Bipolar Current Source

A fixed base reference and emitter resistor set an approximately constant collector current.

Circuit operation

V1 holds Q1 base at 1.5 V. RE converts the base-emitter offset into approximately constant emitter current. RL supplies collector current from 5 V.

Expected behavior

$$I_C\approx\frac{V_{ref}-V_{BE}}{R_E}$$ $$V_C=5\,\mathrm V-I_CR_L$$

The collector current is roughly 0.7 mA. It remains approximately constant only while enough collector-emitter voltage remains for forward-active operation. The ideal reference source makes the current-setting principle easy to see.

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 RE to 2 kΩ; the collector current approximately halves. 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.