Varactor Diode Symbol

Definition: The Varactor Diode symbol represents a PN-junction diode operated in reverse bias as a voltage-variable capacitor — its junction capacitance falls as reverse voltage rises — drawn per IEC 60617 and ANSI Y32.2 / IEEE 315 as a standard diode triangle whose cathode bar is doubled with a parallel capacitor-plate line.

Also known as: varicap, varicap diode, tuning diode, voltage-variable capacitance diode, VVC diode, variable-capacitance diode, epicap.

What the Varactor Diode symbol means

The Varactor Diode symbol denotes a two-terminal device (Anode, Cathode) that exploits the depletion region of a reverse-biased PN junction as the dielectric of a capacitor whose plates are the P and N regions. Increasing the reverse voltage widens the depletion region — pulling the effective plates apart — so junction capacitance DECREASES as reverse bias increases. The relationship is approximately Cj = Cj0 / (1 + VR/φ)^n, where n is about 0.5 for an abrupt junction and 1–2 for hyperabrupt junctions engineered for wide tuning ranges and linear frequency tuning.

Unlike an ordinary diode used for rectification, a varactor is never intended to conduct: it spends its whole life reverse-biased, acting purely as an electronically-adjustable capacitance. This lets a DC control voltage tune an LC resonant circuit with no moving parts, which is why varactors sit at the heart of voltage-controlled oscillators (VCOs), phase-locked loops, electronically-tuned filters, and every classic TV and radio tuner built since mechanical variable capacitors were retired.

How to identify the Varactor Diode symbol

The symbol is a standard diode — triangle (anode) pointing at a bar (cathode) — with the cathode modified to show capacitance: a second line drawn parallel to the cathode bar, forming a capacitor-plate pair at the cathode end. Some drawings keep the diode bar and add the parallel plate slightly separated; others draw the bar itself as two parallel lines. Either way, the diode-plus-capacitor-plate combination is unmistakable next to a plain diode.

IEC 60617 and ANSI Y32.2 / IEEE 315 both use this diode-with-capacitor-bar construction and differ only in minor stylistic details such as an optional envelope circle around the device in older ANSI schematics. Do not confuse the varactor with the Zener diode (bent Z-shaped cathode bar) or the Schottky diode (S-curled cathode bar) — on the varactor the extra element is a straight parallel plate, deliberately evoking the capacitor symbol.

Function in a circuit

In circuit, the varactor is placed across (or in series with) the inductor of a resonant tank, with a DC tuning voltage applied through a high-value resistor or RF choke so the control source does not load the tank. As the tuning voltage rises from about 1 V to 20–30 V, the junction capacitance falls — typically by a ratio of 2:1 to 15:1 depending on junction profile — shifting the resonant frequency f = 1/(2π√LC). Back-to-back series varactor pairs are common in oscillators because they cancel even-order distortion and prevent the RF swing from momentarily forward-biasing a single diode.

The device must always stay reverse-biased: forward conduction destroys the capacitance action and loads the tank, while exceeding the reverse breakdown voltage risks damage. Q factor (typically 100–600 at test frequency) sets how much loss the varactor adds to the tuned circuit, and hyperabrupt types trade some Q for wider, more linear tuning.

Standards: IEC vs ANSI

Terminals / pins

Typical values

Typical tuning varactors: capacitance from about 2 pF to 100 pF at a reference bias (VHF/UHF types such as BB135, BB910, MV209: 2–50 pF), with AM-radio tuning types like the 1SV149 reaching about 500 pF at 1 V. Tuning ratios run 2:1–3:1 for abrupt junctions and up to 10:1–15:1 for hyperabrupt types over a 1–28 V control range. Reverse breakdown voltage is commonly 28–32 V, Q factor 100–600 at 50 MHz test conditions, and reverse leakage under 20 nA. Capacitance follows Cj = Cj0/(1 + VR/φ)^n with n ≈ 0.5 (abrupt) to 1–2 (hyperabrupt).

