What is a Varicap diode?

In electronics, a varicap diode, a varactor diode, a variable capacitance diode, a variable reactance diode or an adjusting diode is a diode type designed to exploit the voltage-dependent capacitance of an inverted p-n junction.

The varators work in a reverse state, so the DC current does not pass through the device. The amount of reverse polarization controls the thickness of the depletion area and therefore the varactor junction capacity. Generally, the thickness of the depletion region is proportional to the square root of applied stress, and the capacity is inversely proportional to the thickness of the depletion region. Thus, the capacity is inversely proportional to the square root of the applied voltage.

All diodes have this variable junction capacity, but varactors are manufactured to exploit the effect and to increase capacity variation.

The example of a cross section of a varactor with the depletion layer formed by a p-n junction. This depletion layer can also be made from a MOS or Schottky diode. This is important in CMOS and MMIC technology.

Generally, the use of a varicap diode in a circuit requires connection to a tuned circuit, usually in parallel with any existing capacitance or inductance. Since a DC voltage must apply reverse bias over the varicap to change its capacitance, it must to be blocked from entering the regulated circuit.

This is accomplished by placing a DC blocking capacitor with a capacity of approximately 100 times the maximum capacitance of the varicap diode in series with it and applying DC from a high impedance source to the node between the varicap cathode and the blocking capacitor.

Example of circuits using varicaps

Since there are no significant DC values ​​in the varicap, the value of the resistor linking the cathode back to the DC control voltage may be somewhere in the range from 22 kΩ to 150 kΩ and the capacitor lock somewhere in the range 5-100 nF. Sometimes, with very high Q circuits, an inductor is placed in series with the resistor to increase the impedance of the control voltage source so that the set circuit does not load and drops Q.

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