electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of

diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diod ...

designed to exploit the voltage-dependent

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...

of a reverse-biased

p–n junction A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contai ...

Applications

Varactors are used as voltage-controlled

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of a c ...

s. They are commonly used in

voltage-controlled oscillator A microwave (12–18GHz) voltage-controlled oscillator A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillat ...

parametric amplifier A parametric oscillator is a driven harmonic oscillator in which the oscillations are driven by varying some parameter of the system at some frequency, typically different from the natural frequency of the oscillator. A simple example of a para ...

s, and

frequency multiplier In electronics, a frequency multiplier is an electronic circuit that generates an output signal and that output frequency is a harmonic (multiple) of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the inpu ...

s. Voltage-controlled oscillators have many applications such as

frequency modulation Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing. In analog freq ...

for FM transmitters and

phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...

s. Phase-locked loops are used for the

frequency synthesizer A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephone ...

s that tune many radios,

television set A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...

s, and

cellular telephone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...

s. The varicap was developed by the Pacific Semiconductor subsidiary of the Ramo Wooldridge Corporation who received a patent for the device in June 1961. The device name was also trademarked as the "Varicap" by TRW Semiconductors, the successor to Pacific Semiconductors, in October 1967. This helps explain the different names for the device as it came into use.

Operation

Varactors are operated in a reverse-biased state, so no DC current flows through the device. The amount of reverse bias controls the thickness of the

depletion zone In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...

and therefore the varactor's junction capacitance. Capacitance change characteristic depends on doping profile. Generally, for abrupt junction profile, the depletion region thickness is proportional to the

square root In mathematics, a square root of a number is a number such that ; in other words, a number whose ''square'' (the result of multiplying the number by itself, or  ⋅ ) is . For example, 4 and −4 are square roots of 16, because . E ...

of the applied voltage, and

is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage. For hyperabrupt junction profile capacitance change is more non-linear, but hyperabrupt varicaps have larger capacitance variation and can work with lower voltages. All diodes exhibit this variable junction capacitance, but varactors are manufactured to exploit the effect and increase the capacitance variation. The figure shows an example of a cross section of a varactor with the depletion layer formed of a p–n junction. This depletion layer can also be made of a MOS or a

Schottky diode The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage ...

. This is important in

CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOS ...

and MMIC technology.

Use in a circuit

Tuning circuits

Generally the use of a varicap diode in a circuit requires connecting it to a

tuned circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...

, usually in parallel with any existing capacitance or inductance. A DC voltage is applied as reverse bias across the varicap to alter its capacitance. The DC bias voltage must be blocked from entering the tuned circuit. This can be accomplished by placing a DC blocking capacitor with a capacitance about 100 times greater than the maximum capacitance of the varicap diode in series with it and by applying DC from a high impedance source to the node between the varicap cathode and the blocking capacitor as shown in the upper left circuit in the accompanying diagram. Varicap Ccts-en

Since no significant DC current flows in the varicap, the value of the resistor connecting its cathode back to the DC control voltage resistor can be somewhere in the range of 22 kΩ to 150 kΩ and the blocking capacitor somewhere in the range of 5–100 nF. Sometimes, with very high-Q tuned circuits, an inductor is placed in series with the resistor to increase the source impedance of the control voltage so as not to load the tuned circuit and decrease its Q. Another common configuration uses two back-to-back (anode to anode) varicap diodes. (See lower left circuit in diagram.) The second varicap effectively replaces the blocking capacitor in the first circuit. This reduces the overall capacitance and the capacitance range by half, but has the advantage of reducing the AC component of voltage across each device and has symmetrical distortion should the AC component possess enough amplitude to bias the varicaps into forward conduction. When designing tuning circuits with varicaps it is usually good practice to maintain the AC component of voltage across the varicap at a minimal level, usually less than 100 mV peak to peak, to prevent changing the diode capacitance too much, which would distort the signal and add harmonics. A third circuit, at top right in diagram, uses two series-connected varicaps and separate DC and AC signal ground connections. The DC ground is shown as a traditional ground symbol, and the AC ground as an open triangle. Separation of grounds is often done to (i) prevent high-frequency radiation from the low-frequency ground node, and (ii) prevent DC currents in the AC ground node changing bias and operating points of active devices such as varicaps and transistors. These circuit configurations are quite common in television tuners and electronically tuned broadcast AM and FM receivers, as well as other communications equipment and industrial equipment. Early varicap diodes usually required a reverse voltage range of 0–33 V to obtain their full capacitance ranges, which were still quite small, approximately 1–10 pF. These types were – and still are – extensively used in television tuners, whose high carrier frequencies require only small changes in capacitance. In time, varicap diodes were developed which exhibited large capacitance ranges, 100–500 pF, with relatively small changes in reverse bias: 0–5 V or 0–12 V. These newer devices allow electronically tuned AM broadcast receivers to be realized as well as a multitude of other functions requiring large capacitance changes at lower frequencies, generally below 10 MHz. Some designs of electronic security tag readers used in retail outlets require these high capacitance varicaps in their voltage-controlled oscillators. TVTunerM 01

