Rick wrote:
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.
Agreed, you can adjust the "noise resistance" of a BJT (the ratio of
its voltage noise to its current noise).
However, minimum noise figure is frequently not the way to obtain the
least added noise voltage from an amplifier. That is the fallacy I
mentioned -- the mistaken notion that increasing the resistance of
the source to achieve a better NF will improve the S/N ratio of the
amplifier output. That will be true only if (i) one can arbitrarily
vary the intrinsic resistance of the source without changing the
source's intrinsic S/N ratio, and (ii) one is stuck with a certain
BJT input device. Neither is almost ever the case in real life
(aside from increasing the source impedance with a transformer).
Furthermore, reducing transistor current to raise the noise
resistance causes undesirable collateral effects (including reduced
bandwidth, which increases phase noise due to baseband noise
modulation of transistor capacitances and generally increases nonlinearity).
BJTs are readily available with noise resistances of less than 50
ohms (see The Art of Electronics, 3rd edition, Chapter 8). The only
reason to increase the source impedance with a transformer is if the
intrinsic source impedance is lower than the lowest available BJT
noise resistance -- for example, in the case of microwave transistors
(remember, this thread started in reference to LN preamps for 100kHz
LORAN antennas).
Best regards,
Charles
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