🚀 go-pugleaf

From article <45753@eerie.acsu.Buffalo.EDU>,
	ferrick@acsu.buffalo.edu (patrick k ferrick):
> OK, all you EE types out there!  Maybe this is kind of a dumb question, 
> but is there a simple device out there that, if given a certain voltage will 
> produce as output a signal proportional to the log of the input?
> 
> In other words,
> 
>    input             output
> (1-1000 VAC) -----> (1-3 VAC)
> 
>
	Well here is a log function for DC that should get you started.
This is from memory so the details are pretty sketchy. Get a basic college
electronics textbook to get the details (and calculations) right. Note that
This only does DC but maybe you can live with that.
	
	This is a classic transform discussed in Circuits I class concerning 
OpAmps. The key element is that a Diode has a logarithmic transfer curve between
non-conduction and conduction (0.5 - 0.65V). thus you have to bias a diode
pretty close to that point and use it as a feedback element in a simple
amplifier. 

	Diode transfer curve:

	i (current)
	|
	|
	|         |
	|         |
	|         |
	|         |
	|        |
	|       /
	|------'
	+----------------------------------- V
	       ^ - 0.6V


	Op amp with logarithmic feedback.
	(the actual values are left as an excercise to the student :-)


                          |\
                          | \
       Linear In -/\/\/\--|+ \ U1
                    R1    |   \
                          |    \____________ Log out
                          |    /       |
                          |   /        |
                    +-----|- /         |
                    |     | /          |
                    |     |/           |
                    |                  |
                    |_____|/|__________|
                    |     |\|
                    /      D1
                    \
                    /
                    \ R2
                    /
                    \
                    |
                   ---
                    -
                    .





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Christopher J. Pikus,               Megatek Corp.
INTERNET:     cjp@megatek.uucp      San Diego, CA
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