Varicap

The capacitance diode or varicap, also known as varactor tuning diode or, is an electronic component. By changing the voltage applied to a variation of the capacity of 10:1 can be achieved. Thus is an electrically controllable capacitance available that has largely replaced the previously common, much larger variable capacitors.

Function

The pn-junction diode having a capacitance which is dependent on the width of the space charge zone. A diode operated in the reverse direction, as occurs at the pn junction, a carrier depletion region, in which an electric field is also caused, by constituting the missing carriers. With increasing voltage increases the width of the charge -free zone so decreases the capacity. By suitable doping capacities ranging from 3 pF to 300 pF can be achieved. The maximum reverse voltage is about 30 V.

Application

The varactor diode is used for tuning of resonant circuits in filters and oscillator circuits, for example, instead of variable capacitors or variable inductors ( variometer ) for frequency selection in radio receivers (eg, radios, television receivers).

In addition to LC resonant circuits connected capacitance diodes are used for automatic fine tuning ( AFC) in receivers or for frequency modulation in FM stations.

The fast frequency control in phase -locked loop (PLL ) is often utilized capacity diodes. This PLL is a key component of all remotely operated radios and cell phones.

Before about in 1970 transistors have been developed with which one could generate outputs of several watts and above 200 MHz, made ​​do with coolable capacitance diodes ( varactors ) for frequency multiplication, for example, 140 MHz to 420 MHz. The distortion of a sinusoidal voltage by means of the strongly non-linear characteristic have been exploited, which occurs in the operation of variable capacitance diodes having more than 20 V AC. By appropriate use of resonant circuits could sift out the desired harmonic.

Vote of LC resonant circuits

For frequency tuning of the LC resonant circuits ( reverse biased ), the capacitance of the diode is set to the voltage that is in parallel with the coupling capacitor C to the condenser. The inductance of the resonant circuit is separated from the alternating current voltage source. They can usually be replaced by a high value resistor of about 100 ohms. To achieve a large range of variation should be only a few volts on the resonant circuit, the AC voltage. Otherwise, the capacitance diode rectifies the AC voltage, generating a bias voltage of several volts own and therefore never reach a large capacity.

The coupling capacitor separates the control voltage of the diode from the resonant circuit.

Thus does not affect the resonance frequency must be chosen. The following applies:

In the series connection of two junction capacitances in parallel with the resonant circuit no additional switching capacity is required, because always one of the two diodes blocking the direct current.

In this circuit, the simpler formula:

However, only half the junction capacitance is active, so it should either select the junction capacitance or the inductance twice. The advantage lies in the improved linearity of the circuit in the LC resonant circuit for large amplitudes. The level of work alternating voltage to varactor diodes must usually be kept very small, so that no unwanted capacitance modulation is carried out by this, since these overlays the kapazitätseinstellende DC.

Capacitance diodes can not be used for tuning resonant circuits with high AC voltages, because their capacity is dependent on the voltage and the maximum reverse voltage is limited. Their temperature dependence is taken into account.