An interesting approach

Andre Kesteloot andre.kesteloot at
Fri Sep 26 12:00:50 CDT 2003

To All from PA0SE

Stewart,  G3YSX wrote:

> 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
> 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
> If my memory serves me right the principle of the inverted valve has 
> been used in the past in a DC valve voltmeter for relatively high 
> voltages causing negligible loading of the voltage being measured.

73, Dick, PA0SE


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