對數放大器滿足射頻功率測量，信號縮放需求

　　對數放大器滿足射頻功率測量，信號縮放需求

　　對數放大器（通常稱為對數放大器，有時稱為對數檢波器）是用于RF電路和電光接口的模擬元件。它的傳遞函數在概念上是簡單的：輸出電壓或電流正比于輸入電壓或電流的對數（圖1）。它實現了這一輸入/輸出關系的輸入范圍為60至80分貝（通常），但它可以是大或寬為120分貝，在某些情況下，一些日志放大器甚至達到160分貝的動態范圍。雖然它被稱為“放大器”，它不是一個習慣意義上的字，它實際上是一個線性對數轉換器。

　　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.