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Darlington pair

Transistors and thyristors · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

Feed one transistor’s emitter current into a second transistor’s base and the gains multiply: β_total ≈ β1·β2, tens of thousands. A tiny base current can switch an amp. The price is a 1.2–1.4 V base drive and a collector-emitter voltage that never falls below about 0.8 V, so it runs warmer than a single transistor.

Darlington pair: the circuit on the simulator's bench

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How it works

βtotal ≈ β·(β+1) + β = 180·181 + 180 ≈ 32,760

IB = (12 − 2·VBE) / RB = (12 − 1.48) / 220 kΩ = 47.8 µA, enough for a 416.7 mA lamp.

Q2 can never saturate: its collector sits at least one VBE plus Q1’s VCE above its emitter, so VCE ≈ 887 mV and the pair dissipates 340 mW. A single saturated 2N2222 drops about 0.1 V.

Try it on the bench

  1. A 220 kΩ base resistor passes only about 50 µA, yet the lamp is fully on. A single 2N2222 would need about 2 mA.
  2. Compare V_CE with the “Transistor as a switch” lesson: about 0.8 V here against 0.1 V there.
  3. Chips like the ULN2003 contain seven Darlingtons for driving relays and stepper motors from logic pins.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Transistors”.

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