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BJT current gain (β)

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

In a bipolar transistor a small base current controls a collector current β times larger: I_C = β · I_B. Here a current source feeds the base, so you can see the gain directly, and see it stop once the collector resistor limits the current (saturation).

BJT current gain (β): the circuit on the simulator's bench

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How it works

β ≈ 180 for this 2N2222. IC = β · IB = 180 × 20 µA = 3.6 mA

VCE = VCC − IC·RC = 8.4 V. The transistor is in its active region, acting as an amplifier.

VBE = 687.9 mV: like any silicon junction, about 0.6–0.7 V.

Try it on the bench

  1. Set I_B to 10, 20 and 40 µA: I_C follows at 180× each time.
  2. Set I_B to 100 µA: β × I_B would be 18 mA, but R_C allows only 12 mA. The transistor saturates and V_CE collapses to about 0.1 V.
  3. Open Tools… → DC sweep: sweep V_cc, plot I_C against V_CE and step I_B (10u, 20u, 30u, 40u) to draw the transistor’s output characteristics.

Parts on the bench

In the course

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

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