Darlington

A Darlington is a compound semiconductor device made from two bipolar transistors stacked in a single package, with the collector of the first transistor connected to the base of the second. This arrangement amplifies the input signal twice over, giving the pair a current gain (beta) typically between 1,000 and 20,000, compared to 50 to 200 for a single transistor. The trade-off is a higher forward voltage drop across the base-emitter junction: approximately 1.4 volts instead of 0.7 volts.

Darlingtons appear in two main forms. Discrete Darlington pairs, such as the 2N6045 or TIP100 series, come as three-lead devices (base, collector, emitter) in TO-220 metal cases or smaller plastic packages. Darlington arrays integrate multiple pairs on a single chip; the ULN2003 is a classic example, packing seven Darlington pairs with built-in collector resistors for driving logic loads. The integrated arrays are popular in control circuits because they allow direct connection to microcontroller outputs without additional components.

Where and why they work

The high gain of a Darlington makes it ideal when small input currents must switch or control large load currents. Stepper motor drivers, relay controllers, and display segment drivers routinely use Darlington arrays. A microcontroller GPIO pin delivering 5 milliamps can switch several hundred milliamps through a Darlington output, eliminating the need for a separate current-limiting resistor on the input. However, the high gain also means the device is sensitive to base-emitter leakage; at elevated temperatures, a Darlington can drift toward partial conduction even with zero input signal.

The high saturation voltage (0.4 to 0.7 volts between collector and emitter) and slow switching speed (typically 5 to 20 microseconds rise time) make Darlingtons unsuitable for audio amplification or high-frequency switching. High-speed logic applications favor MOSFETs or smaller single transistors. For motor control and inductive loads, free-wheeling diodes across the load are essential because the Darlington's slow turn-off can permit dangerous voltage spikes when the current suddenly reverses.

The device is named for Sidney Darlington, who described the configuration in a 1953 patent. Today Darlington pairs remain standard in industrial control systems, particularly in legacy equipment and in simple cost-effective designs where speed is not critical and driving force matters more than precision.