Veractor diodes, also known as varicap diodes, variable reactor diodes or varicap diode is a voltage dependent solid State semiconductor electronics component that offers an internal variable capacitance depending on voltage. Veractor diodes have a negative and positive terminals known as cathode and anode respectively. Just like normal diodes it blocks current in the reverse bias, but it only perfom it intended task only when connected in reverse bias. Therefore, it's different from a Zener diode which can work both in forward and reverse bias. When current is applied to a varicap diode, it blocks it from flowing because of the P-N junction however,charges are accumulated at the P-type and N-type semiconductors region just like a capacitor and the two different semiconductors are separated by the depletion layer. The capacitance of the diode varies proportionally to the amount of current applied to the electrodes(cathode and anode) in the reverse bias. Meaning; the larger the voltage applied, the larger it capacitance while the smaller the voltage applied the smaller the capacitance of the diode. Thus it functions in circuits as a voltage controlled capacitor.
Working principle of a varicap diode.
A varicap diode consists of a P-type and N-type semiconductor joined together just like a junction diode. The veractor diode is made up of P-type and N-type semiconductor. In the N-type semiconductor, free electrons are the majority carriers and holes are the minority carriers. Electric current is conducted by free electrons in the N-type semiconductor while in the P-type semiconductor, holes are the majority carriers and free electrons are the minority carriers, so electric current is conducted by holes in the P-type semiconductor. When a P-type semiconductor is in contact with the N-type semiconductor a P-N junction diode is formed. The P-N junction separates the P-type and semiconductor. At the P-N junction, a depletion region is created a region where mobile charge carriers (free electrons and holes) are absent. The depletion region is made up of positive and negative ions (charged atoms). These positive and negative ions does not move from one place to another place. The depletion region blocks free electrons from n-side and holes from p-side. Thus, depletion region blocks electric current across the p-n junction. When a reverse voltage is applied to the diode it blocks current from flowing. Instead of the diode conducting in the reverse bias, charges are produced at the P-type and N-type semiconductors of the diode, with negative charges formed at the anode and positive charges formed at the cathode just like a capacitor. Charges are formed on the diode due to the dielectric property of the dielectric layer at the p-n junction. The amount of charges produced at the plates depends on the amount of applied voltage. Varicap diode works like a capacitor but only differs because varicap diode's capacitance is voltage controlled.
Circuit applications of veractor diodes.
•Tunning circuits.
Veractor diodes find their major appllications in tunning circuits for VHF(very high frequency) and UHF(ultra high frequency) receivers. They are use to tune receiver circuits into a desired frequency level by varying the voltage across it.
As shown in the circuit diagram above, the frequency of the circuit can be tuned or varied by varying the voltage. The combination of the veractor diode and the inductor creates a parallel resonant circuitry to the output terminals of the AC ground. The capacitors C1 and C2 have no influence on the frequency response of filter since their resistance are very less at the resonant frequency.
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Power Electronics