[sdiy] Log -> process -> AntiLog

[Grant Richter]

There are specialized ships that do all this already in a single temperature
compensated package. You could build a discrete version of the chip using a
CA3046 transistor array. The datasheets cover the design theory. Check out:

http://www.national.com/pf/LH/LH0094.html

and

http://www.analog.com/

and do a search for AD538 "Analog Computation Unit"

And from "Analog Dialog" on tempcos

ABOUT LOG COMPENSATION RESISTORS
Q. Designs of logarithmic circuits*, including those using the AD538 Y[Z/X]m
unit: (for example, Figure 6 from the AD538 Multifunction Unit data sheetÝ)
call for ³ kT/q compensation resistors² . What are they and where do I get
them?

A.    The VBE difference across two opposed silicon junctions, one carrying
a current, I, and the other a current, IREF, is (kT/q) ln (I/IREF). Here,
k/q is the ratio of Boltzmann¹ s constant to the charge on an electron
(about 1/11,605 K/V), and T is the absolute temperature in kelvins.

Although employing similar junctions in isothermal pairs eliminates the
effects of temperature-sensitivity of reverse saturation current, the kT/q
term is still temperature-dependent. To eliminate this dependency in the
application, the logarithmic voltage must be used in a circuit whose gain is
inversely proportional to the absolute temperature of the junctions. Over a
reasonable range of temperatures near 20° C, this may be arranged by the use
of a gain-setting 1-kohms resistor having a positive temperature coefficient
of approximately 3,400ppm/° C- and keeping it at the same temperature as the
junctions. 

A 3,500 ppm/° C resistor is available from Tel Laboratories, 154G Harvey
Road, Londonderry, New Hampshire 03053 (603)-625-8994, Telex:
(710)-220-1844, designated Q-81, and from the Precision Resistor Co. Inc.,
10601, 75th. St., Largo, Florida 33543 [(813)-541-5771 Telex: 821788], as
the PT146. Analog Devices offices in most European countries are aware of
local suppliers of these resistors.


> From: Scott Gravenhorst <music.maker at gte.net>
> Reply-To: music.maker at gte.net
> Date: Thu, 17 Apr 2003 09:18:24 -0800
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Log -> process -> AntiLog
> 
> I want to process the logarithm of a linear CV and then
> convert the result back to a linear CV (voltage, not current).
> 
> I know that a log amp can be made with an opamp, input
> resistor and within the negative feedback loop, a diode.  Is
> there some simple way to get the anti-log voltage output
> function ?  Can I simply swap the diode and resistor so that
> the signal passes through the diode and the resistor is in
> the neg. feedback loop?  If not, then how?
> 
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