Noise Contribution Of Emitter Resistors

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Samuel Groner

Well-known member
Joined
Aug 19, 2004
Messages
2,940
Location
Zürich, Switzerland
Hi

I recently read an article by Baxandall (published in Gayfords "Microphone Engineering Handbook") describing theoretical prediction of transistor noise. He concludes with the following equations for the equivalent noise resistances:

RNv = rbb + 1/(2*gm)
RNi = (2*hfe)/gm

where

gm = (q*Ic)/(k*T)
q = 1.6xe-19 C
k = Boltzmann

How do I include emitter resistors in this equations? Do they show up as series resistors of the base source impedance? This would mean I just add them to the equivalent noise resistances, right?

Thanks for your answers!

Samuel
 
For RNv it's a resistor added to the other two terms.

For RNi---I usually just model the input noise as a current generator at the base, using the shot noise formula for i sub n squared of 2*q*Ib, rather than supposing that is is a resistor. And then also usually apply a fudge factor upwards a bit---bipolars tend to show their excess/low freq current noise earlier than for voltage noise as you go down in frequency.

I'm too lazy to reconcile Peter B.'s expression right now but I'll bet they agree ;-)
 
Where's my post??

I replied to this a couple of hours ago! I guess my post got lost in digi-hell. Anyway, hope I can help you some other time.

/stu
 
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