The article explores the working and characteristics of varactor diodes, focusing on their variable capacitance under reverse-bias operation, the factors influencing their efficiency and Q factor, and key design considerations such as material, packaging, and operational ratings.
A varactor diode is a specially manufactured P–N junction with a variable concentration of impurities in its P-type and N-type materials. In a conventional silicon diode, doping impurities are usually distributed equally throughout the material. Varactors have a very light dose of impurities near the junction, but away from the junction, the impurity level increases. This type of construction produces a much steeper voltage−capacitance relationship. Figure 1 shows a comparison of the capacitance between a conventional silicon diode and a varactor diode. It can be seen that an ordinary diode possesses only a small value of internal capacitance. In general, this capacitance is too small to be of practical value.
Varactor Diode Working
Varactor diodes are normally operated in the reverse-bias direction. With an increase in reverse biasing, the depletion region increases its width. This means less resulting capacitance. A decrease in reverse-bias voltage causes a corresponding increase in capacitance. In effect, the capacitance of a diode varies inversely with its bias voltage. This relationship, however, does not change linearly. Note the nonlinear area between 0 and 0.2 V of the varactor diode in Figure 1. The value of capacitance increases rather significantly when the reverse-bias voltage decreases. The capacitance continues to increase even when the diode is forward biased. In fact, the greatest capacitance is produced just before the forward-bias barrier voltage is reached. This barrier voltage value is approximately 0.6 V for a silicon diode. However, when the barrier is reached, the diode becomes conductive. This causes the diode to respond as a shorted capacitor. Because of this condition, varactor diodes should not be used in the forward-bias region. Their normal range of operation is between zero and the reverse breakdown voltage.
Figure 1. Varactor Diode Characteristics Curve at Ambient Temperature, TA, at 25 oC.
The internal capacitance of a varactor diode changes, to some extent, with temperature. Manufacturers usually take this into account by rating the operational characteristics of the device at some temperature value. The capacitance−voltage (C-V) characteristic curve of the varactor diode in Figure 1 is rated at 25 oC. This shows that these C-V characteristics occur only when the device is operating at or near its rated temperature value. The amount of capacitance change that takes place for a given change in temperature is usually expressed as the temperature coefficient of capacitance. The letters TCc denote this characteristic of the diode. As a rule, the TCc value of a varactor diode has a positive temperature characteristic. This shows that as the temperature increases, the depletion region decreases in width. A decrease in the depletion region causes a corresponding increase in capacitance. A positive TCc value, therefore, shows that an increase in temperature causes a corresponding increase in capacitance.
Electrical Characteristics of Varactor Diode
The electrical characteristics of a representative varactor diode, BB139, are shown in Figure 2. Note the absolute maximum ratings, a housing outline, and the electrical characteristics for an operation at 25◦C. The data listed are for minimum, typical, and maximum operations. Most of these characteristics have some influence on the operational frequency of the device. They are tested at a representative operating frequency. Note, in particular, that this diode has a series resonant frequency (fr) of 1.4 GHz (1 GHz is 1,000,000,000 Hz). The varactor diodes shown are used in very high frequency (VHF) and frequency modulation (FM) applications.
Figure 2. Varactor Diode, BB 139, Datasheet Electrical Characteristics
Varactor Diode Operating Efficiency
When the varactor diode is used as a capacitor in a resonant circuit, its operational efficiency becomes very important. The efficiency rating of a capacitor is a ratio of the amount of energy stored compared with the actual amount of energy used by the capacitor in the storing process. This relationship is called the quality factor, or Q, of a capacitor. A varactor diode has a Q rating very similar to that of a capacitor. The Q of a varactor diode is an expression of the capacitive reactance (XC) divided by the series resistance (RS):
$$Q\ =\ \frac{X_C}{R_S}$$
Since the XC of this expression is frequency dependent, the Q will also change with the frequency. The XC of a capacitor is 1/2πfC. XC is measured in Ohms. For a varactor diode, the XC and RS values of the Q expression are combined into a more workable formula. This is expressed as:
$$Q=\frac{1}{2\pi{f}{CR}_S}$$
Note, in this formula, that Q is inversely related to the values of f, C, and RS. The Q of a varactor diode takes into account such things as internal capacitance, series resistance, and frequency. The series resistance is generally due to lead length and the bulk resistance of the semiconductor material.
