Darlington Pair
Transistor
Two BJTs stacked so the first amplifies the second's base current: gains of 1,000–30,000 from one package. Lets a weak signal switch amps — at the cost of a ~1 V minimum drop.
Schematic symbol
Bipolar transistor (NPN)
How darlington pair appears in a circuit diagram.
Types & variants
Key specs
hFE (gain)
1,000+ — microamps of base current switch amps of load.
Vce(sat)
VHigh for a 'switch': 0.9–2 V. It runs warm — budget for it.
Ic max
ACollector current rating, e.g. 5 A for TIP120.
Vbe(on)
V~1.2–1.4 V (two junctions) — needs more base drive voltage than a single BJT.
Markings
Part number on TO-220/TO-92/DIP body (TIP120, ULN2003AN...).
Standard values
TIP120/121/122 (NPN 60/80/100 V), ULN2003/2803 arrays for steppers and relay banks.
How to choose
Use when your control signal can't source enough base current for a single BJT and a logic-level MOSFET isn't at hand. ULN2003 when you need several channels plus built-in flyback diodes.
Pinout & package
TO-220 — front view, tab at back (TIP122 etc)
Check first: Shown for the TIP120–TIP127 family. Small-signal Darlingtons in TO-92 and arrays like the ULN2003 have entirely different pinouts.
TO-220: B-C-E (TIP series). ULN2003: 7 inputs one side, 7 open-collector outputs the other, COM pin for the diode returns.
Example circuits
- Arduino pin driving a 12 V solenoid via TIP120
- ULN2003 driving a 28BYJ-48 stepper
- Relay bank driver
Common failures
Runs hot because of the high saturation voltage — thermal death without a heatsink; C–E short after inductive spikes when the flyback diode was forgotten.
How to test
Diode mode B→E reads ~1.2–1.4 V (two junctions in series — the giveaway it's a Darlington), B→C ~0.7 V, C–E open both ways.
Substitutes
A logic-level MOSFET is usually the better modern choice (cooler, faster); otherwise two discrete BJTs wired as a Darlington.