Plate resistance is, as someone already stated, the delta Ep/delta
Ip, around an operating point. It is described well in the Radiotron
books, and in Terman's Radio Engineering textbooks. As of yesterday I
bought another copy of Terman (for my home library), 3rd edition. I
now have 1st, 3rd and 4th (when it changed name to Electronic and
Radio Engineering Handbook. I believe that plate resistance is
somewhat related to what audio amplifiers call damping factor in the
output specs for a power amplifier. Damping is affected by the output
transformer performance as well as any feedback in the amplifier, of
course.
In my work, we use plate resistance when we are trying to get an
amplifier that has a low source impedance, to a cavity. The cavity
will be high |z| at resonance. Direct-coupled amplifiers are
sometimes used to drive this cavity (instead of using a 50 ohm
matched input and long cables from a remote PA). If we are trying to
prevent the amplifier from getting perturbed by induced voltage in
the cavity, when ion beams pass through at high velocity, we want it
to be "stiff", having lower output Z. Cathode follower is the
ultimate configuration for this, having only a few dozen ohms up into
HF range. I am running a cathode follower with 450 kW triodes right
now, for a proton storage ring application.
In grounded cathode configuration, large tetrodes such as the
CPI/Eimac 4CM500,000G have very flat constant plate current curves.
Triodes do not, as such. If you calculate Rp from these at the
operating point, you can see that its as low as 500 - 1000 ohms. This
is especially helpful for these applications. Active feedback also
helps to reduce the output Z of an amplifier, as it does with op
amps. High level feedback is often applied around the PA to
accomplish this, but there needs to be enough gain inside the
amplifier to start with. Getting this to work over a bandwidth
without oscillation is the trick.
73
John
K5PRO
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