It is a 40V 4A power pnp with high gain and good saturation (200mV max at 1A).

The specialism is, that it was designed by Motorola with Linear Technology to work with a particular chip, the LT1123, which is a low drop-out (LDO) linear regulator controller.

Together, they really deliver an impressive performance: 200mV at 1A rising to 750mV at 4A.

The chip has no facility for current sensing, which saves the additional voltage drop of a sense resistor, and the trouble of integrating a sense amplifier.

However, it does limit the drive current available to the transistor, and the extra-special thing about the transistor is that it is designed to quickly run out of gain when its collector current reaches 5A (see graph).

Gain limiting, combined with the limited base drive, provides an open-loop current limit for the finished regulator.

Combined limiting current with 30 different transistors

“β-based current limit schemes are normally not practical because of uncertainties in the β of the pass transistor. Here the drive characteristics of the LT1123 combined with the β characteristics of the MJE1123 can provide reliable β-based current limiting,” said Linear Tech at the time, back in 1992.

Neat.