Silicon and Zener Diodes

Published:2012/12/6 19:54:00 Author:muriel | Keyword: Silicon and Zener Diodes | From:SeekIC

Silicon diodes are widely available and inhabit almost every junk box. I have tried using many types of diodes as voltage variable capacitors, but some types, such as rectifier diodes have a better range of variable capacitance than others. As stated before, reversed-biased diodes will decrease in capacitance as the voltage applied to them is increased. Do not expect the Q and thus the stability of a tuned circuit using silicon diodes to be that of better VVC diodes in a similar circuit. One particularly practical example of an application for using silicon diodes is for BFOs. In a BFO, there is typically a crystal and the desired frequency change is relatively small making silicon diodes attractive as the tuning device. Earlier, I referred to W6WTUs HAM Radio Magazine article. From my own experiments, I confirmed Hugh's readings and found that rectifier diodes, are probably the most practical and easy silicon diodes to use as voltage variable capacitors. As far as rectifiers diodes go, the 1N4001 is probably the most commonly used in this application and a web search of HAM Radio electronic sites found its use in three different oscillator projects. From my experiment, the 1N4001 may exhibit a capacitance swing from 35 pF to 4 pF with a corresponding minimum and maximum reverse DC voltage of 0.5 and 12.0 volts. If you increase the voltage above 12.0, a lower capacitance may be realized but its significance is limited as it is getting into the non-linear area of the voltage versus capacitance curve and above normal QRP operating voltages. In addition, the linearity of the change in capacitance versus reverse voltage change is quite poor under ~5 volts for the 1N4001. I plotted the response curve for the 1N4001 and the results indicated that the best control voltage range to use for VFO tuning linearity is 5.0 to 12.0 volts.



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