. Electronic apparatus for biological research . Figure 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 ). 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. A
. Electronic apparatus for biological research . Figure 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 ). 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
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