Electrometer tube
An electrometer tube is a special electron tube with a particularly small grid current . It has been replaced by operational amplifiers in electronic circuits since the 1970s and is no longer used today.
Conventional amplifier tubes have a grid current on the order of a few nA. With tube voltmeters or the tube electrometer for measuring DC voltage at high-resistance voltage sources, such as. B. the pH meter , it is essential that the amplifier tube has the lowest possible grid current. Electron tubes with a grid current on the order of 10 fA to 10 pA are called electrometer tubes . Examples are the 1940 produced by AEG-Osram electrometer tubes T114 and T115a , but also the available yet in the 1960s, T113 and T116 .
Electrometer tubes are usually designed as tetrodes . In contrast to the classic tetrode, however, the first grid serves as a so-called space charge grid . It is operated with a positive voltage of 10 V and accelerates the electrons that have emerged from the cathode towards the anode. In the case of electrometer tubes , the second grid is the control grid , which, with a bias voltage of −3 V, enables the powerless control of the anode current that is typical for electron tubes . The cathode, which is usually only weakly emitting for structural reasons, is supported by the space charge grid. This creates a virtual cathode between the space charge grid and control grid.
Electrometer tubes differ from conventional electron tubes in the following structural changes:
- The high-resistance control grid connection is not led out of the glass bulb via the base, but via a separate connection cap in order to maximize the creepage distance between the grid and other connections of the tube.
- The mechanical fastening and suspension of the control grid inside the tube is usually done using curved glass rods. The reason for the complicated suspension is that the types of glass available in the middle of the 20th century for the construction of electron tubes had a comparatively high electrical conductivity.
- The electrical lead to the control grid is surrounded by a layer of glass inside the tube. This also serves to maximize the creepage distance between the grid and other electrodes. During the manufacturing process, conductive deposits can form on the pinch foot , which can worsen the grid insulation.
- As a cathode material are thoriated tungsten filaments used because oxide cathodes evaporate in use too much material and pure tungsten has no appreciable electron emission at the required low temperatures.
- Not a classic getter .
A further reduction in the grid current can be achieved through external measures:
- Operation with very low anode voltages , max. 6 V . With these low energies, no impact ionization of the inevitable residual gas molecules can take place.
- Operation with the lowest possible cathode temperature in order to prevent evaporation and precipitation of cathode material.
- Residue-free cleaned outer wall of the glass bulb.
- Operation in electrically shielded housings.
- No incidence of light on the tube ( photoelectric effect ).
Due to the low anode voltage and low cathode emission, the slope of electrometer tubes is low. For the T114 tube it is 55 µA / V with an anode current in the range from 100 to 500 µA. The voltage gain is therefore very low and is in the range of 1, with some electrometer tubes even just under 1. Electrometer tubes are therefore primarily used to boost current.
Literature sources
- Josef Schintlmeister: The electron tube as a physical measuring device - tube voltmeter, tube galvanometer, tube electrometer . 4th edition. Springer, Vienna 1944, p. 24 and below .
- Telefunken AG: Handbook special tubes for electronics . Ulm 1965, p. 427 (T113), 429 (T116) .