[time-nuts] how to find low noise transistors

[Charles Steinmetz]

Rick wrote:

>Base spreading resistance can be overcome
>by using a sufficiently high source impedance

This sounds like the all-too-common noise figure fallacy (increasing 
input impedance to get a lower NF).  All this does is raise the 
source impedance's contribution to the total noise -- it doesn't 
reduce the amplifier input noise voltage, and it increases the 
amplifier noise due to input noise current.  The result is more 
output noise, not less, and a reduced signal to noise ratio, despite 
the improved NF.

Even if you use a transformer to raise the source impedance, the 
signal voltage increases by the turns ratio while the impedance 
increases by the square of the turns ratio (thus, the output noise 
due to the amplifier input noise current flowing through the source 
impedance also increases as the square of the turns ratio).

Best regards,

Charles