RE: Noise figures/transistors


Date: Fri Mar 25 1994 - 12:14:30 EST

Daniel from Singapore asks:

> Here's a design question. I am looking at the front-end RF amp
> which uses a 2N3904 and asking myself if I were to replace this with a
> transistor with a lower noise figure, say a 2N5179 or BF689, will I see
> any improvement? Is this mod worth pursuing? How much improvement can I
> expect to see here?

Noise figure is not generally an issue at HF. At frequencies below 50 MHz or
so atmospheric and man-made noise is far louder than any noise generated in
a radio's front end.

Some HF radios can benefit from a low-noise preamp on 10m for demanding
operation (e.g. Mode A), but this is more because their sensitivity falls
off on 10m than because they have noisy front ends.

There is a good discussion of this in (where else? :-) _Solid State Design
for the Radio Amateur_. Noise figures are related to dynamic range and the
minimum discernable signal (MDS), even on HF, but a 2N3904's noise figure is
ample for all but the most demanding purposes.

73 from Burnaby,
laura VE7LDH

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