Bipolar Transistor Switch

Use a small logic-drive current to switch a larger load current.

Circuit operation

A 0–5 V pulse drives the base through 47 kΩ. RC is the 1 kΩ load from 5 V to the collector. A high input turns Q1 on and lowers Vout.

Expected behavior

$$I_B\approx\frac{V_{in}-V_{BE}}{R_B}$$ $$I_{load}\approx\frac{5\,\mathrm V-V_{CE(sat)}}{1\,\mathrm{k\Omega}}$$

Expect almost 5 V when off and a small collector-emitter saturation voltage when on. The load current is about 5 mA. Saturation makes current depend mainly on RC, rather than on the nominal transistor current gain.

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 chart opens with separate traces on a fixed time window; the table retains the same probe order. Zoom to extents includes the complete schematic.

The saved transient runs for 4.0 ms with a maximum step of 1.00 µs. Compare settled cycles after startup and use time-axis cursors to measure delays or periods.

Try a change

Set RB to 220 kΩ; the smaller base current may prevent full saturation under the same load. 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.