The three leaded devices depicted at the top of the page are generally two common cathode connected varicaps in a single package. In the consumer AM/FM tuner depicted at the right, a single dual-package varicap diode adjusts both the passband of the tank circuit (the main station selector), and the

local oscillator In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequen ...

with a single varicap for each. This is done to keep costs down – two dual packages could have been used, one for the tank and one for the oscillator, four diodes in all, and this is what was depicted in the application data for the LA1851N AM radio chip. Two lower-capacitance dual varactors used in the FM section (which operates at a frequency about one hundred times greater) are highlighted by red arrows. In this case four diodes are used, via a dual package for the tank / bandpass filter and a dual package for the local oscillator.

Harmonic multiplication

In some applications, such as harmonic multiplication, a large signal amplitude alternating voltage is applied across a varicap to deliberately vary the capacitance at signal rate to generate higher harmonics, which are extracted through filtering. If a sine wave current of sufficient amplitude is applied driven through a varicap, the resultant voltage gets "peaked" into a more triangular shape, and odd harmonics are generated. This was one early method used to generate microwave frequencies of moderate power, 1–2 GHz at 1–5 watts, from about 20 watts at a frequency of 3–400 MHz before adequate transistors had been developed to operate at this higher frequency. This technique is still used to generate much higher frequencies, in the 100 GHz – 1 THz range, where even the fastest GaAs transistors are still inadequate.

Substitutes for varicap diodes

All semiconductor junction devices exhibit the effect, so they can be used as varicaps, but their characteristics will not be controlled and can vary widely between batches. Popular makeshift varicaps include LEDs, 1N400X series rectifier diodes,Rectifier Diodes As Varicaps http://www.hanssummers.com/varicap/varicapdiode.html Schottky rectifiers and various transistors used with their collector-base junctions reverse biased, particularly the

2N2222 The 2N2222 is a common NPN bipolar junction transistor (BJT) used for general purpose low-power amplifying or switching applications. It is designed for low to medium current, low power, medium voltage, and can operate at moderately high spe ...

and BC547. Reverse biasing the emitter-base junctions of transistors also is quite effective as long as the AC amplitude remains small. Maximum reverse bias voltage is usually between 5 and 7 Volts, before the avalanche process starts conducting. Higher-current devices with greater junction area tend to possess higher capacitance. The Philips BA 102 varicap and a common zener diode, the 1N5408, exhibit similar changes in junction capacitance, with the exception that the BA 102 possesses a ''specified'' set of characteristics in relation to junction capacitance (whereas the 1N5408 does not) and the "Q" of the 1N5408 is less. Before the development of the varicap, motor driven

variable capacitor A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefo ...

s or saturable-core reactors were used as electrically controllable reactances in the VCOs and filters of equipment like World War II German

spectrum analyzer A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...

References

External links

Learning by Simulations
Calculation of the characteristics of a varactor diode for various doping profiles

Trimless IF VCO: Part 1: Design Considerations from Maxim.
Basics of varactor diode with design tips
{{Electronic components Capacitors Diodes