VARIABLE CAPACITANCE DIODES

All diodes demonstrate a capacitance which varies with the voltage applied across them.

This is a fundamental property of semiconductor diodes and the one which is exploited in the varicap diode, sometimes known as a varactor. As shown on the right, the capacitance varies because of the changing width of the depletion zone in the diode's junction. An informative article on varicaps can be found on Wikipedia, headed Varicap.

Varicap diodes can be difficult or expensive to soure but, since all diodes have the potential to be used in a variable capacitance circuit, I wondered whether I could use just any old diode. Doing some information gathering on the web confirmed that others, not least amongst whom is Hans Summers, G0UPL, had had the same idea and got it to work.

Although a good deal of work has already been done to validate the use of ordinary diodes in varicap applications, I was moved to test the concept myself.

Taking Hans Summers' test circuit as a basis I built a test rig. I included a 1k resistor in the line from positive supply to the pot as a protection when the pot is set to a low resistance.

The circuit I used is shown below (click on the image or here to see a larger version of the circuit diagram). The picture to the right shows the test rig complete, constructed on a small piece of matrix board.