Varible Capacitance diode: Varicap
A varactor (Varicap diode) is a diode with a junction specially formulated to have a relatively large range of capacitance values for a modest range of reverse bias voltages. As the reverse bias applied to a diode changes, the width of the depletion layer, and therefore, the capacitance, also changes. The diode junction capacitance (Cj) under a reverse bias of V volts is given by: Cj = Cjo over Square root Von - V: where Cjo = measured capacitance with zero applied voltage. Note that the quantity under the radical is a large positive quantity for reverse bias. As seen from the equation, for large reverse biases Cj is inversely proportional to the square root of the voltage.
A varactor diode is a P-N junction diode that changes its capacitance and the series resistance as the bias applied to the diode is varied. The property of capacitance change is utilized to achieve a change in the frequency and/or the phase of an electrical circuit
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