>
>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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