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
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