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Logarithmic amplifier is realized with op-amp is given in figure bellow.

Simulate logarithmic amplifier.


Logarithmic amplifier provides a logarithmic output for a linear voltage input. In the given schematic, output voltage can be calculated via equaling currents. Input current is


Current IIN flows through diode D. I-V characteristic of the diode is


In here, IS is the saturation current, VT is the thermal voltage and VD is diode voltage.
It means that


Input voltage is positive, and output voltage is negative natural logarithm scaled.
Go to logarithmic amplifier simulation.

External links:

Op-amp Logarithmic amplifier on CircuitsToday

Op-amp Logarithmic amplifier on Wikipedia

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