Hey friends, welcome to the Web kohiki All About Electronics And Electrical. So, in this article, we will learn about the varactor diodes.
What is Varactor Diode
The varactor diodes are the one type of PN junction diodes whose internal capacitance can be changed by applying the reverse bias voltage. So, these diodes are also known as the varicap diodes. And the term varicap refers to the variable capacitance. Similarly, the term varactor refers to the variable reactance.
So, in this diode, as we change the applied reverse bias voltage, its capacitance or in other words its reactance will change. And, this property of the varactor diode is very useful in many applications. Particularly when we want to change the capacitance in the circuit using the externally applied voltage, then these varactor diodes are very useful.
So, because of this property, they are used in the RF communication systems.
this is the symbol of the varactor diode. And the symbol indicates that it is the PN junction diode but unlike the normal rectifier diodes are intended to be used as a capacitor.
Working of the Varactor Diode
So, to understand the working of this varactor diode, first of all, let’s see, how we can change the capacitance of the parallel plate capacitor So, for a parallel plate capacitor, the capacitance can be given as C = eA/ d.
Where A is the area of the plates. while ε is the permittivity of the dielectric material and there this d is the distance between the two plates or the thickness of the dielectric material.
So, for the fixed value of the ε and A, if we change the distance between the two plates or in other words, if we change the thickness of the dielectric medium then the capacitance of the capacitor will change. So, with the same concept, the capacitance of the varactor diode is varied. Now, as I said, the varactor diode is also one kind of PN junction diode.
Now, if you have followed the previous articles on the diode, then you must be aware that in the PN junction diode the depletion region gets formed at the junction where this p-type and the n-type materials meet with each other. So, this p-type material contains an excessive amount of holes while the n-type material contains the electrons. On the other end, if you see this depletion region, then it is depleted of the charge carrier.
So, in a way, this pn junction diode behaves like a capacitor. That means here these p-type and n-type regions act like conducting plates while the depletion region acts as a dielectric medium. And we know that, for the pn junction diode, as we change the applied reverse bias voltage then the width of the depletion region changes.
That means as we increase the applied reverse bias voltage, then the width of the depletion region will also increase. And as the depletion region width increases, then from this equation, we can say that the value of the capacitance will reduce. That means we can say that this capacitance is inversely proportional to the applied reverse voltage.
if we see the exact relation, then it can be given by the following expression. Where V is the applied reverse bias voltage, and Vf is the forward voltage or the built-in voltage of the diode. And here this Co represents the capacitance of the diode whenever it is unbiased. Now, in this equation, the value of n depends on the doping profile of the varactor diode. So, based on the doping profile, there are two types of varactor diodes.
The first one is the abrupt varactor diode So, this is the doping profile of the abrupt varactor diode. So, as you can see, in these p-type and n-type regions, the doping concentration is uniform. But at the junction, there is an abrupt change in the doping profile. So, for this type of junction the value of n = ½.
So usually, the normal pn junction diodes have this abrupt junction. On the other end, this is the profile of the hyper-abrupt varactor diode. And as you can see, on both sides, the doping concentration reduces rapidly with the distance.
for this type of doping profile the value of n =2 So, from this expression, we can say that for the hyper-abrupt junction As the value of this reverse bias voltage increases, then there will be more change in the capacitance. So, in general, for the normal pn junction diode.
if we see the curve of the capacitance versus the applied reverse bias voltage then it will look like this. That means in general any diode can be used as a variable capacitor. But the varactor diodes are optimized and manufactured in such a way that they provide more change in the capacitance with the applied reverse bias voltage. So, now let’s see the equivalent circuit of the varactor diode.
Equivalent Circuit of the Varactor Diode
So, if we apply the reverse bias voltage, then in that condition the equivalent circuit of the varactor diode will look like this. So, here Rr is the reverse resistance of the diode, and this Rs is the ohmic resistance.
Now, typically the value of this Reverse resistance is in Mega Ohms. Because whenever we reverse bias this diode, then only leakage current or the reverse saturation current will flow through the diode.
that current can be represented by this reverse resistance. So, for the varactor diode, to minimize this leakage current or the reverse saturation current, this resistance should be as high as possible. So, this is the equivalent circuit of the varactor diode at the low frequencies.
Specifications of the Varactor Diode
- the first and foremost important specifications are the capacitance range and capacitance ratio. So, if you see the datasheet of any varactor diode then they used to specify the value of the capacitances at the different voltages. For example, as shown in the datasheet, as the reverse voltage changes from 1.2 V to 8V, then the capacitance changes from around 450pF to 25 pF.
- So, basically, this parameter gives an idea, in what range we can change the value of the capacitance for the given diode.
- Then the second important specification is the capacitance ratio. This is commonly expressed as Cx / Cy, where x and y are two ends of the voltage range over which tare measuring the capacitance. For example, over here, the value of x = 1.2 V, while the value of y = 8V.
- it defines, as we change the voltage then how much change can occur in the capacitance value. That means if this ratio is more, then it defines that, as we change the voltage then there will be more change in the capacitance value. For the hyper abrupt varactor diodes, it is possible to achieve ratios which is more than 10. while for the abrupt diodes, usually, it is used to be in the range of 2 to 3. So, for the better tunability of the circuit, large capacitance ratios are desirable.
Reverse Current (Leakage current)
- Another specification is the leakage current or the reverse current. As I mentioned, for the varactor diode, the reverse current or the leakage current should be as minimum as possible.
- Similarly, another important specification is the breakdown voltage. Because these varactor diodes are operated in the reverse bias, condition. So, while operating thee diodes we need to make sure that, the applied reverse bias voltage does not exceed this breakdown voltage. Because if it exceeds, then it can damage Then another very important parameter is the Q-factor.
- So, whenever these varactors are used in the tuning circuits or in the RF filters, then for the sharp response, the diodes with the high Q- factor should be selected. That means for good selectivity, the value of this Q-factor should be as high as possible. So, these are some of the performance parameters for the varactor diodes.
Applications of the Varactor Diode
So, in the end, now let’s see some of the applications of the varactor diodes. As I mentioned earlier, the varactor diode can be used in the tuning circuit.
- 1) Voltage Controlled Oscillator
- 2) Phase and Frequency Modulators
- 3) RF Filters
- 4) FM Receivers and other telecommunication receivers (For tuning)
For example, as shown in the figure, here the varactor diode is employed in the tuning circuit. So, for the given tuning circuit, the resonant frequency fr = 1/2*pi* sqrt (LC’) Where C’= CT + Cc. So, here CT is the capacitance of the varactor diode, while Cc is the coupling capacitor. So, as you can see, as we change the applied reverse bias voltage then we can change the value of this capacitor.
So, in this way, we can tune this circuit to a particular frequency. So, these varactor diodes are used in the FM receivers as well in other communication equipment where the circuit needs to be tuned electronically. Apart from that, they are used in the RF filters, as well as in the voltage control oscillators. So, these are some of the applications of the varactor diode.
So, in summary, I hope you understand, what is the varactor diode, how it works, what are the important specifications and applications of the varactor diode.
So, if you have any questions or suggestions, do let me know here in the comments section below.
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