Darlington Pair Symbol
Definition: The Darlington Pair symbol represents a compound transistor configuration in which two bipolar junction transistors (BJTs) are connected so that the emitter of the first transistor drives the base of the second, producing a combined current gain (h_FE) equal to the product of the individual transistor gains, with three external terminals: Base, Collector, and Emitter.
Also known as: Darlington transistor, Darlington pair, super-beta transistor, compound transistor, TIP120 (common discrete Darlington IC).
What the Darlington Pair symbol means
The Darlington Pair symbol denotes a two-transistor compound device that behaves as a single NPN (or PNP) transistor but with a much higher current gain—typically 1,000 to 20,000—than any single transistor. The emitter current of the first (driver) transistor becomes the base current of the second (output) transistor, so the overall base-to-collector current gain β_total ≈ β_1 × β_2. This extremely high gain allows a very small base current (from a microcontroller GPIO pin or logic gate) to switch or control a large collector current (motors, relays, lamps).
In schematics the Darlington pair symbol signals a high-gain switching or amplifying stage where load current far exceeds the drive capability of the control signal source. Integrated Darlington arrays such as the ULN2003A contain seven pairs in a single package, each capable of sinking 500 mA.
How to identify the Darlington Pair symbol
The Darlington Pair symbol resembles a standard NPN (or PNP) BJT symbol but with a visual hint of the internal dual-transistor structure: either two overlapping transistor symbols inside a transistor envelope, or a single transistor symbol with an annotated internal emitter feedback path. For NPN, the emitter arrow points outward (away from the base), identical to a standard NPN transistor. The three external terminals are Base (B), Collector (C), and Emitter (E). A built-in collector-emitter diode (for inductive load flyback protection) is often shown as an integral diode between C and E.
Function in a circuit
A Darlington pair amplifies base current to collector current with gain β_total = β_1 × β_2 + β_1 + β_2 ≈ β_1 × β_2 for large individual gains. The base-emitter saturation voltage is approximately 1.2–1.4 V (two V_BE drops in series), compared to 0.6–0.7 V for a single BJT—a key disadvantage. The saturation voltage V_CE(sat) is also higher (1.0–2.0 V), increasing power dissipation when fully on. Darlingtons are widely used in medium-power switching where their high input impedance and high gain simplify drive circuit design.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-05 covers transistor symbols. The Darlington pair is represented as an NPN (or PNP) BJT envelope enclosing two transistors or as a single BJT symbol with a note indicating the compound configuration. IEC does not define a unique Darlington-specific glyph separate from the BJT symbol. |
|---|---|
| ANSI/IEEE 315 | IEEE 315 / ANSI Y32.2 uses the standard BJT symbol for a Darlington pair; the internal topology is implied by the device name or part number annotation rather than a unique glyph. |
| Key difference | Both IEC 60617-05 and IEEE 315 use the same BJT-derived symbol for a Darlington pair. The distinguishing information (dual-transistor topology, gain annotation) is conveyed by the component label or a detail inset rather than a unique standardised glyph. |
Terminals / pins
| Pin | Name |
|---|---|
| base | Base |
| collector | Collector |
| emitter | Emitter |
Typical values
Current gain (h_FE): 1,000–20,000. Collector current I_C: up to 8 A for discrete packages (TIP120: 5 A). V_CE(sat): 1.0–2.0 V. V_BE(on): 1.2–1.4 V. Maximum collector-emitter voltage V_CEO: 60–100 V for common types. Common devices: TIP120/121/122 (NPN), TIP125/126/127 (PNP), ULN2003A/ULN2803A (array).
Where the Darlington Pair symbol is used
- Driving DC motors, solenoids, and relay coils from microcontroller GPIO pins that can supply only a few milliamps.
- High-gain audio preamplifier output stages requiring low base drive current.
- ULN2003A/ULN2803A Darlington arrays switching multiple inductive loads (steppers, relays, lamps) from TTL/CMOS logic.
- Power H-bridge motor controllers using Darlington pairs on the low-side switch for high-gain, high-current switching.
- Touch sensor circuits where extremely high input impedance (due to high β) is required to sense very small capacitive currents.
- Boost converter gate-drive circuits in legacy designs requiring high-current base drive for power transistors.
