Inverted-Triode Circuit for measuring of small Currents

The inverted-triode operation for measuring extremely small currents has been published in 1944.

Inverted means: the grid is used as anode, and the anode is used as control element.

Reprint in: HANDBOOK OF INDUSTRIAL ELECTRONIC CIRCUITS, McGraw-Hill, 1948, p. 102

Inverted-triode Circuit for Measurement of Extremely Small Currents

A NEW electronic tube can measure hydrogen-ion contents of chemicals (pH), minute currents produced by phototubes, ion current in mass spectrometers, alloying constituents of steel, and minute quantities that previously required an electrometer or its equivalent.

In this tube the outer electrode, which is normally the plate in an ordinary vacuum tube, is used as the control grid. This inversion minimizes the space-charge effect, thereby making it possible to select a value of grid bias that will result in zero grid current.

Currents as low as 10^-14 ampere can be measured and as low as 10^-16 ampere can be indicated. Direct potentials can be measured to a sensitivity of 10^-4 Volt in circuits having up to 10¹² ohms resistance.

The tube is termed an inverted triode because the outer electrode, which is normally the plate in an ordinary vacuum tube, is used as the control electrode or grid in this tube. This places the control electrode at a maximum distance from the space-charge region surrounding the filament, thus minimizing the amount of electrons collected by the control electrode. In this manner, the current to the control electrode is held at a minimum.

The mesh mounted between the filament and the control electrode is used as the anode. This construction provides more radiating surface to the grid, decreasing its temperature and possible thermionic emission. The control element, or grid, being farthest from the filament, receives less heat and light from the filament, thus decreasing emission from the grid.

The filament is operated at a low temperature to minimize the emission of photoelectrons and of primary electrons from the grid. All the electrodes are operated at rather low voltages to reduce the possibility of ionizing residual gas in the tube, which would cause positive ion current in the grid circuit.

In taking measurements of extremely minute currents, the electrostatic charges that build up on the inside surface of the glass bulb produce a sufficiently high electric field to affect the over-all sensitivity of the tube. This electric field also makes consistent results practically impossible. To eliminate this condition, a small piece of spring wire is mounted with a slight pressure against the inner wall of the glass bulb. The connection is then brought out to a base pin and connected to an electrical ground with respect to the other electrodes. If not thus neutralized, electric fields created by the charge on the glass bulb can easily be of sufficient magnitude to exert a greater control over the electron flow than is obtained from the control electrode.

A microammeter, or a galvanometer, must be used in the plate circuit to measure the small currents. The output may also be fed into a suitable voltage amplifier, in which case the RH507 tube will serve as a coupling device between the source under measurement and the amplifier proper. A typical electrometer circuit using a microammeter, or a galvanometer, is shown.

The tube and all leads from the voltage supply should be shielded carefully from stray magnetic or electrostatic fields. It is also necessary to shield the tube from light as there may be some photoelectric effects while sensitive readings are being taken. It is advisable to mount the tube in a reasonably tight shielded can containing a drying agent, such as calcium chloride or phosphorous pentoxide, to protect it from moisture condensing on the bulb surface.

The filament current is very critical and must be held constant. If there is any drift due to battery or other changes, the plate current will naturally shift, which will affect the constancy of the readings. It is therefore advisable to use only a battery which has been seasoned or has been stabilized so that its voltage has become practically constant.

From: Inverted Triode for Industrial Measurements, Electronics, December, 1944, p. 176.

Regards,
Dietmar

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In the same volume, another similar article can be found on RH507 application as an electrometer circuit.

Electrometer Circuit Using Type RH507 Inverted Triode

IN this circuit, a type-RH507 inverted triode is operated with zero grid current as required for the measurement of currents as low as 10^-11 ampere and will indicate the presence of 10^-16 ampere, which represents a flow of about 625 electrons per second. Direct potentials of 0.0001 volt may be measured in circuits having up to 10¹² ohms resistance.

 

Use of the outer electrode as the control grid minimizes the space-charge effect, making it possible to select a value of grid bias that will bring the grid current down to zero for any anode voltage employed in the range from 4.5 to 9 volts. Good C batteries may be used for both the grid and anode voltage supplies. An air cell battery or No. 6 dry cells may be used for the filament provided that they are seasoned, because the filament current is critical and must be held constant.

 

The tube should be mounted in a reasonably tight shielded can containing a drying agent to minimize surface leakage. All leads from the voltage supply should be shielded from stray magnetic or electrostatic fields, and the tubes should be shielded from light to prevent photoelectric effects while readings are being taken. Wherever possible, all leads from the electrodes should be air insulated. Any feed-through insulators used should be quartz glass or a similar material that offers extremely high resistance to surface leakage.

The high sensitivity of this circuit, when used alone or fed into a suitable amplifier, permits measurement of hydrogen-ion concentrations, currents produced by phototubes when subjected to starlight, ion current in mass spectroeters, and other minute quantities that previously required an electrometer or its equivalent.

W. A. Hayes, RH-507 Inverted Triode, Radio News (Radio-Electronic Engineering edition), January,1945.

It is interesting to see a different schematic of the "inverted triode".

Regards,
Dietmar

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Dear Prof. Dietmar,

Very interesting applications indeed on a little known operating mode of vacuum tubes. What about ion vacuum sensors that use similar ion conduction measurement? It would be nice to have some typical circuits.

Regards, Emilio

 

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Dear Emilio,

you are right, ionization gauges also use inverted triodes for measureing the pressure of residual gases in a valve. I made an article in German (sorry)  on this topic, see post #3.

Regards,

Dietmar

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