I came across an interesting concept yesterday that I thought
worthy of further consideration for an LF active antenna. The
so called inverted vacuum tube amplifier.
Consider a triode, with a positive grid and a negative anode,
with the input signal going to the anode, and the output taken
from the grid. This has a high input impeadance, very low input
capacitance, and low output impeadance. It also is more linear
than a conventional valve amplifier. These are exactly the
characteristics that are needed in an active antenna.
The downside is that although it has a power gain due to the
impeadance transformation, it has a massive voltage loss, because
in this mode the valve mu becomes 1/mu.
At low frequencies the issue with voltage probe active antennas
is not so much the voltage at the probe, but the very high
capacitive potential divider that results from the very low
capacitance of the probe, and the input capacitance of the active
device and its infrastructure. The key test therefore is whether
the voltage attenuation of the value in this mode is less than
the voltage "gain" that results from the lower input capacitance
term in the input potential divider.
Because of it's improvements in linearity, this mode is of interest
to the audio groups, particularly those interested in transformerless
valve amplifiers.
There is a writeup of the technique at
http://members.aol.com/sbench102/inverted.html
The inventor of the technique was none other than Fred Terman, who
wrote it up for proc IRE
F Terman, The inverted vacuum tube, a voltage reducing power amplifier.
Proc. Inst. Rad. Eng. 16:447-61. (1928)
In case anyone thinks that this is a misplaced April Fools Joke, there
ia also a reference to the paper in his book Radio Enginneering, although
neither of the editions that I have give any useful theory on the operation.
If anyone has access to a copy of that paper, I would appreciate a
copy, otherwise I will try to get one the next time I am in the
IEE library in London.
Stewart G3YSX
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