SOFT VALVES 



rapidly accomplished. If R is large the charging phase may be made inde- 

 finitely slow. If V is made large the charge is made quicker — an effect which 

 may be combated by further increase in R — but, more important, only the 



Figure 7.11 



first part of the exponential is used and this is approximately linear. The 

 output is thus of the 'quasi-triangular' form o{ Figure 7.11. Quasi-triangular 

 waveforms occur frequently in electronics. 



HOT CATHODE DIODE 



In the cold cathode diode conduction only becomes appreciable when the 

 energy possessed by positive ions, created by a few chance colhsions, becomes 

 sufficient to knock further electrons out of the cathode to produce a sustained 

 discharge; this only happens when the potential difference is rather large, 

 between 60 and 150 V, according to the type of tube. If the cathode tempera- 

 ture be raised to dull red heat, usually by making it in the form of a ribbon 

 through which a special heating current is passed, there is an adequate supply 

 of electrons thermionically emitted to represent a very considerable current. 

 If the anode be made negative with respect to the cathode, these electrons 

 are all turned back to the cathode and no current ffows. If the anode be 

 made slightly positive with respect to the cathode, some electrons pass to the 

 anode, and others cloud around the cathode to form a negative 'space charge' 

 which tends to prevent the emission of further electrons, and a small current 

 flows. This phase in the operation of the valve is known as 'space charge 

 limited'. When the anode potential is raised further to a critical value — the 



Ionization 

 potential 



Figure 7.12 



ionization potential — which is between 10 and 25 V, depending on the gas 

 used, the average electron energy is suddenly sufficient to ionize the gas 

 fiUing. When this happens the positive ions travel towards the cathode and 

 neutralize the space charge. There is now nothing to prevent any current 

 being passed by the diode that the external circuit dictates, up to the limiting 



126 



