On Wed, 24 Sep 1997 11:15:02 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> From: sears@rell.com
To: <topband@contesting.com>
>> Date: Tue, 23 Sep 97 14:59:47 +0000
>
>> Tom, you wrote:
>
SNIP..
>I need "speaking clearly lessons".
>
>What I was referring to was this. If a FIXED amplifier gain block
>like an MMIC is used, the traditional way to reduce gain is
>to add an attenuator pad. There is no access to the emitter of
>the internal transistor.
>
>With a discrete amplifying system, it is possible (through use of
>lossless feedback) to move the gain and impedances all around while
>optimizing input impedance for best noise figure... all without
>noticably decreasing any important performance parameter.
Tom, I suspect that the lossless feedback circuit is unknown in most ham
circles. It was patented by Anzac (dont know if it is now public) and
only one very small paragraph showed in the ham magizines that I am aware
of. It also was in the defunct RF Design magazine and I believe a UK
publication also.
You are absolutely correct, the feedback does not degrade the NF to any
meaningful degree.... hence the "lossless" designation.
With a 2N5109 the IP3 point can be in the +20dB region if you run enough
current and a heat sink.
I had several of mine measured (28 thru 222MHz) at a VHF conference and
the NF were in the 1.5 dB region up thru 222MHz. I use several as a
general purpose amp with gains in the 8 to 10dB region all around the
shop and the shack.
With a NEC NE41632B the NF was even better.
The 2N5109 will run 1.5W output as a QRP CW amp so it is pretty robust as
a Beverage preamp.
How about some quality ideas on front end protection....no pilot lamps
please! I have often thought about using a biased PIN...comments???
73 Carl KM1H
>
>Speaking in this context, proper use of transformer coupled feedback
>(or even conventional resistor feedback) can improve noise figure and
>dynamic range over the methods of reducing gain (i.e. attenuators or
>reduced supply voltage) used with MMIC's
>
>With small loop antenna's or other antennas with **very** low
>efficiency in quiet environments it might be necessary to use
>noiseless feedback and an antenna mounted amplifier, but in most
>cases on 160 it is sufficient to use conventional emitter
>degeneration and voltage feedback with resistors (that do slightly
>decrease NF).
>
>I'd never use an MMIC in this application. It's a poor choce for
>far too many reasons.
>
>73, Tom W8JI
> -
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>
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