A Darlington transistor is a pair of bipolar transistors connected to provide a very high-current gain from a low-base current. The emitter of the input transistor is always wired to the base of the output transistor; their collectors are tied together. As a result, the current amplified by the input transistor is further amplified by the output transistor. A Darlington is often used where a high gain is needed at a low frequency. Common applications include audio amplifier output stages, power regulators, motor controllers and display drivers.
Also known as a Darlington pair, the Darlington transistor was invented in 1953 by Sidney Darlington at Bell Laboratories. During the 1950s and 1960s, it was also called a super-alpha pair. Darlington recognized the many advantages of this design for emitter-follower circuits and patented the concept.
The Darlington transistor's usually low-power, high-gain nature can make it very sensitive to small changes in input current. Darlingtons are often used in touch and light sensors for this reason. Photodarlingtons are designed specifically for light-sensitive circuits.
The output side is often high-power, lower-gain. With a very high-power transistor, it can control motors, power inverters and other high-current devices. Medium power designs are often used with integrated circuit (IC) logic to drive solenoids, light-emitting diode (LED) displays and other small loads.
The Darlington transistor design provides several advantages over the use of standard single transistors. The gain of each transistor in the pair is multiplied together, giving a quite high-total current gain. The maximum collector current of the output transistor determines that of the pair—it can be 100 amps or more. Less physical space is required, as the transistors are often packaged together in one device. Another advantage is that the overall circuit can have very high input impedance.
The transistor generally follows the same design rules as a single transistor, with a few limitations. It requires a higher base-emitter voltage to turn on, typically twice that of a single transistor. Its turn-off time is much longer since the output transistor base current cannot be actively shut off. This delay can be reduced by wiring a discharge resistor between the base and emitter of the output transistor. Darlingtons are not well-suited for high-frequency applications because of this lag time, however.
The saturation voltage of a Darlington transistor is also higher, often 0.7v DC for silicon instead of about 0.2v DC. This sometimes causes higher-power dissipation, as the output transistor cannot saturate. At higher frequencies, a larger phase shift is also possible, which may lead to instability under negative feedback.
A Darlington transistor schematic often portrays the pair of transistor elements wired together inside of a single large circle. A Complementary Darlington or Sziklai transistor uses opposite types of transistors together. When many low-power pairs are needed in a circuit, a Darlington transistor array IC can be used. Drivers often utilize these since they typically include diodes to prevent spikes when the loads are switched off. Many Darlington circuits are also constructed with pairs of individual discrete transistors wired together.