The Vacuum Tube as a Detector and Amplifier
Presented at Meeting of the Radio Club of America, Columbia University, January 16, 1920
|A large part of the practical wartime development work of the Western Electric Co. in vacuum tubes is creditable to Mr. Clement. This article therefore may be accepted as an authoritative presentation of the subject in question, and fills a need we have long felt in QST.--Editor.|
If the filament is placed in an evacuated bulb, it is obvious that the electrons will penetrate the space surrounding the filament to a greater distance than they will in air, because of the removal of the large gas molecules. Suppose now an electrode in the form of a plate is introduced in the bulb and a potential positive with respect to the filament is applied to it. The negative electrons will flow to the plate. Instead of thinking of a flow of electrons from the filament to the plate, we generally think of a flow of current from the plate to the filament; that is, from the positive to the negatively charged body. If a negative potential were applied to the plate, no electrons would be attracted to the plate, and hence no current would flow because of the lack of these carriers of negative electricity. If an alternating potential were applied to the plate it is obvious that current would only flow through that part of the cycle when the plate was positive with respect to the filament.
The two-element tube, due to its unilateral conductivity, has found some application as a rectifier. The General Electric Company has built some commercial types of rectifiers and some tubes have been built for potentials of 180,000 volts.
Fleming recognized that this device could be used to rectify or detect radio frequency signals. He called the rectifier for this purpose a receiving valve. The device was not very generally used as it was not far superior to the ordinary crystal detectors which required no external battery.
In order to study the action of these valves under different conditions, a circuit shown in Figure 1 is used. The data in Table I shows the effect of filament current (which is a measure of the filament temperature) on the plate current for different plate potentials. These results are shown in the curves of Figure 2. For a constant plate potential, we observe that the plate current increases with filament current until saturation point is reached, after which no rise in the plate current takes place.
On the section of the 60 volt curve, Figure 2, AB, the plate voltage is drawing to the plate all of the electrons emitted, but beyond the saturation point the filament is supplying more electrons than the plate potential can draw to it. This is due to the resultant charge of the cloud of electrons between the filament and the plate which causes the excess of electrons to be returned to the filament. This is sometimes spoken of as the space charge effect.
As is to be expected the saturation points for the 30 and 15 volt currents occur at lower filament temperatures. In general, tubes should be operated beyond the temperature saturation points as then a small change in filament current produces practically no change in plate current.
The voltage-current characteristics of the valve were taken at three different filament temperatures. This data is contained in Table II and plotted on curves in Figure 3. As the plate voltage is increased the plate current rises until the saturation point is reached, beyond which an increase of voltage does not produce an increase of current.
Below the saturation point the filament is emitting more electrons than can be drawn to the plate at the plate voltage applied, due to the space charge effect. Beyond the saturation point all of the electrons emitted by the filament are drawn to the plate. The saturation occurs at lower plate currents when the filament current is less because fewer electrons are available.