Varactor Diodes

Varactor diodes, also known as "variable reactance diodes", are made by using the characteristic that the junction capacitance changes with the applied voltage when the pN junction is reverse biased. When the reverse bias voltage increases, the junction capacitance decreases, and conversely, when the reverse bias voltage decreases, the junction capacitance increases. The capacitance of the varactor diode is generally small. The maximum value is tens of picofarads to hundreds of picofarads, and the ratio of maximum capacitance to minimum capacitance is about 5:1. It is mainly used in high-frequency circuits as automatic tuning, frequency modulation, equal modulation, for example, as a variable capacitor in the tuning loop of a television receiver.

Effect

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, communication and other circuits.

Figure 1. Varactor diode

Characteristics

Varactor diode is a reverse bias diode, and the PN junction capacitance can be changed by changing the reverse bias voltage on its PN junction. The higher the reverse bias voltage, the less junction capacitance, and the relationship between the reverse bias voltage and the junction capacitance is nonlinear, as shown in Figure 2.

Figure 2. Varactor diode and reverse bias

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

(a) The reverse bias voltage increases, resulting in a decrease in capacitance;

(b) The reverse bias voltage decreases, resulting in an increase in capacitance.

The capacitance error range is a specified range of capacitance for a varactor diode. The data table will show the minimum, nominal, and maximum values, which are often plotted on the graph.

Working principle

Varactor diode is a kind of special diode. When a forward bias is applied, a large amount of current is generated, 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, due to the leakage current generated when the forward bias is applied, so the reverse bias is supplied in applications.

In fact, we can think of it as a PN junction. We think that if a reverse voltage V (Varactor diode is used in reverse) is added to the PN junction, the electrons in the N-type semiconductor are led to the positive pole, the holes in the T-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 to d, which changes with the change of the reverse voltage V. As a result, 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), while the reverse voltage decreases, the depletion layer width d becomes narrower, 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. Achieved the goal.

Main uses:

Low-power diodes used for automatic frequency control (AFC) and tuning are called varactor diodes. By applying a reverse voltage, the capacitance of the PN junction changes. Therefore, it is used for automatic frequency control, sweep oscillation, frequency modulation, and tuning. In general, silicon diffusion diodes are used, but alloy diffusion, epitaxial bonding, and double diffusion diodes can also be used, because the capacitance of these diodes varies greatly in terms of voltage. The junction capacitor changes with the reverse voltage VR, replacing the variable capacitor, and is used as a tuning loop, oscillation circuit, and phase-locked loop, and is often used in the channel conversion and tuning circuit of the high-frequency head of the TV, mostly made of silicon materials.

Figure 3. Varactor diodes are used to tune circuits

As shown in Figure 3, the reverse voltage of the diode (D) is changed by changing different R2, which causes the capacitance of the diode to change. Thus, changing the resonant frequency of the varactor diode in it can be adjusted to the full range of variation in the required capacitance in the parallel resonant bandpass filter.