[sdiy] Log amplifier circuits

[Harry Bissell]

Thanks to all who replied and sent links or attachments. I found several circuits that do exactly
what I expected.  The overall results are virtually identical (I expect that with some more value tweaks, they would
be identical.

I also think I remember a circuit using the LM13700 that was supposed to be a log amp (but is really exponential). I'll
have to mine the data sheet and see if I might try something with that (it could be temperature compensated, at least good
enough for me... and I'm using other LM13700 packages

H^) harry


----- Original Message -----
From: Harry Bissell <harrybissell at wowway.com>
To: synth diy <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 16 Jan 2012 11:37:19 -0500 (EST)
Subject: [sdiy] Log amplifier circuits

Hello all. Playing with LTspice I used a ln (natural logarithm) math function and it worked
well for me. I tried to make a log amp using a diode or transdiode configuration, but the dynamic
range is much more limited than the math function was.

I tried to play around with the input resistor value (more of less current into the log amp) but that
didn't have much effect.  It seems like the values are limited to voltage ranges right around the
forward drop of the diode(s) or transistors.  

anyone have any ideas or circuit references for me to look at ?

tia

H^) harry
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-- 
Harry Bissell & Nora Abdullah 4eva