
The graceful clipping is nice here as is the lack of ugly opamp common
mode problems.

2. Line output stage typically driving into one or more mixer
inputs or, at worst case, a power amp input.  If it can handle a 10
Kohm load and electrical abuse from patching errors it's fine.  

3. Balanced line input stage for a 3280 OTA-based circuit in a
standalone, rack mounted unit.  The input jack connects to a 100 K
level control pot, then to a pair of FET buffers, and then to the
differential inputs of the 3280.  I need a buffer between the level
control pot and the OTA because the OTA input impedance is kind of low
(I'm using the linearizing diodes here), I need to isolate the OTA
inputs from impedance changes due to the input and the level control,
and I also need to trim the offset voltage on the 3280.  This is easy
with the FET buffers.  

4. Buffering the OTA output for a VCF.  

So a 500 ohm or 1000 ohm output impedance is not a problem for my
applications.  

   What about a complemetary 2-stage follower, not with BJTs but
   MOSFETs for the second stage?  Either building an ordinary
   darlington configuration for each half (nJFET source follower into
   nMOSFET source follower, negative threshold voltage of JFET and
   positive threshold voltage of MOSFET would sort of fit, but might
   have to be matched), ame for lower half with pJFET and pMOSFET.  Or
   maybe better, a "pseudo darlington" for each half, nJFET of upper
   part gets a drain resistor and the voltage across that goes to gate
   of pMOSFET, lower half same with pJFET and nMOSFET.  Just an idea,
   neither tested not thoroughly thought over. But there is something
   like that with nJFET and pnpBJT for sure, so ...

Very reasonable.  The only problem I see is that you need to match all
four transistors together.  

  -- Don