NPN Transistor (Bipolar)

A small base current controls a much larger collector current in the forward-active region.

Circuit operation

I1 supplies 20 µA to the base. RC connects the collector to 5 V; the emitter is grounded. The table shows the base-emitter voltage, collector-emitter voltage, and both branch currents.

Expected behavior

$$I_C\approx\beta I_B=100(20\,\mu\mathrm A)=2\,\mathrm{mA}$$ $$V_{CE}\approx5\,\mathrm V-I_C(1\,\mathrm{k\Omega})=3\,\mathrm V$$

Q1 operates in its forward-active region. Increasing base current eventually pulls the collector into saturation, where the simple current-gain equation stops applying.

Reading the results

The first voltage probe is green and the second is red. Additional voltage traces use distinct colors. Current probes, when present, start with blue and then purple. The operating-point table opens with comfortable row spacing. Zoom to extents includes the complete schematic.

The operating-point table opens automatically with the steady-state voltages and currents.

Try a change

Set I1 DC current to 30 µA; collector current increases from about 2 mA to about 3 mA while the transistor remains forward-active. Restore the saved values after comparing the result.

Model limits

These are generic transistor models, not a specific manufactured part. The bipolar models use nominal forward current gain 100 with illustrative junction capacitances and transit times. Device mismatch, temperature drift, layout parasitics, breakdown and detailed noise are not represented. The schematic and component properties expose the values used.