The Varactor diodes belong to the family of semiconductor devices. These are special purpose diodes, also called a varicap diodes or tuning diodes or variable capacitance diodes, designed to work always in reverse bias state.
But unlike a zener diode they don’t allow the flow of current in zener breakdown regions. These diodes have many applications in the electronics and wireless or radio communication industrial applications. In this article we will discuss varactor diode basics and its principle of operation.
The contents of the lecture are;
- The Design and Basics of the Varactor Diode
- Characteristics and Parameters of Varactor Diode
Capacitance tolerance range
You can watch the following video tutorial which explains the varactor diode working
The Design and Basics of the Varactor Diode
As discussed above, a varactor diode works as a variable capacitor. But how can we do variation in the capacitance of the tuning diode. To understand this point let me remind you a little bit about a simple PN junction diode.
In a simple PN junction diode (Silicon or Germanium), we do doping to make n-type and p-type regions. The N-type region consists of more free electrons as compared to the p-type region.
When we do doping, a PN junction is being created between the n type and p type area. When we create p-type and n-type regions, more free electrons are produced in the n-type region and more holes are produced in the p-type region. But after the creation of PN junction,a depletion region is formed. So no electron can flow from n-side to the p-side due to barrier potential and vice versa. (See figure-1)
Fig-1: PN Junction Diode
This seems like a situation of a parallel plate capacitor. In a parallel plate capacitor, there is a dielectric material between the two plates so no electrons can flow from one plate to the other.
Similar situation can be taken in case of a diode. N-region acts as one plate of a capacitor and p-region acts as the second plate of a capacitor whereas depletion region acts as dielectric material. So by assuming the conditions mentioned above a varactor diode can operate as a variable capacitor. (See figure-2)
Fig-2: A diode as capacitor
This is how a diode can be assumed as a parallel plate capacitor. If you look closely at the varactor diode symbol in figure-3. The symbol is a combination of a diode and a capacitor. Now let’s find out how we can vary the capacitance of a varactor diode.
Fig-3: Varactor Diode Symbol
Characteristics and Parameters of Varactor Diode
We already know we can find the capacitance using the following equation.
where C is the capacitance, A is the area of the plates, epsilon () is the dielectric constant and d is the distance between the plates. From this equation it is clear that if we increase the area of the plates the capacitance increases if we decrease the area of the plates the capacitance decreases.
Similarly by changing the distance, the capacitance changes. if we increase the distance between the plates the capacitance decreases if we decrease the distance between the plates the capacitance increases.
To achieve reverse biasing, let’s say we connected the positive terminal of the battery with cathode and the negative terminal of the battery with anode. So the diode is reversed biased. (See figure-4)
Fig-4: Varactor Diode Biasing
When we increase the reverse voltage the depletion region increases its mean the distance between the plates has been increased whereas the area of the plates has been decreased. (see figure-4)
Fig-4: Increase in reverse bias voltage
So with the increase of the reverse voltage the capacitance will decrease whereas with less reverse voltage the capacitance will increase or in other words for a small reverse voltage the capacitance will be high as compared to a large reverse voltage. (see figure-5)
Fig-5: Reverse Voltage vs Capacitance
In a varactor diode, these capacitance parameters are controlled by the method of doping near the pn junction and the size and geometry of the diode’s construction. varactor capacitances are typically available from a few picofarads to several hundred picofarads.
What is Capacitance Tolerance Range and Capacitance Ratio?
Let’s understand this with the help of the varactor diode data sheet (given below) of zetex 830 series. Minimum, nominal and maximum capacitances are shown for various varactor diodes. It means these diodes can show capacitances between these ranges.
These values are taken at 3V of reverse bias voltage. It means if we apply 3 volt at these diodes they can exhibit the capacitance between these ranges. But these ranges should not be considered as the values of different capacitances when we vary the reverse voltage.
Because those values are measured using another parameter known as capacitance ratio (CR) which is the ratio of the capacitance at minimum voltage to the capacitance at maximum voltage.
You can see in the table the capacitance ratio is shown in the form of C2/C20. This shows these minimum and maximum capacitance ratios are taken by varying the voltage from 2V to 20V. C2 is the capacitance at 2V whereas C20 is capacitance at 20V
Advantage of Capacitance Ratio
Let’s take an example of 831B to understand the capacitance ratio concept and its advantage. You can see in the data sheet above, the minimum capacitance ratio given is 4.5. This means the value of the capacitance of this diode changes with the factor of 4.5 when we increase the reverse voltage from 2 volt to 20 volt.
We can use this factor to find the capacitance at different voltage ranges. Let’s say the capacitance at 2 volt is 15.0 picofarad (pF). Now if we consider capacitance ratio 4.5, the capacitance at 20V can be found by dividing 15 pF with capacitance ratio of 4.5. The capacitance at 20V will be 3.33 pF.
This means if we increase the voltage from 2 volt to 20 volt the capacitance of the diode will change from 15 picofarad to 3.3 pF.
If we consider factor 6.0, then the capacitance at 20V will be 2.5 pF. It means the capacitance will vary from 15 pF to 2.5 pF
Some Questions and their answers
Where varactor diode is used?
The major use of varactor diodes is in VHF, UHF and cellular communication products. Where it is used in the tuning circuits. By changing the capacitance it allows the resonant frequency to be adjusted as variable voltage.
Who invented the varactor Diode?
Erik Kollberg and Anders Rydberg collectively worked to invent this device in 1989 at Chalmers University of Technology.
What is varactor diode modulator?
The varactor diode modulator is a direct method to generate FM transmission. The modulating voltage varies junction capacitance. So the oscillator’s frequency is adjusted accordingly.
Why capacitance of varactor diode varies?
As we have discussed above, the capacitance varies due to change in reverse bias voltage. When we change the reverse bias voltage, the depletion region changes resulting in change of area of p and n types regions.
How to use varactor diode?
If we use single varactor diode in radio communication products. A part of AC signal can forward bias the varactor diode. Hence it produces harmonic distortion. So to avoid distortion mostly two varactor diodes are connected in a back-to-back configuration manner.