A precision rectifier is similar to a common inverting op-amp configuration
with a diode in the feedback loop.  This allows the op-amp to correct for
the 0.6V forward bias step and diode non-linearity, reducing it by the open
loop gain of the op-amp.  Many op-amp application sheets have examples of
this simple circuit, I know National does, check out:

   http://www.national.com/ms/LB/LB-8.pdf

HTH - Martin R. Green
elimar@bigfoot.com

----------
From:   Martin McCormick[SMTP:martin@DC.CIS.OKSTATE.EDU]
Sent:   Thursday, September 25, 1997 9:53 AM
To:     PICLIST@mitvma.mit.edu
Subject:        OT Precision Rectifiers Was Help connecting a microphone to a PI
C

 Steve Smith writes:
>Use a simple duel op-amp to give some gain to the mic and then drive a the
>second op-amp as a presicion rectifier to charge a cap on an analouge
input

        What kind of amplifier configuration is a precision rectifier?
I may already have a good idea, but I figure some of you are experts and
can
set me straight.  The circuit that comes to mind is an op amp set for some
particular gain in the usual way with the trick being that the AC signal
is fed to a pair of diodes such that positive half-cycles go to one input
and negative half-cycles go to the inverting input.  Is that all there is?

Martin McCormick