Re[2]: [AMPS] Re: TopBand: Followup Beverage Box

Wed, 24 Sep 1997 11:15:02 +0000

> From:          sears@rell.com
> Date:          Tue, 23 Sep 97 14:59:47 +0000

>      Tom, you wrote:

>      "Rolling a pre-amp out of discrete components allows the designer to 
>      add negative feedback to reduce gain, and that 
>      improves dynamic range and noise figure! Solid State Design for the 
>      Radio Amateur covers this, as did Ulich Rhode in some of his RF Design 
>      articles in the late 70's and early 80's."
>      
>      One mistake, adding negative feedback (either shunt or series) will 
>      never improve noise figure. The addition of series feedback will give 
>      you low frequency stabilization, raise noise figure, and the ability 
>      to move S11 closer to the optimum Noise Figure match, which will 
>      improve your input match while trying to source match for Noise 
>      Figure. Shunt feedback will again improve stability, reduce gain 
>      depending on the amount of feedback, and raise noise figure depending 
>      on the amount of feedback.

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.

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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