The action of the latter in controlling the anode current can be explained as follows.Īs we now know from the foregoing, the cathode is normally surrounded by a cloud of electrons known as the space charge. The only true variable is, therefore, the anode voltage. The amount of electron emission depends upon the temperature of the cathode, which is fixed by the voltage supply to the heater, this being a constant value. The magnitude of the current flowing in a diode depends upon the number of electrons emitted and the magnitude of the voltage applied to the anode (known as the anode voltage Va). The Americans, however, never cottoned on to this terminology and always refer to them as 'vacuum tubes'. It also explains the choice of the word 'valve' to describe the device, since a valve is, by definition, a one-way device. This explains why the valve only conducts in one direction, from cathode to anode, and not vice versa. Since electrons are negative charged particles, they will only be attracted to the anode if this is given a positive potential with respect to the cathode. These correspond to the two electrodes of the original diode valve mentioned above, the cathode being the electrode that is heated and emits electrons, and the anode being the electrode that collects the electrons (notice also that these terms have passed forward into semiconductor phraseology - cathode, anode, emitter, collector!). (a) Construction of a modern diode valve (indirectly heated type), (b) circuit symbol for a diode valve. The diode valve is so called because it has just two electrodes – the cathode and the anode.
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