Re: Noise figures/transistors

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From: Ray Anderson (Raymond.Anderson@EBay.Sun.COM)
Date: Fri Mar 25 1994 - 11:19:51 EST


Daniel (ASIRENE@v9001.ntu.ac.sg) writes:

>Hi,

> 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?

> Tks.

>Daniel

        You didn't say what frequency band the receiver in question was designed
for, but seeing as it presently uses a 2N3904 device in the front end I'll
assume it is an HF low-band receiver and not for VHF use.

        The performance of most receivers used in HF service is limited by
atmospheric noise and not usually by the receiver noise figure. If the present
circuit gives a noise figure of say 10dB then reducing it to say 3dB probably
wouldn't produce any noticable difference. On 10 meters this argument might
be debatable as the atmospheric contribution is becoming less and the receiver
performance is becoming more important. At VHF frequencies a good receiver noise
figure is of paramount importance to realize good weak signal reception.

        The other thing that must be considered is that even if improving the
receiver noise figure would improve actual performance, you usually can't just
change transistors to one with a lower rated noise figure. This is because the
noise performance of a transistor is very much dependent on the impedances it
see's on its input and output (primarily the input). To make things even more
interesting, the correct impedance to produce maximum gain is almost certain
not to be the impedance that is required for best noise performance. The bottom
line is that when a major change in transistor type is made, the matching
networks will probably need to be redesigned also in order to realize all the
performance the device is capable of.

        So I guess the short answer to your question is: unless you have an
extraordinarily bad receiver, you probably won't notice much if any improvement
in performance on the HF bands by lowering the noise figure. And if you were to
change transistors on the front end amplifier, there are other changes which
need to be made in order to get the best performance from the device.

        Good Luck es 73's de WB6TPU
                                Ray


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