BASIC LOG AMP CONCEPTS AND TERMINOLOGY-analog application note
The term “Logarithmic Amplifier” (generally abbreviated to “log amp”) is something of a misnomer, and “Logarithmic Converter” would be a better description. The conversion of a signal to its equivalent logarithmic value involves a nonlinear operation, the consequences of which can be confusing if not fully understood. It is important to realize that many of the familiar concepts of linear circuits are irrelevant to log amps. For example, the incremental gain of an ideal log amp approaches infinity as the input tends to zero, and a change of offset at the output of a log amp is equivalent to a change of amplitude at its input—not a change of input offset. For the purposes of simplicity in our initial discussions, we shall assume that both the input and the output of a log amp are voltages, although there is no particular reason why logarithmic current, transimpedance, or transconductance amplifiers should not also be designed.