DIODE CIRCUITS 



A diode* is a device which exhibits a markedly lower resistance to the passage 

 of electric current in one direction than in the other (Figure 6.1). If the current 

 passed by a diode is plotted as a function of the potential difference across 

 the terminals, the graph is found to comprise two more or less straight lines 

 possessing different slopes, and the slopes of these lines give the forward 

 resistance and backward resistance of the diode. The figure of merit for 

 diodes is the ratio of these slopes, known colloquially as the 'front-to-back 

 ratio'. 



If the diode is thermionic, its back resistance is effectively infinite and the 

 arrangement is therefore a perfect valve {Figure 6.2). Non-thermionic diodes 



Thermionic 

 diode 



M- 



Rt = 



Semi-conductor 

 diode 



Figure 6. 1 



Figure 6.2 



Figure 6.3 



make use of semi-conducting materials such as copper oxide, selenium, 

 germanium and silicon. With these the backward resistance is regrettably 

 finite, practical front-to-back ratios being upwards of 100 {Figure 6.3). In 

 this chapter we enumerate and discuss some of the functions of diodes. 



RECTIFICATION 



Apart from valve heaters, electronic apparatus is powered largely by direct 

 voltages. Where apparatus is to be run from the supply mains, nowadays 

 mainly alternating voltage, the necessary conversion is done by power rectifica- 

 tion. Again, it often happens that a signal is represented by the amplitude 

 of an alternating voltage and we wish for some reason to convert the signal 

 to a direct voltage. Here again we use the rectification process, this time 

 signal rectification. Signal rectifiers are also sometimes called detectors, not 

 a good name, but one rooted in the history of wireless telegraphy. In brief, 



* Strictly, a diode is any two-electrode device, but here we restrict the term to those in 

 which use is made of asymmetric resistance properties. 



Certain devices sometimes called diodes — notably the cold cathode soft diodes — are so 

 named in the sense of having two electrodes, and are not relevant here. 



96 



