[time-nuts] how to find low noise transistors

[Richard (Rick) Karlquist]

On 7/20/2015 8:12 AM, Charles Steinmetz wrote:
> 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
>

When you raise the source impedance, you also have to reduce the
collector current.  Your analysis didn't take that into account.

Refer to page 83 of the first edition of "Low Noise Electronic Design".
Equation e. states that optimum noise figure is a function of the ratio 
between base spreading resistance and (beta)(r-sub-e).  If base 
spreading resistance is high, you make r-sub-e high by reducing 
collector current.  Equation f. states that doing that will increase
optimum source resistance.

Rick Karlquist N6RK