Varicap Diode

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Symbol of the Varicap diode.

The Varicap diode known as variable capacity diode or varactor, is a diode that takes advantage of certain construction techniques to behave in the event of variations in the applied voltage, like a variable capacitor. Reverse biased, this electronic device presents characteristics that are extremely useful in tuned circuits (L-C), where capacity changes are necessary.

Operation

When a Varicap diode is reverse biased, it is said that the potential barrier or junction formed by the materials N and P from the point of union of the junctions produces a capacitance. Seen in a metaphorical and practical way, it is the equivalent of two plates of a capacitor that separate as the supply voltage increases. This increase in voltage causes a decrease in the final equivalent capacity at the diode terminals (the greater the distance between plates, the lower the final capacity). For this reason, the concept is clear that the greatest capacity that a diode of this nature can provide is found at a point of low supply voltage (not zero), while the minimum final capacity will be determined by how much inverse voltage it can withstand between their terminals. Without going to extreme values, the most common are usually between 3 or 4 picofarads and 50 picofarads for examples such as the NXP BB148 diode. With a voltage of less than one volt, it reaches its maximum capacity, reaching its minimum value with 12 or 13V, as we can see in the graph obtained from its data sheet.

In order to measure the capacity of these diodes, you can use the MBR formula:

${displaystyle C_{d}={dfrac {C}{sqrt {1+2|V_{d}|}}} pF}$

where C = capacitance of reverse biased diode (Farads)

V_d= reverse polarization voltage of the diode (Voltios), ( ${displaystyle |V_{d}|}$ is the magnitude of the reverse polarization voltage of the diode, always positive)

C_d= C

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