BB172X Discrete Semiconductor Products

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Variable capacitance diodes, also known as varicaps or varactors, are three-terminal devices that can be used in a variety of electronic applications. A device such as the BB172X comprises two layers of a semiconductor junction between two electrodes, layered like a sandwich. As a voltage is applied to the terminals, the junction is altered, resulting in a variation in capacitance between the two electrodes.The BB172X is intended for operation from RF to low frequency applications up to 8 GHz and is also utilized in switching circuits and modulators. It also has low power losses, high breakdown voltage, low noise, high reverse transfer capacitance as well as good linearity.The working principle behind the BB172X can be explained as follows. By applying a positive bias to the anode, a field is built up between the cathode and anode. This creates a potential barrier to the flow of electrons across the junction. At the same time, the depletion region formed by one of the two layers of semiconductor materials creates a region of negative carriers.Now, when an external voltage is applied to the terminals, the two depletion layers form a junction which produces a variation in capacitance depending on the polarity of the external voltage. In the event of a positive voltage being applied, the depletion region does not stop the flow of electrons and the capacitor has a higher capacitance than when with no voltage applied. On the other hand, in the case of a negative voltage, the depletion region prevents the flow of electrons and the capacitor has a lower capacitance. The relationship between the capacitance of the junction diode and the applied voltage is linear and this allows the capacitor to be used in a wide variety of applications. One such application is within an oscillator circuit. Here the action of the combination of the oscillator and the BB172X creates an output in the form of a sine wave. In a DC –AC conversion application, an AC signal is generated using the BB172X capacitance as the input and the oscillator as the controlling element. Another important application is in tuning circuits that are utilized in radio frequency circuits for broadcasters and receiver. This circuit comprises of two sets of elements with the first set including an antenna and a tank circuit formed by a coil and a capacitor. The second set comprises of the more complex elements such as a variable capacitor, impedance matching capacitor and the BB172X. In this circuit, the impedance tuning capacitor and the variable capacitor creates an effect on the frequency of the signal. On the other hand, the BB172X is the element which controls the signal level and impedance.In summary, the BB172X variable capacitance diode is a highly versatile element used within a range of electronic applications. Its linear capacitance – voltage relationship as well as its high breakdown voltage, low noise and power loss characteristics makes it suitable for a range of frequencies and circuits that includes oscillator, DC-AC conversion and tuning circuits.

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