Varactor or varicap diodes are used mainly in radio frequency (RF) circuits to be able to provide a capacitance that can be varied by changing a voltage in an electronics circuit. This can be used for tuning circuits including radio frequency oscillators and filters.
Although both names: varactor and varicap diode are used, they are both the same form of diode. The name varactor meaning variable reactor, or reactance, and varicap meaning variable capacitance (vari-cap).
Operation of a variable capacitor
They key to understanding how a varactor or varicap diode works is to look at what a capacitor is and what can change the capacitance. As can be seen from the diagram below, a capacitor consists of two plates with an insulating dielectric between them
... the capacitance and the amount of charge that can be stored depends on the area of the plates and the distance between them.... |  |
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... the capacitance and the amount of charge that can be stored depends on the area of the plates and the distance between them.... | |
The capacitance of the capacitor is dependent upon the area of the plates - the larger the area the greater the capacitance, and also the distance between them - the greater the distance the smaller the level of capacitance.
A reverse biased diode has no current flowing between the P-type area and the N-type area. The N-type region and the P-type regions can conduct electricity, and can be considered to be the two plates, and the region between them - the depletion region is the insulating dielectric. This is exactly the same as the capacitor above.
As with any diode, if the reverse bias is changed so does the size of the depletion region. If the reverse voltage on the varactor or varicap diode is increased, the depletion region of the diode increases and if the reverse voltage on varactor diode is decreased the depletion region narrows. Therefore by changing the reverse bias on the diode it is possible to change the capacitance.
Change of varactor diode capacitance with reverse bias
Varactor or varicap circuit symbol
The varactor diode or varicap diode is shown in circuit diagrams or schematics using a symbol that combines the diode and capacitor symbols. In this way it is obvious that it is being used as a varacor or varicap capacitor rather than a rectifying diode.
Circuit symbol for a varactor diode / varicap diode
When operated in a circuit, it is necessary to ensure the varactor diode remains reverse biased. This means that the cathode will be positive with respect to the anode, i.e. the cathode of the varactor will be more positive than the anode.
Varactor circuits
It is impossible to show all the circuits in which varactor / varicap diodes may be used. However it is worth providing one example to show how these diodes may be used in a typical circuit. Effectively a capacitor is replaced with the varactor diode, but it is necessary to also ensure that the tune voltage, i.e. the voltage used to set the capacitance of the diode can be inserted into the circuit, and that no voltages such as bias voltages from the circuit itself can affect the varactor diode.
Voltage controlled oscillator using a varactor diode
Within this circuit D1 is the varactor diode that is used to enable the oscillator to be tuned. C1 prevents the reverse bias for the varactor or varicap diode being shorted to ground through the inductor, and R1 is a series isolating resistor through which the varactor diode tuning voltage or bias is applied.
Specifications
The most important characteristics of the diode are its capacitance and the range of capacitance that can be achieved. Normally two voltage points are specified, one at the top of the range and the other near the bottom at the minimum useable voltage. It is obviously important to select a diode which combines the correct capacitance range for the available tuning voltage range. The higher voltage specified is normally the maximum reverse bias and this should not be exceeded otherwise breakdown may occur.
Diodes typically operate with reverse bias ranging from around a couple of volts up to 20 volts and higher. Some may even operate up to as much as 60 volts, although at the top end of the range comparatively little change in capacitance is seen.
Some varactor circuits can use quite high voltages. With voltages of 60 volts
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