OTA question

[Don Tillman]

   From: JH-3-Man
   Date: Thu, 12 Oct 95 10:40:00 PDT

   Thanks for the answer. I use the 13600/13700 almost every time,
   and I have used all three methods (discrete, follower, inverter)
   in different projects myself - but what I was really looking for
   was some theory about the two alternatives I named (in *some*
   applications the level shift of a discrete buffer is not that good).
   I was really asking about pro's and cons of the two named
   alternatives, and to make it easy I named a 3080. I might add that
    opamps I use to use the TL07x series.
   So given these to components (for what reason ever) - what would
   the pro's and con's for either configuration?

(You certainly did ask about the pros and cons.  I was just being
lazy.)

Okay, the OTA is performing the same function in both, except that, as
you noted, in (1) the OTA output pin is seeing the output voltage and
in (2) the OTA output pin is held at ground and the OTA is effectively
cascoded.  Obviously (2) is better in that the OTA output transistors
get to deal with fewer nonlinearities, but I don't think that's an
issue because those transistors are part of current mirrors, which are
mighty linear.  I can't say for sure without a complete chip schematic
(or a microscope) but it's quite likely that the current mirrors are
the more complex variety and are already cascoded.

We're all pretty familiar with the vehavior of the opamp part of the
circuit.  

So, I'm thinking it doesn't make much difference.

  -- Don