|[AMRadio] 4-1000A Grounded Grid Class-C|
k4kyv at charter.net
Thu Oct 3 14:21:46 EDT 2013
> From: Rob Atkinson <ranchorobbo at gmail.com>
> So is a grid driven class C final different from the cathode driven
> example (as pertains to feed through issues)
> because grid driven takes less drive power for a given output? I mean
> do you escape the feed through power problems with grid driven because
> the drive power is a lot less?
No, the amount of driving power required has nothing to do with it. The
feed-through is inherent to the g-g circuit. With grounded grid, the
output appears from the plate of the tube to ground at the plate tuning
network just as with a grid driven amplifier, but the grid drive is
inserted between the cathode and ground. The actual output delivered by
the tube still runs from plate to cathode, effectively placing the input
(rf drive) in series with the output load as far as the tube is
concerned, "ground" merely being a reference point in the series circuit
between cathode and plate. Thus with a steady carrier driving the
cathode with respect to ground, when the actual plate output from the
tube reaches zero at 100% negative modulation peaks, the steady carrier
feeding the input remains in series with the load, allowing most of that
driving power to appear at the plate tuning network (total driving power
minus the power consumed in the grid circuit).
Driving the negative modulation peaks past cut-off during overmodulation
still results in splatter and distortion, whether the rf that appears in
the transmitter output at cut-off is zero at baseline, or at some fixed
minimum determined by the amount of feed-through. In the latter case,
the scope may not show modulation in excess of 100%, but you are still
attempting to exceed the modulation capability of the transmitter.
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