Darlington transistors are circuits that combine two bipolar transistors in a single device. They provide high current gain (hFE) and require less space than configurations that use two discrete transistors. In Darlington pairs, transistor collectors are tied together and the emitter of the first transistor is directly coupled to the base of the second transistor. The total gain, which is often 1000 or more, is the product of the gain of the individual transistors. Compared to single transistor configurations, Darlington transistor pairs have more phase shift at high frequencies and can become unstable with negative feedback more easily. Darlington transistors also have a higher base-emitter voltage, which is the sum of both base emitter voltages. Sidney Darlington, an engineer at Bell Laboratories in the 1950s, is credited with first combing two transistors on a single chip.
Selecting Darlington transistors requires an analysis of performance specifications. The common emitter current gain (hFE), the ratio of collector current to base current (ß), characterizes the amplifying ability of bipolar transistors. Collector-to-emitter breakdown voltage (VCEO) is the maximum voltage than can be applied continuously in the reverse direction of the collector junction when the emitter is open. Other important considerations include collector-to-base breakdown voltage (VCEO) and maximum collector current (ICmax). Current gain bandwidth product (fT) is the frequency at which the common emitter current gain is in unity. Power dissipation (PD), which is usually expressed in watts or milliwatts, is the total power consumption of the device. Operating temperature (TJ) is the junction’s full-required range of ambient operating temperatures. Some Darlington transistors support a specific temperature range and feature mechanical and electrical specifications that are suitable for commercial or industrial applications. Other devices meet screening levels for military specifications (MIL-SPEC).