In conventional operation, the reverse-biased voltage of the diode blocks DC from passing through the device. The internal resistance of a diode is a form of opposition to an AC signal. Since AC passes through the capacitance of a reverse-biased diode, its value is influenced by the RS that it sees. As a rule, the RS of a varactor diode is quite small at high frequencies. The datasheet in Figure 2 shows RS to be 0.35 Ω at a frequency of 600 MHz. This means that a varactor diode offers some measurable amount of opposition to an AC signal. This opposition is frequency dependent. The Q of a varactor diode is often expressed as a number value at an operational frequency. On the datasheet, the Q is 150 at 100 MHz.
Typically, these devices have a rather high value of Q. In general, the Q increases at lower frequencies and decreases in value at higher frequencies. This means that the device works very efficiently at some designated range of AC frequency. If the varactor is subjected to frequencies higher than the designated operating frequency, the Q will drop in value. If the normal operating frequency is exceeded, the Q can be reduced to a value of 1. The frequency at which this occurs is called the cutoff frequency (fc). This operating condition is an important selection characteristic for the varactor diode. The cutoff frequency of a varactor diode can be calculated by the following formula:
$$f_c=\frac{1}{2\pi{CR}_S}$$
Varactor Diode Construction
Varactor diodes are a rather broad general class of semiconductor devices that are designed for a variety of different applications. As might be expected, the construction of the device has a great deal to do with its use in different applications. The material used in its construction, for example, dictates the frequency response. Silicon is used largely for devices that operate on frequencies up to 1 GHz. Gallium arsenide is generally used for devices that operate on frequencies in excess of 1 GHz. The type of housing or packaging of a device is another important selection factor. A variety of different packages are available.
The DO-35 package shown on the datasheet in Figure 2 is preferred for most low-power and low-frequency applications. This type of package is glass enclosed with axial leads that can be soldered into a circuit. The power rating of the device is in the range of 500 mW. Power rating is another important selection consideration. Power ratings range from 500 mW to 50 W for stud-mounted packages. The power rating refers to the ability of a varactor diode to dissipate heat. Generally, this is based on the series resistance (RS) value of the device. The power dissipated is in the form of AC energy that is being manipulated. AC can pass through a reverse-biased diode that responds as a capacitor.
Varactor Diode Symbols
A number of schematic symbols are used to represent the varactor diode. Figure 3 shows three popular methods of representation. The symbol on the left has a small capacitor inside the circle surrounding the diode. This symbol tends to be used more frequently than the other two. Industrial electronic circuits and specialized engineering schematics can use the other two symbols.
Figure 3. Varactor diode symbols.
Varactor Diode Review Questions
1. The depletion region of a varactor diode serves as the _____ material of a capacitor.
2. The P-type and N-type materials of a varactor diode serve as the _____ of a capacitor.
3. The _____ of a varactor diode is changed by altering the reverse-bias voltage of the junction.
4. The junction of a varactor diode is normally _____ biased when operated as a voltage-variable capacitor.
5. An increase in the reverse-bias voltage of a varactor diode causes the junction capacitance to (increase, decrease).
6. A decrease in the reverse bias of a varactor diode causes the junction capacitance to (increase, decrease).
7. The C-V characteristics of a varactor diode change to some extent with _____.
8. _____ is commonly used in the construction of a varactor diode that will control frequencies below 1 GHz.
9. The _____ rating of a varactor diode refers to its ability to give off or dissipate heat.
10. Varactor diodes are commonly used as the _____ component of an LC resonant circuit.
11. The Q factor of a varactor diode is an expression of _____.
12. When the Q of a varactor diode drops to a value of 1, it is called the _____ frequency.
13. Varactor diodes generally have a rather (high, low) Q rating.
Answers
- dielectric
- conductor plates
- capacitance
- reverse
- decrease
- increase
- temperature
- Silicon
- power
- tuning
- operating efficiency
- cutoff
- high