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Theoretical Question about noise figure

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GSarris

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High everybody
I am studying circuit noise behavior and i am a little confused. I read on Gray, Hurst et.al that the noise factor has a meaning only when the source inpedance is ohmic. But in Guillermo Gonzalez's Microwave transistor amplifiers in an Appendix, noise figure for a non ohmic source impedance is calculated which means that the noise figure has meaning even for a complex source impedances... I bet they are both right but i cannot understand Gray's point.
Can you help me please?
Thanx in advance,
George
 

godfreyl

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Perhaps a clearer way to put it is:
The "noise figure" has a meaning only when the source impedance has a resistive component.

So it's meaningful with a complex source impedance, but not with a purely reactive one, or with a short circuit or open circuit.

The key point is that the source has to contribute some thermal noise, since the noise figure is essentially a comparison between the amplifier's noise and the source's noise.
 
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