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[AMPS] Linearizing Linears, feedback or feedforward

To: <amps@contesting.com>
Subject: [AMPS] Linearizing Linears, feedback or feedforward
From: 2@vc.net (measures)
Date: Mon, 25 Sep 2000 23:51:10 -0700
>
>> We know that negative feedback in audio amplifiers improves linearity and
>> reduces distortion.  What would it take to improve RF linears with
>> feedback? In thinking about it for a bit (not enough time, no doubt).  It
>> seems that RF feedback will introduce lots of problems with stability
>> because the feedback will vary in  phase over frequency...producing an
>> oscillator. Linearizing the envelop might improve things, but it doesn't
>> do anything about the phase component...might even worsen it.
>
>Grounded grid amplifiers already have a very high level of almost 
>perfect negative feedback as long as the grid is properly grounded.
>
>Some tubes have long grid leads that add some amount of  
>uncontrolled feedback, and some amplifiers add some uncontrolled 
>feedback through intentional "floating" of the control grid through 
>low value capacitors.
>
>> In the commercial world, broadband, low IMD amplifiers use feed forward
>> techniques...precompensate the input to produce a clean output.  The
>> principle seems clear but how it applies to a bandswitched amp and how a
>> ham with just a grid dipper can set it up seems murky.
>
>You can add negative feedback to a grid driven tetrode with the 
>addition of an unbypassed resistor in the cathode.
>
True enough.  However, with a grid-driven tetrode, cathode R subtracts 
from the cathode/screen potential on voice peaks, so R must be kept 
fairly low.  
Eimac recommends 11-ohms of Rc for the 4CX600JA.  A cathode feedback R is 
ideal for g-g config.  
>
cheers
end

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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