For conditions represented by G it is evident that if the grid potential is varied symmetrically, the resulting anode current changes will be asymmetrical. For such conditions the tube may therefore be used as a rectifier of wireless signals, the process being termed anode rectification. When used in this way a typical circuit would be that shown in fig. 6 which may be compared with the use of the diode as illustrated in fig. 3.
The other method of using a three-electrode valve as a rectifier of wireless oscillations is illustrated in fig. 7. High frequency oscillations produced by signals received on the aerial system are transferred to the grid via the condenser C. Due to the fact that the grid collects electrons during the positive half-cycles of potential, but does not do so during the negative half-cycles, the mean potential of the grid becomes negative in the course of a signal and a diminution of anode current the telephone responds to this as a result. When the signal ceases the negative grid charge leaks away via the resistance R.
If, for a tube with characteristics shown in fig. 5, the anode potential is maintained at ioo volts, the relation between grid potential and anode current is, for a fair range of grid potential, sensibly linear. For such a value of anode potential the tube the fall of potential across the resistance R.
The Use of Amplifying Valves in Cascade.—Thermionic valves are frequently used in cascade to secure still higher ampli fication. The type of linkage used between one valve stage and the next depends on the particular type of electrical signal to be amplified. In the majority of cases this signal is either a high frequency alternating potential, such as a radio-frequency wire less signal, or a low frequency alternating potential such as a wireless signal after rectification. Thus in a broadcast receiver could be used as an amplifier. Since, however, the grid when posi tive attracts a certain proportion of the electric current and yet when negative does not do so, a certain asymmetry would prevent the anode current changes from reproducing faithfully the grid potential variations. To obtain distortionless amplification it is usual to use a higher anode potential (e.g., 140 volts) and shift the point representing the conditions of operation into the region of negative grid potential (e.g., to J) by using negative grid bias.
For operating conditions such as are indicated by the point J in fig. 5 the characteristics are approximately linear and for suffi ciently small variations may be represented by the equation where ko, and are constant. ki is the differential mutual
conductance while is the differential anode conductance. From (I 2) the amplification factor is evidently 7 also the differential R2 internal resistance Ri is equal to If a resistance R is connected in the anode circuit of a triode a change of grid voltage brings about a change in voltage across we have high-frequency amplifying stages before the detector stage and low-frequency amplifying stages after the detector stage.
Two methods of linkage for amplifying valves are shown in figs. 9 and io. In the first case a transformer is used to connect the anode circuit of the first valve with the grid circuit of the second. In the second case the changes of potential that occur across the resistance in the anode circuit of the first valve are communicated to the grid of the second valve by means of the condenser C. This method of linkage is known as resistance capacity coupling.
Both of these methods may be used for either high or low frequency amplification, but the type of transformer used in the one case and the values of resistance and capacity used in the other depend upon the frequency of the impulses to be amplified. In transformer coupling air core coils are normally used for radio frequency amplification and iron core transformers for low or audio-frequency amplification. In the case of resistance-capacity coupling the values of resistance and capacity used depend on the particular type of valve used, but in this connection it should be remembered that the linkage ca pacity is always considerably higher for low-frequency ampli fication than it is for high frequency amplification.
The Secondary Electron Tube or Dynatron.—When electrons of sufficient energy strike a metal surface secondary electrons are emitted by the sur face. As the speed of the imping ing electrons is increased, the number of secondary electrons emitted for each electron impact is increased and may reach values higher than unity. As the ma jority of secondary electrons possess very small velocities, the chief factor deciding whether the electrons leave or return to the surface from which they are emitted is the direction of the electric field at that surface. Sec ondary emission takes place at the surfaces of both anode and grid in a three-electrode valve when these electrodes are bom barded by electrons, but the effect, however, is most strikingly marked in the case of the anode because of the much greater value of the current which reaches that electrode.