对数放大器满足射频功率测量，信号缩放需求

　　对数放大器满足射频功率测量，信号缩放需求

　　对数放大器（通常称为对数放大器，有时称为对数检波器）是用于RF电路和电光接口的模拟元件。它的传递函数在概念上是简单的：输出电压或电流正比于输入电压或电流的对数（图1）。它实现了这一输入/输出关系的输入范围为60至80分贝（通常），但它可以是大或宽为120分贝，在某些情况下，一些日志放大器甚至达到160分贝的动态范围。虽然它被称为“放大器”，它不是一个习惯意义上的字，它实际上是一个线性对数转换器。

　　Figure 1： The logarithmic amplifier， or converter， produces an output current， or voltage signal （Y axis）， which is proportional to the logarithm of the input current， or voltage signal （X axis）; note that it is undefined for inputs equal to or below zero， although log amps have ways around this limit.

　　It may seem counterproductive to deliberately place a nonlinear component in a system， given all the emphasis on component and circuit linearity and resultant low distortion over a wide range. The log amp is part of the design， but it is usually not directly in the signal chain. In RF circuits， it is often part of the closed-loop control which adjusts the receive-channel gain via automatic gain control （AGC） to maintain a near-constant signal level in the channel by measuring input signal strength over a wide range （RSSI—received signal strength indicator）， Figure 2， as well as to control transmitted power. In optical circuits， it is used to monitor the current to the laser diode and adjust for changes due to temperature and other operational factors.