[sdiy] Expo converter ??

[Don Tillman]

   > Date: Fri, 17 Jan 2003 23:41:27 -0500
   > From: harrybissell <harrybissell at prodigy.net>
   > 
   > What are the advantages/disadvantages of the NPN/PNP expo
   > converter favored by ARP synths... vs the more well known NPN
   > pair and opamp servo ???
   > 
   > For what parameters are the NPN / PNP matched ???
   > 
   > All dissertations cheerfully entertained...

For folks who are unfamiliar with it, the ARP exponential converter is
a wickedly simple circuit consisting of an NPN emitter follower
followed by a PNP common emitter stage.  The emitter follower's Vbe
offset biases the base of the common emitter stage.  The two Vbe's
track over temperature, but you still need a temperature dependent
resistor for accurate scaling.

My opinion, for what it's worth...

The advantages are that it's incredibly simple, it's fast, it's
accurate, and there are very few ways it can screw up.

Disadvantages?  It may be slightly less accurate than the opamp servo
approach.  On the other hand, the accuracy could be improved if the
emitter follower was loaded with a constant current source instead of
a resistor.  And if the common emitter was cascoded.

For matching: I'm a big fan of customizing the matching procedure to
mimic the environment the transistors will be operating in.  So I
would match the NPN and PNP by measuring the Vbe with the collector
shorted to the base running at the appropriate reference Ic (probably
near 1ma).

  -- Don

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