OTOH, the other cure for high base spreading resistance is
to simply parallel multiple devices. This avoids the bad
side effects you mention. The other key noise parameter
in a BJT is RF current gain, and this cannot be "cured"
by any circuit design tricks.
Rick Karlquist N6RK
On 7/23/2015 8:29 PM, Charles Steinmetz wrote:
Rick wrote:
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.
I replied:
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).
I should also have mentioned:
Reducing transistor current also frequently reduces beta (sometimes by a
large factor, depending on the transistor's beta vs. current curve and
where you are on it). This directly affects (beta)(r-sub-e) and,
therefore, directly reduces the noise figure. I've pasted in the beta
vs. collector current graphs for the ubiquitous 2N3904 and 2N4401 to
illustrate this. Some transistors are better than these over a useful
range of collector currents, others are much worse. The beta of PNPs,
which are generally quieter than NPNs, also generally falls off faster
with reduced collector current.
Note that these are static (DC) curves, which are good approximations
for the 1/f region. The curves in the white noise region, even at
relatively low frequencies like 1 kHz, generally fall off faster than
this as current is reduced, so the effect of reduced beta on in-band
noise figure is greater.
Best regards,
Charles
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