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[Amps] Plate Resistance factor for PAs TSPA

To: amps@contesting.com
Subject: [Amps] Plate Resistance factor for PAs TSPA
From: "John T. M. Lyles" <jtml@lanl.gov>
Date: Tue, 11 Apr 2006 10:59:18 -0600
List-post: <mailto:amps@contesting.com>
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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