This is quite good..thnksHave a look at this Wikipedia entry:
http://en.wikipedia.org/wiki/Operational_amplifier_applications
Scroll down to "Logarithmic output", the 2nd to the last schematic.
The parameters Is and Vt are part of the transistor physics. Actually, they are not really constant but depend very strongly on temperature. The parameter R, which is very nearly constant with temperature changes (relatively speaking), is easily changed. So, for a given temperature (held constant), you can scale IsR as needed, and the Vt you can scale by putting an amplifier or attenuator in front of the stage.This is quite good..thnks
however what are the constants doing there ?
like if we calculate logarithm of 2 then its 0.30
but if we calulate it through the derived equation , its 0.150 (if R=100k, Is = 50nA, and 0.025)
why this difference ??? please explain
The parameters Is and Vt are part of the transistor physics. Actually, they are not really constant but depend very strongly on temperature. The parameter R, which is very nearly constant with temperature changes (relatively speaking), is easily changed. So, for a given temperature (held constant), you can scale IsR as needed, and the Vt you can scale by putting an amplifier or attenuator in front of the stage.
Basically, if you need a pure natural logarithm, you can make it act that way. Or, if you want a base 10 or base 2 logarithm, you can do that too. But often, the scaling is arbitrary, or definable as needed, and it isn't necessary to make precise mathematically perfect functions.
Note that because of the temperature dependence, that simple circuit is not practical at all. There are better circuits, but even these need to be implemented with all transistor on one integrated circuit chip so that temperature is nearly equal for all devices, hence allowing compensation techniques to be used.
Since R is multiplied by Is, then R is the way to control Is, effectively. Within limits, you can scale the value of Is to the voltage you need from RIs which scales the input voltage Vin.... what about thermal voltage and reverse current ? the reverse current is in nano meter , but how to keep such value of thermal voltage that give accurate log of input voltage ?
like , log of 2 is 0.300 but according to multiSIM ITS DIFFERENT, how to set the values of Vt and Is in multisim ?? and how to set them practically for accuracy ?
is Vt 25mV in multisim ???
Yes, you can do that, but keep in mind that you are not being mathematical if you do that. For example ln(1)=0, but ln(-1)=i*pi. Or, more generally, ln(-x)=ln(x)+i*pi. This is an esoteric point and may or may not be a problem, but you seem intent on trying to make a proper natural logarithm function, rather than just a logarithmic scaling conversion, as is more typical.2nd question,
the second question , the circuit will only work for the positive half of the cycle if i want to make it work for a complete cycle can i connet a reverse diode in parallel with the forward diode ?