Gerald,
Feedback does change the input impedance, both in terms of real and imaginary
components. The equations are certainly in earlier Editions of Terman's 'Radio
Engineering', if not the later editions. Obviously, the swamping effect of
passive grid helps reduce the variations, but as the magnitude of the variation
depends on the gain, then any non-linearity will affect (and probably degrade
by Murphy's Law!) the IMD.
73
Peter G3RZP
========================================
Message Received: Nov 18 2013, 01:42 PM
From: TexasRF@aol.com
To: g8on@fsmail.net, wlfuqu00@uky.edu, km1h@jeremy.mv.com,
donroden@hiwaay.net, amps@contesting.com
Cc:
Subject: Re: [Amps] Passive grid stability calculation for a tetrode?
Hi All, a late input here: I have been wondering what the feedback
power/voltage does to the input vswr. Seems there would be two standing waves
present: one from the input circuit mismatch (if any) and the feedback voltage
of hopefully a smaller magnitude.
The resulting combined vswr would depend on the relative phase of the two
standing waves, right?
It seems that there would be some good benefits to adding a neutralizing
circuit even for a passive grid design.
All of my tetrode experience has been with cathode driven designs. I have a
TH347 PA on 1296 MHz that has a fully adjustable input matching circuit.
No amount of tweaking will make the input vswr much better than about 2:1. I
always assumed it was because of the unneutralized feed back from anode to
cathode. By tuning the driver and input match for maximum PA power output,
it all seems to work well so it has been left as is.
73,
Gerald K5GW
In a message dated 11/15/2013 12:17:40 P.M. Central Standard Time,
g8on@fsmail.net writes:
Actually Bill, the formula was derived using tuned circuits, where Rp =
QXLp and Rg = QXLg. As you move off resonance, the impedance and gain
naturally drops. However, it is worth keeping the value of H around 1 to 1.25
maximum.
The input impedance should be figured as the swamp resistor in shunt with
121pF, although as that is a lower impedance, it will actually increase
stability. Whether or not the driver can stand the SWR (and be stable!) is
another matter.....
73
Peter G3RZP
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