The simplest logarithmic amplifier is one with a diode connected between output (anode) and inverting input(cathode) and the input voltage is fed through a resistor to (-) input. It takes advantage of the expotential dependence of diode current on voltage to take the logarithm of its input signal.
But it is not perfect and very temperature dependant.
Much better circuit with its full description you can find here:
**broken link removed**
Regards,
IanP