What are Varactor Diodes?

Catalog

Ⅰ Introduction

The capacitance of the varactor diode is generally small, and its maximum value is tens of picofarads to hundreds of picofarads. The ratio of the maximum capacitance to the minimum capacitance is about 5:1. It is mainly used for automatic tuning, frequency modulation, and equalization in high-frequency circuits, such as variable capacitors in the tuning loop of a television receiver.

The main features of varactor diodes are small size, dust and moisture resistance, shock and vibration resistance, and long life. For example, the use of varactor diodes in the tuner circuit of FM radios not only replaces the tuning mechanism of ordinary variable capacitors in a brand-new way but also derived many new technologies such as automatic tuning and pre-selection of programs. These technologies have improved the FM radio to a new level in terms of quality and ease of use.

Ⅱ Features of varactor diodes

Varactor diode

The external conditions of the varactor diode are similar to those of the Zener diode. It must work in the reverse voltage bias region (the Zener diode works in the reverse breakdown state). When a reverse voltage is applied across the varactor diode, the PN junction inside becomes thicker, as shown in Figure. The higher the reverse voltage, the thicker the PN junction. Because the PN junction prevents the current from passing, the varactor diode is in an off state when it works. Here, the PN junction is equivalent to the insulating medium between the two plates of the ordinary capacitor. The P-type semiconductor and the N-type semiconductor are respectively equivalent to the two plates of the ordinary capacitor. That is to say, the varactor diode in the off state, its internal A structure equivalent to a parallel plate capacitor is formed, and this "capacitor" is called junction capacitance. The P-type and N-type semiconductors of ordinary diodes are relatively small, and the junction capacitance formed is small and can be ignored; while the area of P-type and N-type semiconductors is deliberately increased during the manufacture of varactor diodes, thereby increasing the junction capacitance. Under reverse bias conditions, the capacity and varactor effect is greatly enhanced.

The formation of varactor junction capacitance

The varactor diode can be regarded as a small-capacity variable capacitor. To change the frequency, we can connect the varactor diode to the tuning loop and control the reverse voltage applied to the varactor diode. The electronic tuner made of varactor diode has a simple structure, reliable contact, and convenient manufacture, and can realize the purpose of remote control and precise tuning. At present, it has been widely used in color TV sets, FM receivers, and various communication devices. In addition, varactor diodes can also be used for various purposes such as frequency modulation, sweep oscillation, automatic frequency fine-tuning, and control.

The function of the varactor diode is to use the principle of variable capacitance between PN junctions to make semiconductor devices, which are used as variable capacitors in high-frequency tuning and communication circuits.

Varactor diodes are reverse biased diodes. Changing the reverse bias voltage on the PN junction can change the capacitance of the PN junction. The higher the reverse bias, the less the junction capacitance, and the relationship between the reverse bias and the junction capacitance is nonlinear.

Varactors and reverse bias

Diagram of the relationship between the capacitance value of the varactor diode and the reverse bias value:

(a) The reverse bias voltage increases, causing the capacitance to decrease;

(b) The reverse bias voltage decreases, causing the capacitance to increase.

The capacitance tolerance range is the capacitance range of a specified varactor diode. The data table will display the minimum, nominal and maximum values, which are often drawn on the graph.

Ⅲ Varactor diodes symbol

The new standard circuit symbol of the varactor diode is shown below. The graphic symbol is made by adding a capacitor symbol next to the ordinary crystal diode symbol, which vividly shows that this is a special varactor diode.

Varactor diodes symbol

Note: The polarity of "+" and "-" next to the graphic symbol of the varactor diode (it is added for the convenience of explaining the problem, it is not added when drawing the circuit diagram), does not mean that the tube is connected in the reverse direction when it is connected to the circuit. The polarity of the working voltage refers to the polarity of the tube itself.

The varactor diode must be connected to the reverse DC voltage in the circuit, that is, the negative pole of the tube is connected to the high potential in the circuit, and the positive pole is connected to the low potential, so as to ensure its normal operation. The text symbols of varactor diodes are exactly the same as ordinary crystal diodes and are often represented by "VD" or "V". If there are multiple components of the same kind in the circuit diagram, you can add a number after the text symbol to show the difference, such as VD1, VD2.

Ⅳ The main parameters of varactor diodes

The main parameters of varactor diodes include junction capacitance, junction capacitance variation range, maximum reverse working voltage, capacitance ratio, and Q value, etc., and their specific definitions are as follows.

① Junction capacitance. This refers to the capacitance of the PN junction inside the varactor diode under a specific DC reverse voltage. For example, the junction capacitance of a 2GB12 varactor diode is 15~18pF under a 3V reverse voltage and 2.5~3.5pF under a 30V reverse voltage.

②The range of junction capacitance. This refers to the variation range of the junction capacitance when the DC reverse voltage of the varactor diode changes from 0V to a certain voltage value. For example, the junction capacitance of the 2CC13A varactor diode varies from 30 to 70 pF.

③The highest reverse working voltage. This refers to the maximum DC reverse voltage value allowed to be applied at both ends of the varactor diode during normal operation. It is not allowed to exceed this value during use, otherwise, the pipe may be broken down. For example, the maximum reverse working voltage of the 2CCIB varactor diode is 20V, and the maximum reverse working voltage of the 2CCIF varactor diode is 60V.

④Capacitance ratio. This refers to the ratio of the maximum capacitance to the minimum capacitance within the variation range of the junction capacitance, and it reflects the capacitance variation capability of the varactor diode.

⑤Q value. This is the quality factor of the varactor diode, which reflects the loss of loop energy when the tube is connected to the circuit. For example, the Q value of the 2CCIB varactor diode is not less than 2, and the Q value of the 2CC17B varactor diode is not less than 100. The Q value varies with frequency and bias voltage. At a certain frequency, the larger the Q value, the smaller the loss of the varactor diode and the better the quality of the varactor diode.

