Once I've seen a grid driven amplifier in which a ferrite broadband
transformer 50/200 ohm was used, probably to develop a bigger voltage across
grid without increasing the driving power. The 200 ohm port, that one going
to the grid, was also terminated in a 200 ohm non inductive resistor but its
size wasn't sure good to dissipate large power and was probably there to
establish a load impedance, and nothing else.
Personally I find quite useless to develop grid driven PAs if then a good
linearity is only achieved lowering the gain to values typical of other, but
much sympler, configurations (when tubes allows).
Also dissipating the unnecessary driving power in a load before amplifying
makes me thinking if the shortest line between two points is still a stright
one.
73,
Mauri I4JMY
-----Messaggio Originale-----
Da: George T. Daughters <gdaught6@leland.Stanford.EDU>
A: <amps@contesting.com>; <johnf@futurenet.co.za>
Data invio: martedì 4 gennaio 2000 10.11
Oggetto: Re: SV: SV: [AMPS] Final..
> My understanding is that one DOES NOT deliver power to the grid of an
> AB1 amplifier. The driving voltage is always such that the grid
> is never driven into the region where grid current flows. All one
> does is charge and discharge the "input capacitance" of the tube.
> If one refers to one of the ARRL handbooks, in the tube
> charactersitics section, one finds "driving power = 0".
>
> So the grid of a tube biased for AB1 is NOT resistive. A good way to
> make the input of such an amplifier appear as a constant resistive
> load to the exciter (a good thing!) is to put a fixed non-inductive
> resistor from the grid to (RF) ground. The resistor dissipates
> (wastes!) the drive power. Another benefit of such a resistor is
> that it enhances the stability of the amplifier.
>
> 73,
>
> George T. Daughters, K6GT
>
>
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