Bipolar transistor noise: The equivalent noise resistance

A large number of very detailed noise analyses are available for bipolar noise and some of them are very detailed indeed. However, for a quick design assessment of noise it is more useful to use the bipolar equivalent noise resistance. This is given by : RN = rb + 1/2gm. Here rb is the base series resistance and gm is the transconductance. What this expression is telling us is that to minimize noise use a stripe geometry for the bipolar to reduce rb and increase the current to increase gm. However, in the case of variable gain amplifiers this is a challenge to say the least. Most variable gain bipolar amplifiers use a varying gm. So as the current changes in the device the nose resistance also changes. A compromise has to be made.


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