Ⅴ Working principle of varactor diodes

Varactor Diodes are a kind of special diodes. When a forward bias is applied, a large amount of current is generated, and the depletion region of the PN (positive and negative) junction becomes narrower. And the capacitance becomes larger, resulting in a diffusion capacitance effect; when a reverse bias is applied, a transition capacitance effect will occur. However, since the leakage current will be generated when the forward bias is applied, the reverse bias is supplied in all applications.

Varactors are also called voltage-controlled varactors, which are semiconductors that change the junction capacitance according to changes in the supplied voltage. In other words, as a variable capacitor, it can be used in resonance circuits such as FM tuners and TV tuners, and FM modulation circuits.

In fact, we can think of it as a PN junction. We think that if a reverse voltage V is applied to the PN junction (varicap diodes are used in reverse), the electrons in the N-type semiconductor are directed to the positive pole, and the holes in the P-type semiconductor are led to the negative electrode, and then a depletion layer with neither electrons nor holes is formed. The width of the depletion layer is set as d, which changes with the reverse voltage V. In this way, when the reverse voltage V increases, the depletion layer d becomes wider, and the capacitance C of the diode decreases (according to C=kS/d). when the reverse voltage decreases, the depletion layer width d becomes narrower and the capacitance of the diode becomes larger. The change of the reverse voltage V causes the change of the depletion layer, thereby changing the junction capacity C of the voltage-controlled varactor.

The equivalent circuit of the varactor diode

Figure (a) the equivalent circuit of the Varactor Diodes (b) the simplified equivalent circuit of the Varactor Diodes, where RP is the junction resistance of the reverse bias voltage; LS' is the external lead inductance; LS is the internal lead inductance; CC is the packaging capacitance; RS is the diode body resistance; CJ is the junction capacitance.

Usually, the inductance and package capacitance in the equivalent circuit can be omitted. The simplified equivalent circuit is shown in Figure(b). Generally, the relationship between the varactor diode and the applied voltage can be expressed as

Varactor diodes are semiconductor devices made using the principle of variable capacitance between PN junctions. They are used as variable capacitors in high-frequency tuning and communication circuits.

Varactors come in various packaging forms such as glass package, plastic package, the metal package (gold package), and leadless surface package, as shown in Figure. Generally, low-power and medium-power varactors are sealed with glass, plastic, or surface packaging, while varactors with a higher power are mostly sealed with gold.

Varactor diode package form

Ⅵ Application of varactor diodes

The material is mostly silicon or gallium arsenide single crystal, and epitaxial technology is used. The larger the reverse bias voltage, the smaller the junction capacitance. Varactors have a series of resistance related to the resistivity of the substrate material. For different purposes, varactor diodes with different C and Vr characteristics should be selected, such as electronic varactor diodes dedicated to resonance circuit tuning, parametric varactor diodes suitable for parametric amplifiers.

Low-power diodes used for automatic frequency control (AFC) and tuning are called varactor diodes. By applying a reverse voltage, the electrostatic capacitance of the PN junction is changed. Therefore, it is used for automatic frequency control, sweep oscillation, frequency modulation, and tuning. Although silicon-based diffusion diodes are used, specially manufactured diodes such as alloy diffusion type, epitaxial bonding type, double diffusion type, etc. can also be used. These diodes have a particularly large rate of change in capacitance for voltage. The junction capacitance changes with the reverse voltage VR and replaces the variable capacitance. It is used as a tuning loop, an oscillation circuit, and a phase-locked loop. It is often used in the channel conversion and tuning circuit of a TV tuner.

Typical application circuit of varactor diode 1

Typical application circuit of varactor diode

The capacitor C1 is connected in series with the junction capacitance of the varactor diode VD1 and then connected in parallel with L1 to form an LC parallel resonance circuit. The positive DC voltage is applied to the negative pole of VD1 through resistor R1. When the magnitude of this DC voltage changes, the magnitude of the reverse bias voltage added to VD1 changes, and the size of its junction capacitance also changes. So the resonance frequency of the LC parallel resonant circuit also changes.

A reverse DC bias voltage is applied to the varactor diode. When the reverse bias voltage changes, the junction capacitance of the varactor diode changes.

The junction capacitance of the varactor diode and the total capacitance of C1 in series are connected in parallel with L1. The junction capacitance of the varactor diode is just a variable capacitance capacitor in this LC parallel circuit. Changing the capacitance of the varactor diode can change the resonant frequency of the LC parallel resonant circuit.

Typical application circuit of varactor diode 2

Tuning circuit of varactor diode

Varactors are electronic variable capacitors. In other words, the capacitance exhibited by the variable diode is a function of the reverse bias potential. The figure below is a typical LC oscillator circuit tuned by a varactor diode. The function of the circuit coupling inductor L2 is to input the radio frequency signal to the oscillation circuit when the oscillation circuit is used as a radio frequency amplifier. The main LC oscillating circuit includes the main inductor L1 and a series circuit of capacitors C1 and CR1. In addition, the stray capacitance Cs, which are widely present in electronic circuits, must be considered. The function of the capacitor C2 is to filter the tuning voltage Vin.

Because LC tuning is a function of the resonant frequency of the tank circuit as a function of LC, the ratio of the maximum to the minimum resonant frequency of the tank circuit changes with the square root of the capacitance ratio. The capacitance ratio here refers to the ratio of the capacitance when the reverse bias voltage is the smallest to the capacitance when the reverse bias voltage is the largest. Therefore, the tuning characteristic curve (bias voltage-resonant frequency) of the circuit is basically a parabola.

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