Where the Varactor Diode symbol is used

• Voltage-controlled oscillators (VCOs) in phase-locked loops, frequency synthesizers, and RF transceivers
• Electronic tuning of radio and TV receiver front ends, replacing mechanical variable capacitors
• Voltage-tuned bandpass filters and preselectors in communications receivers
• FM modulators, where audio applied to the varactor deviates an oscillator's frequency directly
• Automatic frequency control (AFC) loops that trim a local oscillator onto the correct frequency
• Antenna matching and tuning networks in software-controlled and remotely-tuned RF systems

Example

In a 100 MHz VCO schematic, a BB135 varactor has its Cathode pin fed with a 1–12 V tuning voltage through a 47 kΩ resistor and its Anode pin connected through a DC-blocking capacitor across the tank inductor; raising the control voltage from 1 V to 12 V drops the junction capacitance from roughly 18 pF to 4 pF, tuning the oscillator up in frequency, and the PLL adjusts this voltage to lock the output to the reference.

Key facts

• Capacitance moves INVERSELY with reverse voltage: more reverse bias widens the depletion region and lowers capacitance, following Cj = Cj0/(1 + VR/φ)^n.
• The varactor operates only in reverse bias — it is a capacitor, never a rectifier; forward conduction defeats its purpose and loads the circuit.
• Abrupt-junction varactors (n ≈ 0.5) give tuning ratios around 2–3:1; hyperabrupt junctions (n ≈ 1–2) reach 10:1 or more and can give near-linear frequency-versus-voltage tuning.
• The symbol is a diode with a capacitor plate parallel to the cathode bar — straight plate for varactor, bent bar for Zener, curled bar for Schottky.
• Back-to-back series varactor pairs are used in oscillators to cancel even-order distortion and stop RF peaks forward-biasing the junction.
• Q factor (typically 100–600) determines added tank loss; hyperabrupt types trade Q for tuning range.
• The tuning voltage is applied through a high-value resistor or RF choke so the control source does not damp the resonant circuit.
• Common part numbers: BB109, BB135, BB910, MV209, MVAM108, 1SV149 (AM tuning, ~500 pF).

Frequently asked questions

How does a varactor diode symbol differ from a normal diode symbol?

A varactor keeps the standard triangle-and-bar diode symbol but adds a second line parallel to the cathode bar, forming a capacitor-plate pair. That plate is the visual cue that the device is used as a voltage-variable capacitance rather than a rectifier. A Zener's bar is bent into a Z and a Schottky's is curled into an S — the varactor's extra element is a straight parallel plate.

Does varactor capacitance increase or decrease with voltage?

It decreases as reverse voltage increases. Higher reverse bias widens the depletion region — effectively pulling the capacitor plates apart — so capacitance falls according to Cj = Cj0/(1 + VR/φ)^n. A typical tuning diode might present 30 pF at 1 V and 5 pF at 25 V. Maximum capacitance occurs at the lowest usable reverse bias.

Why must a varactor always be reverse-biased?

The capacitance effect exists only while the depletion region is intact, which requires reverse bias. If the diode is forward-biased — even momentarily by a large RF swing — it conducts, the depletion capacitance vanishes, the tank is loaded by the conducting junction, and distortion spikes. Designers keep a minimum reverse bias (often 1 V or more) and frequently use back-to-back pairs so the RF peaks cannot forward-bias either device.

What is the difference between abrupt and hyperabrupt varactors?

The doping profile of the junction. Abrupt-junction varactors have a capacitance exponent n ≈ 0.5, giving a modest 2–3:1 tuning ratio with high Q. Hyperabrupt varactors are doped so n ≈ 1–2, giving much larger tuning ratios (10:1 or more) and, over part of their range, an approximately linear frequency-versus-voltage characteristic prized in VCOs — at the cost of somewhat lower Q and tighter usable voltage range.

Where are varactor diodes used in real circuits?

Anywhere a capacitance must be adjusted electronically: the tank circuit of a VCO inside a PLL frequency synthesizer, electronically-tuned radio/TV front ends, voltage-tuned filters, FM modulator stages where audio directly deviates the carrier, AFC loops, and remotely-tuned antenna matching networks. Any circuit that once needed a mechanical trimmer capacitor adjusted by hand is a candidate for a varactor plus a DAC.

Can I use an ordinary diode as a varactor?

To a degree — every reverse-biased PN junction has voltage-dependent capacitance, and hobby circuits sometimes use a 1N4007 or an LED as a crude tuning diode. But purpose-made varactors have engineered doping profiles for a specified Cj0, tuning ratio, high Q, and tight tolerance, none of which ordinary rectifiers guarantee. For a repeatable VCO or filter, use a real tuning diode.

Related symbols

• Capacitor symbol
• Crystal Oscillator symbol
• Diode symbol
• Schottky Diode symbol
• VCO Block symbol
• Zener Diode symbol

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