🚀 go-pugleaf

In article <913@cseg.uucp> gmg@hcx.uucp (Greg M. Garner) writes that he is
looking for a high speed log amplifier.

As Frank Reid has pointed out, the simple diode in the feedback path of an
op-amp design is limited due to gain-bandwidth considerations.  This circuit
is often known as the transdiode configuration.  The main problem with this
circuit is one of closed-loop stability; since gain is a function of signal
level, there is a choice in the design between stability at high signal levels
and bandwidth at low signal levels.

For a good discussion of this problem consult:

   "Nonlinear Circuits Handbook", ed. Daniel Sheingold, Analog Devices, Inc.
   Norwood, MA, 1974.

This book is recommended highly for assistance in your application.  It also
discusses analog multiplication and powers and roots.  You may find exactly
what you need in this book.

The wideband logarithmic amplifier made by AD (AD9521) is likely not what you
are looking for.  This circuit should more properly be known as a wide band
log detector.  It is used in many radar applications where its DETECTED output
provides a DC current logarithmically proportional to the AC signal level at
the input.  Plessey Semiconductors make a number of circuits similar to these.
I do not know if it is still made but the Plessey SL1531  *might*  do what you
need.  This is a five stage log IF strip with a 50 db dynamic range with +/-
1 db error.  Its typical frequency range is 10 to 160 MHz, however.  Some other
Plessey devices cover the 3 to 500 MHz ranges but have much smaller dynamic
range.  You should also be able to find a number of discrete and hybrid log
amplifiers intended for use in radar applications where compression of the IF
frequency is needed.  Check any of the numerous microwave journals for ads.

BTW, Plessey makes a number of interesting IC's that I would love to play with.
Unfortunately, I have never found a source for small quantities of these.  Good
luck with the project.
                                            Barry  WA4VZQ

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|  ___  ________  |
| |  / /        | |  Dr. Barry L. Ornitz  UUCP:..rutgers!rochester!kodak!ornitz
| | / /         | |  Eastman Kodak Company
| |< < K O D A K| |  Eastman Chemicals Division Research Laboratories
| | \ \         | |  P. O. Box 1972
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