SV: [AMPS] Linearizing Linears, feedback or feedforward

[measures]

>
>Tom says;
>
>	>It adds uncontrolled RF feedback with undesired phase shift to the 
>	>PA. Feedback level varies with drive power, and from band to band.
>
>
>Capacitive feedback in grounded cathode circuits can work well - BUT:-
>
>In the 'standard' Bruene neutralising circuit, where a grid tuned circuit has
>one end to the grid and the cold end via a capacitor to ground, with the
>neutralising cap from plate to the 'hot' side of this capacitor, NFB can be
>achieved by suitable choice of capacitor values.

>
>If the phasing effects Tom mentions are not to screw things up, a deal of 
>care
>is needed in choosing the capacitor to ground - it MUST have low 
>inductance. 

yea, verily

>The
>neutralising gets screwed up if it doesn't too. Both sides of the tuning cap
>MUST be floating too.
>
What tuning cap?  The Bruene circuit uses no tuning capacitor.  The 
tuning is done with a roller coil to cancel grid reactance.  Add a tune 
cap. to the grid and you get a master oscillator.  In order for the 
Bruene circuit to work, the ratio of grid-C to feedback C must equal the 
ratio of the capacitors used in the bridge.  Typically, this c. 130 to 1. 
 

end




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


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