SV: [AMPS] Linearizing Linears, feedback or feedforward

Tue, 26 Sep 2000 08:55:38 +0200

Collins has used negative rf feedback over the tube final and driver stages
for years. More of this in W6SAIs RADIO HANDBOOKs. The SB 220 har rf feed:
the grid capacitors and grid to anode capacitances are used as a rf voltage
divider. Look also into the old 1954 SINGLE SIDEBAND PRINCIPLES AND CIRCUITS
by PAPPENFUSS et al if it is still avaiable, who knows?

Prosit de Hans SM5KI
>Från: "Tom Rauch" <>
>Till: <>, "jeff millar" <>
>Ämne: Re: [AMPS] Linearizing Linears, feedback or feedforward
>Datum: tis 26 sep 2000 05.17

>> 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.
>73, Tom W8JI
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