Varicap, Varactor or Tunning Diodes !

Varicap, Varactor or Tunning Diodes !
Varicap, Varactor or Tunning Diodes !
Varicap, Varactor or Tunning Diodes !
Varicap, Varactor or Tunning Diodes !
A varicap diode, also known as varicap, varactor, tuning diode or charge-storage diode, is an electronic semiconductor component where the capacitance is formed by a p-n junction. As the voltage across the diode increases, so does the width of the diode's depletion zone and the capacitance decreases. Appropriate doping can achieve capacitances over a range of 3 pF to 300 pF. The circuit symbol for a varicap diode is illustrated opposite.

Varicap, Varactor or Tunning Diodes !

By adjusting the applied voltage, it is possible to achieve a capacitance variation of 10:1. The capacitance can thus be electrically controlled. The maximum reverse bias voltage is about 30 V.

Varicap diodes are used to tune resonant circuits and filter circuits (e.g. instead of variable capacitors and inductors). For this purpose, the varicap is connected in parallel with a capacitor via a decoupling capacitor CK as shown in the diagram below. The inductor LB ensures that the resonant circuit is isolated in terms of alternating current from the tuning voltage Ua and prevents short-circuiting of the capacitor's AC voltage via the tuning voltage. The decoupling capacitor isolates the diode's tuning voltage from the oscillating circuit. The inductor LB can usually be replaced by a large resistance (as on the experiment card).

Varicap, Varactor or Tunning Diodes !

Use of two series-connected varicap diodes, as shown in the circuit below, eliminates the need for a decoupling capacitor, because one of the two diodes always blocks the flow of direct current.

Varicap, Varactor or Tunning Diodes !

The resonant frequency here is determined by the following equation:

Varicap, Varactor or Tunning Diodes !

CS is the voltage-dependent capacitance of one diode. Because the diodes are connected in series, only half the depletion layer capacitance is active, so that either the capacitance of the depletion or the inductance need to be twice as large compared with their respective sizes in the first circuit. However, an advantage of this second circuit is its more linear characteristic at large amplitudes.

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