Colpitts Oscillator Symbol
Definition: The Colpitts Oscillator symbol represents an LC feedback oscillator circuit block — named after Edwin Colpitts who patented it in 1918 — that generates a sinusoidal output frequency determined by the formula f = 1 / (2π√(LC_eq)), where L is the tank inductance and C_eq is the series combination of the two feedback capacitors C1 and C2, operating from a VCC supply pin to produce a continuous RF or audio-frequency sine wave output.
Also known as: Colpitts oscillator, Colpitts LC oscillator, capacitive voltage divider oscillator, Colpitts VCO, colpitts circuit, LC oscillator (capacitive tap).
What the Colpitts Oscillator symbol means
The Colpitts Oscillator symbol represents a self-sustaining oscillator block with two pins: VCC (power supply input, left) and OUT (sinusoidal frequency output, right). Internally, the circuit consists of an amplifier (typically a BJT, FET, or op-amp) and a resonant LC tank network in which the feedback voltage is taken from a capacitive voltage divider formed by C1 and C2 across the inductor L. The ratio C1/C2 sets the feedback fraction, and the inductance L and the series capacitance (C1 in series with C2) set the resonant frequency.
The Colpitts topology is distinguished from the Hartley oscillator by using a capacitive tap (two capacitors) for feedback rather than an inductive tap. This makes it more suitable for high-frequency (RF) operation where inductors with multiple taps are difficult to wind precisely. Colpitts oscillators are common in radio transmitters, receivers, signal generators, and as reference oscillators in synthesisers.
How to identify the Colpitts Oscillator symbol
The Colpitts Oscillator symbol is a rectangle labelled 'Colpitts Oscillator' or 'Colpitts' with a VCC pin on the left (power supply) and an OUT pin on the right (sinusoidal output). At the discrete-component level, the circuit shows a transistor (common-base or common-emitter BJT, or common-gate FET) with an LC tank in which an inductor L connects from the collector (or drain) to the junction of C1 and C2, with C1 from the junction to ground and C2 from the base/gate to ground.
Function and usage boundary
The Colpitts Oscillator generates a continuous sinusoidal signal at the resonant frequency f = 1 / (2π√(L × C1×C2/(C1+C2))). Sustained oscillation requires that the amplifier loop gain be at least unity (Barkhausen criterion) and that the phase shift around the loop be 0° (or 360°). The capacitive divider C1/C2 provides the required 180° feedback phase shift (in a common-emitter configuration), and the amplifier provides the additional 180° inversion. The output frequency is highly stable for a given LC combination and can be tuned by making L or C2 variable (forming a variable-frequency Colpitts VCO).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a specific Colpitts oscillator symbol. In IEC-standard schematics the circuit is drawn at the component level (transistor + L + C1 + C2) or as a labelled functional block. IEC 60068-2 series covers environmental testing for oscillator circuits used in products. |
|---|---|
| ANSI/IEEE 315 | IEEE 315 / ANSI Y32.2 does not define a dedicated Colpitts oscillator symbol; the circuit is represented as a composite of standard component symbols or as a labelled rectangular block per IEEE Std 315-1975 functional block conventions. IEEE Std 1249 covers crystal oscillator measurements (relevant for crystal-controlled Colpitts). |
| Key difference | Both IEC and ANSI/IEEE use a labelled rectangular block symbol when the Colpitts oscillator is treated as a functional module. At the component level, both standards use the same transistor, inductor, and capacitor symbols. There is no distinct glyph difference between IEC and ANSI representations. |
The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.
Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| vcc | VCC |
| out | OUT |
Where the Colpitts Oscillator symbol is used
- AM/FM radio receivers — local oscillator in the superheterodyne front-end mixing the incoming RF signal with a Colpitts-derived LO frequency
- RF signal generators — Colpitts VCO as the core frequency-generating element in bench instruments
- Wireless transmitters — Colpitts oscillator at the carrier frequency before amplitude or frequency modulation
- Crystal-controlled oscillators — adding a crystal in series or parallel with the LC tank for high stability (Pierce variant is a crystal Colpitts)
- PLL frequency synthesisers — Colpitts VCO in the feedback loop of a phase-locked loop for programmable frequency generation
- Impedance measurement (RF bridges) — low-noise Colpitts reference oscillator as the signal source
- Educational electronics — classic LC oscillator experiment demonstrating Barkhausen criterion and resonance
Key facts
- A Colpitts oscillator generates a sinusoidal output frequency f = 1/(2π√(L × C1C2/(C1+C2))), where the feedback is taken from a capacitive voltage divider C1/C2 across the tank inductor L.
