A single op-amp antilog amp?

Paul Perry pfperry at melbpc.org.au
Thu Sep 24 12:03:47 CEST 1998


At 10:48 PM 23/09/98 -0700, Jim J at technotoys wrote:

>I have seen the basic topology for an antilog amp - diode in place of the
>input resistor in an otherwise standard op-amp amplifier - but I don't
>remember how to do the math to calculate the resistor values. Obviously the
>input voltage to the antilog amp must be limited to a range of 0 to about
>0.7 volts - but how do I calculate the current through the diode (and hence
>the feedback resistor)?

how indeed....
you canput a pot in series with the diode & connect across a battery
(perhaps a fixed resistor as well to protect the diode0 and then measure 
I and V across the diode as you move the pot settings... then plot it all
and you will be the wiser.... this device will be very temperature 
sensitive, BTW.

Would like to hear people's opinions on this. There is  table in "The Art of
Electronics" for zillions of different diodes, if you are looking for a high 
impedance (likely) try a diode made by connecting the base to the collector
of a 
transistor, possibly.

paul perry melbourne australia

incidentally, you will notice that the diode resistance changes as you hold it! 
just from your fingers warming it up.. remember thids if you are ever matching 
transistors/diodes.




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