The minimum noise figure of an amplifier is found (working with the input
match and not the with the output), seaching for the source resistance and
varying the bias of the amplifying device until the best NF comes out.
Actually, I never found the best NF coming out tuning for the best return
loss.
Under my experience, 11.76 dB of gain is not a sparkling gain. A typical
well designed GG amplifier, including PS in the good design, and using a
decent tube (or tubes) to conservatively deliver such power level without
beeing (much) over the compression point, is typically around 13 dB of gain
below 30 MHz, more in monoband decks.
Unless the amplifier is quite "peculiar" (read as accepting large
compromises or very economical in some aspects) a good 1500W GG amplifier
shouldn't need much more than 75 W of drive to reach 1500 W out. If this
doesn't occur, the PS is not undersized and the tube is not compressing, the
reason is in the losses and/or mismatches of the input or the output
networks that are wasting a part of the available energy.
Matching losses and any other type of losses can stay hidden and often
neglected at amateur powers and duty cycle. Increasing the driving power and
accepting unnecessary heating and higher consumtpion, the QSO can be done
in spite of everything.
73,
Mauri I4JMY
-----Messaggio Originale-----
Da: Jon Ogden <jono@enteract.com>
A: Maurizio Panicara <i4jmy@iol.it>; Roger D. Johnson <n1rj@pivot.net>
Cc: Amps Reflector <amps@contesting.com>
Data invio: venerdì 1 ottobre 1999 16.40
Oggetto: Re: R: [AMPS] Conjugate matching and amplifiers
>
> Maurizio Panicara wrote:
>
> >Although selective mismatching in RF amplifiers is not an heresy, but a
> >practical, logical and controlled way of decreasing (fixing) stage gain,
> >efficiency is something unavoidable at high power level in order not to
> >waste energy and not to stress components without any reason.
>
> Good points, Maurizio!
>
> Another case in a low signal level environment where you want to mismatch
> and amplifier is in a low noise amplifier configuration. Minimum noise
> figure and a conjugate match don't go together. Minimum noise figure is
> generally achieved at some mis-matched impedance. However, in a receive
> path, we are concerned with noise performance and NOT with power transfer.
>
> The problem is that most hams (US at least) have the idea in their mind
> that the amplifier must produce 1500 Watts output with 100 Watts of
> drive. So they design around that. This is a shortsighted and silly
> idea. It certainly doesn't result in optimal efficiency and optimal
> performance.
>
> 73,
>
> Jon
> KE9NA
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