Other diodes: Diode types
When choosing a varactor diode from the information on the datasheet, the varactor specifications need to be carefully assessed to see whether its performance meets the needs for the circuit in which it is intended for uses.
In view of the fact that the varactor diode is a semiconductor diode, many parameters are the same as those of other diodes. However there are several other specifications that are unique to the operation of the varactor diode.
As the varactor requirements can be a balance between several competing factors it is necessary to understand the specifications and what they mean.
Varactor reverse breakdown specification
The reverse breakdown voltage, VB of a varactor diode is an important specification because the voltages which can be used to drive these diodes to provide the required range of capacitance can be quite high in some instances.
The capacitance decreases with increasing reverse bias, although as voltages become higher the decrease in capacitance becomes smaller. To achieve the required minimum capacitance and the overall capacitance range it may be necessary to apply some relatively high voltages.
In view of the high voltages that may need to be applied, it is wise to choose a varactor diode that has a margin between the maximum voltage it is likely to expect, i.e. the rail voltage of the driver circuit, and the reverse breakdown voltage of the diode in the specification datasheet. By ensuring there is sufficient margin, the circuit is less likely to fail.
It is also necessary to ensure that the minimum capacitance specification is achievable with the maximum driver voltage. Again a good margin should be left to accommodate variations in the parameters between devices.
Diodes typically operate with reverse bias ranging from around a couple of volts up to 20 volts or possibly 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. Also as the voltage on the diode increases, it is likely that specific voltage supplies for the circuits driving the varactor diodes will be required.
In some diodes there is a sharp point at which breakdown occurs, very much akin to the curve of a Zener diode, and this is where avalanche breakdown occurs. Less high performance diodes will have a much softer breakdown characteristic and diodes with this type of breakdown often offer lower levels of performance.
The reverse breakdown voltage is normally measured at the point where there is 10µA of reverse current. As varactors are often driven from a high impedance source, this limit is very acceptable.
When selecting a varactor diode, the reverse breakdown voltage parameter VB is of importance and the diode should not be run too close to the maximum. It is wise select a diode with a maximum reverse voltage which is a very minimum of 5 volts greater than the maximum voltage which will be applied. A greater margin is advisable as this will improve reliability.
Varactor diode reverse current specification
The reverse current , IR, is another measure of the basic performance of the diode. If the leakage current rises too high, then it will affect the drive circuit which is normally a high impedance. It will also reduce the Q of the tuned circuit in which most diodes are located.
Capacitance range and capacitance ratio specification
The actual capacitance range for a varactor diode depends upon a number of factors: the area of the junction; the width of the depletion region for a given voltage, etc.
It is found that the thickness of the depletion region in the varactor diode is proportional to the square root of the reverse voltage across it. In addition to this, the capacitance of the varactor is inversely proportional to the depletion region thickness. From this it can be seen that the capacitance of the varactor diode is inversely proportional to the square root of the voltage across it.
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.
One of the key parameters for a varactor diode is the capacitance ratio. This is commonly expressed in the form Cx / Cy where x and y are two voltages towards the ends of the range over which the capacitance change can be measured.
For a change between 2 and 20 volts an abrupt diode may exhibit a capacitance change ratio of 2.5 to 3, whereas a hyperabrupt diode may be twice this, e.g. 6.
However it is still necessary to consult the curves for the particular diode to ensure that it will give the required capacitance change over the voltages that will be applied. It is worth remembering that there will be a spread in capacitance values that are obtainable, and this must be included in any calculations for the final circuit.
Maximum frequency of operation specification
There are a number of items that limit the frequency of operation of any varactor diode. The minimum capacitance of the diode is obviously one limiting factor. If large levels of capacitance are used in a resonant circuit, this will reduce the Q. A further factor is any parasitic responses, as well as stray capacitance and inductance that may be exhibited by the device package. This means that devices with low capacitance levels that may be more suitable for high frequencies will be placed in microwave type packages. These and other considerations need to be taken into account when choosing a varactor diode for a new design.
As a particular varactor diode type may be available in a number of packages, it is necessary to choose the variant with the package that is most suitable for the application in view.
Varactor Q specification
An important characteristic of any varactor diode is its Q. This is particularly important in a number of applications. For oscillators used in frequency synthesizers it affects the noise performance. High Q diodes enable a higher Q tuned circuit to be achieved, and in turn this reduces the phase noise produced by the circuit. For filters the Q is again very important. A high Q diode will enable the filter to give a sharper response, whereas a low Q diode will increase the losses.
The Q is dependent upon the series resistance that the varactor diode exhibits. This resistance arises from a number of causes:
- the resistance of the semiconductor in the areas outside the depletion region, i.e. in the region where the charge is carried to the "capacitor plates".
- some resistance arising from the lead and package elements of the component
- some contribution from the die substrate
The Q or quality factor for the diode can be determined from the equation below:
Cv = the capacitance at the measured voltage
R = the series resistance
From this it can be seen that to maximise the Q it is necessary to minimise the series resistance. Varactor diode manufacturers typically use an epitaxial structure to minimise this resistance.
When designing the circuit, the Q of the circuit can be maximised by minimising the capacitance.
These are the main varactor specifications that need to be considered outside the basic diode specifications. These parameters detail the performance of the varactor as a variable capacitance diode and enable its suitability to be gauged in this role.
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