It is therefore the reciprocal of the slope of the static characteristic, and varies with the operating point, although fairly constant over much of the space charge limited region. It is defined as shown below,ĭefining anode slope resistance for a diode valve.Īnd is the value of resistance obtained by dividing a small change in anode voltage by the corresponding change in anode current. It is worth introducing this parameter at this time since it is one that we shall make use of later in discussing the performance of more complex valves. No significant current flows when the anode is negative with respect to the cathode.This is because the anode is collecting electrons at the same rate as they are being emitted by the cathode. In the temperature limited region there is little change in la even though there are large changes in Va.In the space-charge limited region, the characteristic is nearly linear (actually following the 'three-halves' power law: Ia is proportional to Va -3/2).

This is known as the 'splash current' and is the result of a few high energy electrons that manage to cross the inter-electrode gap even without an attracting potential. The current is not exactly zero when the anode voltage is zero, but has a value (Iao) of a few micro-amperes.The static characteristic of a diode valve.Ībove three curves have been drawn for different values of cathode temperature, although in practice, as explained earlier, the cathode is held at a constant temperature. For the diode, these illustrate clearly the dependence of anode current upon anode voltage. We now start getting into the ways in which specifications for thermionic devices are presented. The picture below shows an illustration of this. This gives rise to a continuous current flow around the circuit, which may be detected by an ammeter placed in, say, the anode lead. Electrons arriving at the anode flow to the positive supply terminal, while at the same time an equal number of electrons leave the negative supply terminal for the cathode. The gaps created by these electrons leaving the space charge are filled by further emission from the cathode. With zero anode voltage there is no current flow, and there is a state of equilibrium between the electrons being emitted and those falling back onto the cathode's surface, The application of a small positive voltage to the anode causes some of the space charge electrons to be attracted to the anode, resulting in a small anode current flow. 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.

Anode and cathode of diode