On 6/27/2015 4:07 AM, Jeff wrote:
On 26/06/2015 13:24, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Both definitions are correct and mean the same thing; a negative NF,
when expressed in dB, would be when the SNRout is less than the SNRin.
However, the big but is that an negative NF is not possible.

I don't think both definitions mean the same thing. If the amplifier
adds *any* noise it increases the NF above zero by the conventional
definition. The only way the NF can be negative is if the amplifier
removes noise from the input, or in other words, increases the SNR.

What he seems to be suggesting is that NF is the ratio of the signal
noise to the amplifier noise.


It only appears to be the case due to the fact that the OP is not
comparing like with like, the test method used is only valid if the
system impedance remains the same. You cannot compare oranges with lemons.



--

Rick