[sdiy] RE: cem3396 and Log->lin convertor

[ASSI]

On Sunday 18 April 2004 16:43, Scott Gravenhorst wrote:
> Usually, the expo converter does this,
> but it converts a voltage to a current.  Is this as easy as pushing
> the current through a fixed resistor and taking the voltage from the
> two ends of the resistor?

This would of course work, but might have problems with the common mode 
range of the difference amplifier. For an npn expo it is easier to have 
it suck the current from the virtual ground of an op amp (negative 
input) and have the op amp produce the output voltage on the feedback 
resistor. Indeed this is the standard schematic when you look up an 
exponential amplifier in any books talking about op amp nonlinear 
circuits, which generally are voltage in, voltage out. This produces 
only a positive CV, of course. With another resistor to the virtual 
ground like for an inverting summing amplifier you can level shift the 
ouput, you may want to use another inverting amp to have the zero level 
follow the CV.


Achim.
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