A Darlington transistor is a composite transistor structure composed of two bipolar junction transistors (BJTs) cascaded together. By connecting the emitter of the first transistor to the base of the second transistor, it achieves a significant increase in current gain. Its total current gain (β) is the product of the gains of two transistors (β ₁× β ₂), which can typically reach thousands to tens of thousands of times, far exceeding that of a single transistor.
core features
High current gain: can drive high current loads (such as motors, relays, LED arrays).
High input impedance: Suitable for driving high impedance loads and reducing the burden on signal sources.
Low output impedance: reduces the impact of load on circuit performance and improves stability.
High saturation voltage drop: Due to the superposition of voltage drops of two transistors, it is usually higher than that of a single transistor.
Slow switching speed: limited by the base charge release time of the second transistor, it is not suitable for high-frequency applications.
key parameter
Current gain (hFE): The total gain is the product of the gains of two transistors (e.g. β ₁=100, β ₂=100, then total β=10000).
Maximum collector current (ICmax): determines the load driving capability (e.g. BD681 can reach 100A).
Maximum dissipated power (Ptot): affects heat dissipation design (such as TIP3055 with a Ptot of 2W).
Saturation voltage drop (VCE (sat)): typically between 1-2V, affecting efficiency.
Packaging types: including TO-220, TO-3, etc., suitable for different power and heat dissipation requirements.
application area
Power amplifier: an audio power amplifier that drives speakers.
Motor drive: controls the start, stop, and speed regulation of the motor.
Switching power supply: efficient conversion and voltage regulation.
Relay driver: Using CMOS circuits to drive high-sensitivity relays.
LED intelligent display screen: As a row/column driver, it controls the pixel emission of the LED matrix board.
Classification and packaging
Combining by polarity:
Same polarity connection (NPN+NPN, PNP+PNP).
Polarized connection method (NPC+PNP, PNP+NPN).
By function:
Ordinary Darlington tube.
High power Darlington transistors with protective circuits (such as BD677 series).
According to packaging:
Plug in type (such as TO-220): suitable for high-power scenarios, easy to dissipate heat.
Surface mount (such as SOT-23): suitable for portable devices and high-density circuit boards.
development trend
Integration: Integrated design with other circuit components or modules to reduce costs and improve reliability.
High frequency: By improving processes and materials, the operating frequency range is increased to meet the needs of high-frequency applications.
Low power consumption: Optimize the design to reduce power consumption, energy consumption, and heat generation.
Modularization: Combining multiple Darlington transistors into modules for easy system deployment and maintenance.
Typical application cases
ULN2003A integrated chip: includes seven Darlington transistor channels, can be directly driven by TTL or CMOS logic gates, and has internal suppression diodes, suitable for driving inductive loads such as relays and stepper motors.
Darlington transistors play an important role in electronic circuits by achieving high current gain and good driving capability through their unique cascade structure. With the continuous advancement of technology, its application fields will be further expanded and its performance will continue to improve.