SMV1405-040LF vs SMV1275-079LF
| Part Number |
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| Category | Diodes - Variable Capacitance (Varicaps, Varactors) | Diodes - Variable Capacitance (Varicaps, Varactors) |
| Manufacturer | Skyworks Solutions Inc. | Skyworks Solutions Inc. |
| Description | DIODE VARACTOR 30V 20MA 0402 | DIODE VARACTOR |
| Package | Cut Tape (CT) | Tape & Reel (TR) |
| Series | - | - |
| Operating Temperature | -55°C ~ 125°C (TJ) | -55°C ~ 125°C (TJ) |
| Mounting Type | Surface Mount | Surface Mount |
| Package / Case | 0402 (1005 Metric) | SC-79, SOD-523 |
| Supplier Device Package | 0402 | SC-79 |
| Diode Type | Single | Single |
| Voltage - Peak Reverse (Max) | 30 V | 10 V |
| Capacitance @ Vr, F | 1.45pF @ 4V, 1MHz | 1.81pF @ 4V, 1MHz |
| Capacitance Ratio | 4.1 | 1.8 |
| Capacitance Ratio Condition | C0/C30 | C1/C4 |
| Q @ Vr, F | 3200 @ 4V, 50MHz | - |
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1. Is a diode a variable capacitor?
The diode itself is not a variable capacitor, but a varactor diode has the characteristic of variable reactance and can be regarded as a special variable capacitor.
A varactor diode is a special diode made by using the characteristic that the size of 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; conversely, the junction capacitance increases. The capacitance of this diode is generally small, with a maximum value of tens of picofarads to hundreds of picofarads, and the ratio of maximum capacitance to minimum capacitance is about 5:1. Varactors are mainly used in high-frequency circuits for automatic tuning, frequency modulation, and phase modulation, such as as variable capacitors in the tuning circuit of a television receiver.
The working principle of a varactor diode is based on the reverse bias and charge accumulation effect of a PN junction. When a reverse voltage is applied to a PN junction, the charge accumulates to form a barrier layer similar to a capacitor. As the reverse voltage changes, the thickness of the barrier layer changes, resulting in a change in the junction capacitance. This phenomenon of dynamic capacitance change with reverse voltage enables varactor diodes to play an important role in high-frequency circuits. -
2. What is a variable capacitor?
Variable capacitor, also known as variable capacitor, is a capacitor whose capacitance can be adjusted within a certain range. Its working principle is to change the capacitance value by changing the relative effective area or distance between the pole pieces. Common variable capacitors include mechanical, electronic and piezoelectric types.
Classification
Mechanical variable capacitor: Change the capacitance value by rotating or moving the metal plate.
Electronic variable capacitor: Change the capacitance value by changing the reverse voltage of the PN junction.
Piezoelectric variable capacitor: Use certain crystal materials to produce polarization when subjected to mechanical stress, thereby changing the capacitance value. -
3. What effect does capacitance have on diodes?
The effect of capacitance on diodes is mainly reflected in the following aspects:
Capacitance effect: Diodes have capacitance effect, mainly including barrier capacitance (CB) and diffusion capacitance (CD). Barrier capacitance is formed by the difference in conductivity on both sides of the PN junction, and its capacitance changes with the applied voltage. Diffusion capacitance is the charge accumulation effect formed by the diffusion of carriers in the P region and the N region when the diode is forward biased.
Equivalent circuit: In order to simplify circuit analysis, the diode can be simulated by an equivalent circuit. In a DC circuit, the ideal diode model can be regarded as an ideal switch, which is equivalent to a closed switch when forward biased and an open switch when reverse biased.
Series effect: When a capacitor is connected in series with a diode, a short current spike may occur, especially in a DC circuit. But this usually does not directly cause the capacitor to burn out unless the current is very large or the capacitor quality is very poor. In an AC circuit, the impedance of the capacitor changes with frequency, which may affect the working state of the circuit, but will not directly cause the capacitor to burn out.
Parallel effect: When a capacitor is connected in parallel with a diode, the connection of the capacitor does not interfere with the normal operation of the diode, nor does it directly cause the capacitor to burn out. In this case, the capacitor mainly plays a filtering or energy storage role. -
4. What is the use of varactor diodes?
The main uses of varactor diodes include the following aspects:
High-frequency tuning and communication: The varactor diode uses the characteristics of the PN junction capacitance changing with the applied voltage to be used as a variable capacitor in high-frequency tuning and communication circuits to achieve functions such as automatic tuning, frequency modulation, and phase modulation.
Wireless communication: In wireless communication systems, varactors are often used in tuning circuits, such as receivers and transmitters that tune VHF and UHF bands. By changing the reverse bias voltage, the resonant frequency of the circuit can be adjusted, thereby achieving frequency selection.
RF signal processing: In RF signal processing, varactors are used to achieve frequency modulation and phase modulation. They can be used as part of a tunable filter to select signals of a specific frequency.
Oscillator: In an LC oscillator, a varactor can be used as a tunable capacitor to adjust the oscillation frequency. This application is very common in radio equipment.
Automatic gain control: In some communication systems, varactors can be used for automatic gain control to adapt to different signal strengths by adjusting the gain of the circuit.
Tuning circuit: In a tuning circuit, a varactor can be used to adjust the resonant frequency of the circuit to match different signal sources.
Analog multiplier: Varactors are used as variable capacitors in analog multipliers to achieve signal multiplication.
Electronic music equipment: In electronic music equipment, varactors can be used to modulate sound and produce special sound effects.
Precision Electronics: In precision electronic devices, varactor diodes can be used to achieve precise voltage control and signal processing.
Sensors: In some types of sensors, varactor diodes can be used to detect physical quantities such as pressure and temperature, reflecting changes in the measured value by changing the capacitance value.
