Darlington pair

A Darlington pair consists of two bipolar junction transistors (BJTs) connected in a specific configuration where the emitter of the first transistor feeds directly into the base of the second, with both collectors tied together. This arrangement creates a single composite device that amplifies current far more powerfully than either transistor alone. The result is a component that behaves like a single transistor but with a current gain (beta) equal to the product of the two individual transistors' gains, typically reaching 1000 to 20000 or higher.

The practical effect is striking: you can switch large currents or drive heavy loads using only tiny base currents. Where a standard transistor might require 10 milliamps of base current to switch a load, a Darlington pair may need only a few microamps. This makes them invaluable for relay drivers, stepper motor controls, power switching circuits, and anywhere space or signal budget is tight. Darlington pairs come packaged as discrete components (two separate transistors wired together by the user) or as integrated Darlington pairs in single packages, with part numbers like the TIP120 series available from multiple manufacturers.

Trade-offs and limitations

The amplification advantage comes with penalties. A Darlington pair exhibits a higher saturation voltage, typically 0.6 to 1.4 volts between collector and emitter when fully on, compared to 0.1 to 0.3 volts for a single transistor. This voltage drop wastes power as heat, matters in low-voltage circuits, and limits the device's usefulness in precision analog applications. Switching speed is also slower because charge must move through two semiconductor junctions rather than one. The composite base-emitter voltage is roughly 1.4 volts instead of 0.7 volts, which constrains design in low-voltage logic circuits.

Darlington pairs are also prone to thermal runaway if base drive current rises with temperature, since beta increases as the device heats up. Modern designs often include a resistor between base and emitter (typically 100 ohms to a few kilohms) to improve stability and prevent unwanted switching from leakage current or noise. Some integrated Darlington packages include this resistor internally.

The name honors Sidney Darlington, who patented this transistor configuration at Bell Labs in 1953. Today, despite the rise of MOSFETs and modern integrated circuits, Darlington pairs remain common in legacy industrial equipment, audio amplifiers, and any application where driving a high-impedance base with minimal current is the design requirement. Understanding their behavior is essential for field service and retrofit work on older machinery.