Q.What is a logarithmic amplifier?

A logarithmic amplifier produces an output voltage proportional to the logarithm of its input. It exploits the exponential relationship between the current through a semiconductor diode (or transistor) and the voltage across it. In the circuit the input $V_i$ reaches the inverting $(-)$ node through an input resistor $R_i$, and the feedback element is a diode. Because the diode's forward voltage is proportional to the logarithm of the current through it, the output takes the form

An anti-logarithmic (exponential) amplifier performs the reverse operation. Here the positions are swapped — a diode sits at the input and a resistor provides feedback — so the output is proportional to the anti-log (exponential) of the input voltage. …