Example
An Arduino UNO (GPIO current limit: 40 mA) must drive a 24 V DC solenoid drawing 800 mA. A TIP120 Darlington (NPN, β_total ≈ 1000) is connected with its Base through a 1 kΩ resistor to the Arduino GPIO, Collector to the solenoid (with a flyback diode across the solenoid), and Emitter to ground. The GPIO sources approximately 3–4 mA, which drives 800 mA through the solenoid via the TIP120's 1000× gain.
Key facts
- A Darlington Pair is a two-BJT compound transistor circuit with overall current gain β_total ≈ β_1 × β_2, typically 1,000–20,000, far higher than a single transistor.
- The three external terminals are Base (B), Collector (C), and Emitter (E); an NPN Darlington's emitter arrow points outward, and a PNP Darlington's emitter arrow points inward, following standard BJT conventions.
- The base-emitter on-voltage is approximately 1.2–1.4 V (two V_BE drops in series), about twice that of a single BJT, which increases minimum supply voltage requirements.
- V_CE(sat) is typically 1.0–2.0 V for a Darlington, compared to 0.1–0.3 V for a single BJT—a disadvantage in power-dissipation-sensitive applications.
- Integrated Darlington arrays (e.g. ULN2003A: 7-channel, 500 mA per channel; ULN2803A: 8-channel) include built-in flyback diodes and are standard for driving inductive loads from CMOS/TTL logic.
- IEC 60617-05 and IEEE 315 both represent the Darlington pair using the standard BJT symbol; the dual-transistor topology is identified by the component label rather than a unique glyph.
- Darlington pairs have slower switching speed than single BJTs or MOSFETs because the internal emitter-follower stage increases charge storage, making them less suitable for high-frequency switching (>10 kHz) applications.
Frequently asked questions
What does the Darlington pair symbol look like?
The Darlington pair symbol resembles a standard NPN or PNP BJT transistor symbol with three terminals: Base, Collector, and Emitter. For NPN the emitter arrow points outward; for PNP it points inward. Some schematics show two overlapping transistor symbols inside the envelope to indicate the dual-transistor structure. A flyback diode between Collector and Emitter is often included in the symbol for integrated types.
What does a Darlington pair do in a circuit?
A Darlington pair provides extremely high current gain (typically 1,000–20,000) by cascading two BJTs so the emitter current of the first becomes the base current of the second. This allows a microcontroller or logic gate, which can only supply milliamps, to control loads drawing several amperes.
What is the difference between a Darlington pair and a single transistor?
A Darlington pair has approximately the product of the two individual transistor gains (β_total ≈ β_1 × β_2), producing much higher current gain than a single transistor. The trade-off is a higher saturation voltage (V_CE(sat) ≈ 1–2 V vs. 0.1–0.3 V) and a higher base-emitter turn-on voltage (1.2–1.4 V vs. 0.6–0.7 V), which increases power dissipation.
What are the pins of a Darlington pair?
A Darlington pair has three external pins: Base (B, the control input), Collector (C, the high-current path), and Emitter (E, the return current path). Integrated Darlington ICs such as the TIP120 also expose these three terminals in a TO-220 package.
What is the ULN2003A and how does it relate to the Darlington pair?
The ULN2003A is a 7-channel integrated Darlington array: each channel contains one NPN Darlington pair capable of sinking up to 500 mA at up to 50 V, with an internal flyback diode. The device is designed to interface TTL/CMOS logic directly to inductive loads such as relays, stepper motor coils, and lamps.
Can a Darlington pair be used with a 3.3 V microcontroller?
Yes, but the 1.2–1.4 V base-emitter voltage of a Darlington pair is a larger fraction of the 3.3 V GPIO output than it would be with a 5 V system. A suitable base resistor must be calculated to ensure sufficient base current. For 3.3 V systems, a logic-level MOSFET is often preferred over a Darlington because MOSFETs have no V_BE drop and switch fully with a 3.3 V gate voltage.
Which standard defines the Darlington pair schematic symbol?
IEC 60617-05 (semiconductor device symbols) and IEEE 315 / ANSI Y32.2 both use the standard BJT symbol (NPN or PNP) to represent a Darlington pair. Neither standard defines a unique Darlington-specific glyph; the part number or a 'Darlington' label next to the symbol identifies the configuration.
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