Darlington Pair

Two transistors cascaded so their current gains multiply.

Intermediate

The emitter of the first transistor drives the base of the second:

$\beta_{total} \approx \beta_1 \beta_2$

Gains of 1000 or more let a tiny current switch heavy loads (e.g. the ULN2003 array drives relays and stepper motors).

Drawbacks: the base–emitter drop doubles (~1.2–1.4 V), and the saturation voltage is higher (~0.7–1 V), so more power is wasted than with a single BJT or a MOSFET.

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