Darlington Pair Current Gain

Formula

Description

A Darlington pair connects two transistors so the first one’s emitter current drives the second’s base, multiplying their current gains. The result is an extremely high effective beta, allowing a tiny base current to control a large load current—useful for power switching, relay and motor drivers, and high-input-impedance buffers.

Variables

• βtotal — Combined current gain
• β1 — First (driver) transistor gain
• β2 — Second (output) transistor gain

Practical Notes

Two β=100 transistors give β≈10000. The cost is a doubled base–emitter drop (~1.2–1.4 V) and slower switching; a Baker clamp or speed-up components help turn-off.