光纤通信接收机中MOSFET前置放大器的热噪声

THERMAL NOISE OF MOSFETS IN FIBER OPTICAL RECEIVER AMPLIFIER

用一种简单明了的新方法推导了ＭＯＳＦＥＴ热噪声电流功率谱密度的通用计算公式，据此计算得到了光纤通信接收机中ＭＯＳＦＥＴ热噪声电流功率谱密度为ＳＩＤ＝４ＫＴａｇｍｓａｔ或４ＫＴｒｇｄｏｈｍ，其中ａ和ｒ是由器件衬底掺杂浓度、栅下氧化层电容大小、漏极电压等参数决定的因子．本文证明了对于光接收机中工作在放大状态的ＭＯＳＦＥＴ，取ａ＝２和ｒ＝２／３能较好地符合实际情况，而现有一些光纤通信文献中取ａ＝２／３则是不符合实际情况的．

MOSFETs are usually used in the preamplifier of fiber optical receiver for the low and moderate bite communicaltion systems beccause of their higher input impedance and negligible gate leakage current. The preamplifier noise is a critical factor in determing the receiver sensitivity,which is the main figure of merit for a fiber optical receiver. In this papar,the universal expression of the thermal noise spectral density of MOSFETs is derived by using a new method. From this expression,the thermal noise spectral density of MOSFETs operating in strong inversion is given by SID= 4 kTagmsat or 4 KTrgdohm,where both a and rdepend on the acceptor impurity concentration in the substrate, the oxide thickness, and the drain voltage etc. For commercial MOSFETs,the acceptor impurity concentration can be from 1015cm -3 to 1017cm-3, therefore a is about three times greater than 2/3 given by the many literatures concerned with fiber optical receiver design,and r is about equal to 2/3. The theory is in agreement with experimental results.