- The symbol has two pins: VCC (supply voltage, left) and OUT (sinusoidal frequency output, right).
- The Colpitts topology is distinguished from the Hartley oscillator by using a capacitive tap (two series capacitors) instead of an inductive tap (centre-tapped inductor) for feedback.
- Sustained oscillation requires the Barkhausen criterion: loop gain ≥ 1 and total phase shift = 0° (or 360°); the C1/C2 divider provides 180° feedback, and the amplifier provides the other 180°.
- The feedback fraction is approximately C2/(C1+C2) — increasing this ratio increases the feedback but reduces output amplitude and may cause distortion.
- The Pierce oscillator (used in crystal oscillators for microcontrollers) is a series-resonant variant of the Colpitts topology with a crystal replacing the inductor.
- Frequency stability depends on the Q-factor of the LC tank: high-Q inductors and low-loss capacitors give lower phase noise; crystal-controlled Colpitts oscillators achieve Q factors of 10,000–100,000.
Frequently asked questions
What does the Colpitts oscillator symbol mean in a circuit diagram?
The Colpitts oscillator symbol represents a sinusoidal signal generator block that produces a continuous output frequency determined by its internal LC tank network (inductor L and two feedback capacitors C1 and C2). The VCC pin provides the power supply and the OUT pin delivers the sinusoidal output. The frequency is f = 1/(2π√(L × C1C2/(C1+C2))).
What does the Colpitts oscillator symbol look like?
As a functional block, the Colpitts oscillator symbol is a rectangle labelled 'Colpitts Oscillator' with a VCC pin on the left and an OUT pin on the right. At the component level it shows a transistor (or FET/op-amp) with an LC tank where the inductor L connects across the series combination of capacitors C1 and C2, with the junction of C1 and C2 as the feedback point.
What is the difference between a Colpitts and a Hartley oscillator?
Both are LC feedback oscillators with a tank circuit and an amplifier. The Colpitts oscillator uses a capacitive voltage divider (two capacitors C1 and C2 in series across the inductor) to derive the feedback signal. The Hartley oscillator uses an inductive tap (a centre-tapped inductor or two series inductors) to derive the feedback. The Colpitts is preferred at higher frequencies (VHF/UHF) because single capacitors are easier to specify precisely than tapped inductors.
How do I calculate the Colpitts oscillator frequency?
The oscillation frequency is f = 1/(2π√(L × C_eq)), where C_eq = C1 × C2 / (C1 + C2) (the series combination of C1 and C2). For example, with L = 2.5 µH, C1 = C2 = 100 pF: C_eq = 50 pF; f = 1/(2π√(2.5×10⁻⁶ × 50×10⁻¹²)) = 1/(2π × 3.54×10⁻⁸) ≈ 4.5 MHz.
What is the Barkhausen criterion for the Colpitts oscillator?
The Barkhausen criterion requires that the loop gain magnitude equals 1 and the total phase shift around the feedback loop is 0° (or a multiple of 360°). In a common-emitter Colpitts, the transistor provides 180° phase inversion; the capacitive feedback network provides the remaining 180° at the resonant frequency. If the loop gain is less than 1, oscillations decay; if greater than 1 without a limiting mechanism, the output clips and becomes distorted.
What is the Pierce oscillator and how does it relate to the Colpitts?
The Pierce oscillator is a crystal-controlled variant of the Colpitts topology. The crystal replaces or supplements the inductor in the tank circuit, operating between its series and parallel resonant frequencies in an inductive region. The two Colpitts capacitors (C1 from inverter output to ground, C2 from inverter input to ground) complete the tank. Pierce oscillators are used in virtually every microcontroller and digital IC that requires a crystal-based clock reference, including the CMOS inverter oscillators in ATmega, STM32, and PIC devices.
Can a Colpitts oscillator be tuned?
Yes. A Colpitts oscillator can be tuned by making C1, C2, or L variable. Using a varactor diode (voltage-controlled capacitor) in parallel with C1 or C2 creates a voltage-controlled oscillator (VCO) where the output frequency varies with an applied control voltage. This Colpitts VCO topology is widely used in phase-locked loop (PLL) synthesisers in radio and telecommunications equipment.
Related symbols
- 555 Timer IC symbol
- Astable Multivibrator symbol
- Crystal Oscillator symbol
- LoRa Module symbol
- nRF24L01 RF Module symbol
- Wien Bridge Oscillator symbol
Physical breadboard, PCB, and panel mapping
No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.
Place the Colpitts Oscillator symbol directly on a wiring diagram or schematic in the free online editor — no download required.