Variable Capacitance Diode (Varicap Diode, also called Varactor Diode) – a special type of semiconductor diode. It is designed to exploit the changes in voltage-dependent capacitance of a reverse-biased “p-n junction”, therefore its ability to use junction barrier capacity has been improved. In other words, Varicap can be described as a voltage-controlled capacitor. Varicap Diodes are mostly manufactured from Silicon (Si) and Gallium Arsenide (GaAs). With its manufacture it is strived to achieve the highest value of voltage sensitivity (re-tuning diode factor).
Its capacitance can change in a narrow range, usually from about 6 pF (picofarads) to 20 pF with the voltage change from 2 to 20 V. The main parameters describing the varicap diode are its two limiting capacities: Ctmin – minimum capacity and Ctmax – maximum capacity. As mentioned above, these parameters are dependent on voltage. At high voltage, capacity has minimal value, whereas at small voltage – maximum.
Variable Capacitance Diode detailed description
Variable capacitance diodes Apply changes in capacitance of The p-n junction below the effect of the applied voltage. They generally use polarization from the negative way. This mechanism occurs in virtually any semiconductor diode, polarized from the negative way. On the other hand, the capacitive diode is especially adapted for this function, since it’s distinguished by the method by which in which the semiconductor is doped at the area of the p-n junction, and consequently includes a corresponding concentration of current carriers.
From the barrier coating There’s a dipole charge, shaped by the dopant ions. Since the diameter of the barrier coating and also the worth of this uncompensated charge within this layer vary with fluctuations in the polarization voltage, so the proportion of the boost in charge into the corresponding rise in voltage is understood to be the barrier capacitance. This type of junction is comparable to a normal billed planar capacitor. As is well known, the capacitance of the a capacitor is dependent upon the space between the charged facings. After the obstruction voltage rises, the region of the junction increases, consequently its capacitance declines, while as soon as the barrier voltage declines, the capacitance increases.
Variable capacitance diodes are employed at the part of this Characteristic curve Ct=f(U), bounded by the minimal capacitance Ctmin along with also the capacitance Ctmax. The capacitance Ctmin comes from the junction breakdown voltageas well as the capacitance Ctmax has been decided by the rise in energetic conductivity of this diode, which raises as the conduction current IF increases.
For average diodes, the capacitance changes in a dozen to over 100pF. The aim is to achieve the Greatest possible pruning variable, characterized by The formulation:
Sensitivity, defined by the formula, is also an important parameter:
Component division
- Varicap (Variable Capacitance) – low power component, characterized by capacity varying from 10-500pF, used specially in automatic tuning systems of resonant circuits, usually in amplifiers systems,
- Varactor (Variable Reactance) – high power component, characterized by capacity varying from 0.2 to 20pF. They are mainly used in high frequency devices, e.g. to reproduce frequencies.
Limiting properties
Technical properties and permissible limits for the varicaps:
- Cj – junction diode capacitance at reverse bias and reverse frequency (usually at a voltage value close to maximum),
- Rs – series diode resistance or its Q value at a determined reverse bias and reverse frequency,
- VRmax – the maximum constant reverse voltage,
- VRMmax – the maximum peak constant reverse voltage,
- IFmax – the maximum constant bias,
- Tj – permissible junction temperature.
Technical properties and permissible limits for the varactors:
- Cj – junction diode capacitance at determined reverse bias and reverse frequency (usually for UR = 0, so for maximum capacity),
- IR – reverse current at determined UR reverse voltage (usually at UR -Urmax)
- Maximum frequency at a given reverse voltage,
- VRmax – the maximum constant reverse voltage,
- Pmax – the maximum value of dissipation power,
- Tj – permissible junction temperature.
Varicap characteristics are shown on Fig. 2. below.
Applications
They’re used, by way of instance, to develop voltage-tuned resonant circuits within electronics.
Frequency modulation can be carried out from the machine of an voltage-tuned generator, the frequency of that should shift directly into the value of this voltage. This kind of outcome can be obtained by changing the resonant frequency of the LC circuit of the generator using a modulating signal. It may be accomplished by linking in parallel into LC circuit capacitive (or inductive) reactance linearly based on significance of modulating signal voltage or current.
In the easiest instance, a capacitive diode initially polarized in the adverse direction may be utilized to listen to the resonant circuit. The change of the polarization resulting from the modulating signal will alter the capacitance of the diode, thus restraining the generator. The diagram below shows an instance of the Colpitts generator circuit in the WB figuration, where among those split capacitances was substituted using a capacitive diode polarized at the adverse direction (at the circuit: UCC, an HV choke and resistor R3).
The immediate value of this diode polarization voltage will be the amount of the primary polarization voltage and also the regulating voltage. This ion alters the capacitance of the diode and for that reason the frequency of the generator. Whenever the modulating signal is zero, the generator generates a carrier waveform with frequency fN. Since the modulating voltage rises, the diode capacitance increases along with the generator frequency declines.
PSpice model library
Capacitive diode models are available on the official PSpice manufacturer’s website. There are more than 200 component models to download for free.
Capacitive diode models for PSpice are available for download at this link:
https://www.pspice.com/discrete/diodes/variable-capacitance?page=2
References:
http://www.kueitwn.pollub.pl/images/Inz_Mat_instr/Cwiczenie%20nr%2011%20-%20instrukcja.pdf
http://www.eres.alpha.pl/elektronika/print.php?type=A&item_id=374
