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Diodes - Variable Capacitance (Varicaps, Varactors)

BB208-02,115 is a type of varicap diode, also known as varactor diode. It is a variable capacitance diode where the capacitance is variable with applied voltage.

Application

The most common use of variable capacitance diodes is in oscillator circuits where they are used to set the resonance frequency. They are also known as voltage controlled capacitors or voltage variable capacitors (VVCs). These diodes are also used in RF tuners for automatic gain control and for frequency multiplication and detection. Other applications include FM discriminators, phase comparators, logic energy storage and voltage dividers.

Working Principle

The working principle of a variable capacitance diode is based on the fact that the depletion region width of the diode is proportional to the applied reverse bias voltage. The width of the depletion region is the distance between the N-type and P-type layers, and this width is reduced when the reverse bias voltage is increased. When a capacitor is connected across the diode, the capacitance is proportional to the depletion region width. So, if the reverse bias voltage is increased then the capacitance of the capacitor is also increased.

A varicap diode has two leads, the anode and the cathode. When a reverse bias voltage is applied between the anode and the cathode, the depletion region height increases, resulting in a decrease in capacitance. The capacitance can be varied over a wide range by varying the reverse bias voltage applied to the diode.

Advantages

The main advantage of a varicap diode is that it allows for a wide range of capacitance values. The capacitance can be varied over a large range with small changes in the reverse bias voltage. This makes it ideal for use in oscillator circuits or for resonant frequency adjustment.

Another advantage of varicap diodes is that they are easy to use, due to their simple design. They do not require the use of any external components and are very small in size. This makes them perfect for applications where size and weight are critical constraints.

Disadvantages

The main disadvantage of a varicap diode is that the capacitance is dependent on the applied voltage, which can cause some problems. If the voltage is too high, the diode will be damaged, and if it is too low then the capacitance will be insufficient for the required purpose. Another problem is that the diodes can be noisy due to the large capacitance changes.

In addition, the capacitance of these diodes is dependent on temperature, so they are not suitable for use in extreme temperatures.

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